CY2161 - Engineering Chemistry – II

1 CY2161 - Engineering Chemistry – II

CY2161 - Engineering Chemistry – II

Lecture Notes for all units

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UNIT I ELECTROCHEMISTRY

Electrochemistry

Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor (a metal or a semiconductor) and an ionic conductor (the electrolyte), and which involve electron transfer between the electrode and the electrolyte or species in solution.

If a chemical reaction is driven by an external applied voltage, as in electrolysis, or if a voltage is created by a chemical reaction as in a battery, it is an electrochemical reaction. In contrast, chemical reactions where electrons are transferred between molecules are called oxidation/reduction (redox) reactions

METALLIC AND ELECTROLYTIC CONDUCTORS All substances do not conduct electrical current. The substances, which allow the passage of electric current, are called conductors. The best metal conductors are such as copper, silver, tin, etc. On the other hand, the substances, which do not allow the passage of electric current through them, are called non-conductors or insulators. Some common examples of insulators are rubber, wood, wax, etc. The conductors are broadly classified into two types, Metallic and electrolytic conductors.

Metallic conduction Electrolytic conduction

(i) It is due to the flow of electrons.

(i) It is due to the flow of ions.

(ii) It is not accompanied by decomposition of the substance.(Only physical changes occurs)

(ii) It is accompanied by decomposition of the substance. (Physical as well as chemical change occur)

(iii) It does not involve transfer of matter.

(iii) It involves transfer of matter in the form of ions.

(iv) Conductivity decreases with increase in temperature.

(iv) Conductivity increases with increases in temperature and degree of hydration

http://en.wikipedia.org/wiki/Chemistry

http://en.wikipedia.org/wiki/Chemical_reaction

http://en.wikipedia.org/wiki/Solution

http://en.wikipedia.org/wiki/Electrical_conductor

http://en.wikipedia.org/wiki/Metal

http://en.wikipedia.org/wiki/Semiconductor

http://en.wikipedia.org/wiki/Electrolyte

http://en.wikipedia.org/wiki/Voltage

http://en.wikipedia.org/wiki/Electrolysis

http://en.wikipedia.org/wiki/Battery_(electricity)

http://en.wikipedia.org/wiki/Molecule

http://en.wikipedia.org/wiki/Redox


due to decreases in viscosity of medium.

The electrolyte may, therefore, be defined as the substance whose aqueous solution or fused state conduct electricity accompanied by chemical decomposition. The conduction of current through electrolyte is due to the movement of ions. On the contrary, substances, which in the form of their solutions or in their molten state do not conduct electricity, are called non-electrolytes. Galvanic (Voltaic) Cell: converts chemical energy of oxidants and reductants into electrical energy

Electrodes: are conductors used to permit the flow of electrons in an electrochemical cell. One electrode is the anode, the other is the cathode.

Anode: Oxidation occurs at anode Anode is negative Anode disintegrates

Cathode:Reduction occurs at cathode Cathode is positive Solid deposits on cathode

Salt bridge: allows for migration of ions to complete the electrical circuit Electron Flow: from anode to cathode

Electrons flow from negative to positive Spontaneous Reaction: E0 for the galvanic cell is positive

Example : The Daniell Cell

A galvanic (voltaic) cell is a device that uses REDOX reactions to produce electricity.

In 1836 Professor John Daniell adopted a two-cell approach to produce electricity. The Daniell Cell is divided into 2 half-cells connected by a wire and a salt bridge to complete the electrical circuit.

Anode: Zn ---> Zn2+ + 2e Eo = +0.76V

Cathode: Cu2+ + 2e ---> Cu Eo = +0.35V

Cell: Zn + Cu2+ ---> Zn2+ + Cu Eocell = +1.11V

At the negative anode, zinc is oxidised to zinc ions.

http://www.ausetute.com.au/redox.html



http://www.ausetute.com.au/calcelemf.html



The zinc anode disintegrates in time. At the positive cathode, copper ions are reduced to copper atoms. Copper is deposited on the copper cathode in time. Electrons flow from the zinc anode to the copper cathode. In the overall REDOX reaction zinc is donating electrons to copper ions. This REDOX reaction occurs spontaneously, Eo is positive. This REDOX reaction produces 1.11V of electricity.

Reversible and Irreversible cells

REVERSIBLE AND IRREVERSIBLE CELLS

Daniell cell has the emf value 1.09 volt. If an opposing emf exactly equal to 1.09 volt is applied to the cell, the cell reaction,

Zn + Cu2+ --> Cu + Zn2+

stops but if it is increased infinitesimally beyond 1.09 volt, the cell reaction is reversed.

Cu + Zn2+ --> Zn + Cu2+

Such a cell is termed a reversible cell. Thus, the following are the two main conditions of reversibility:

(i) The chemical reaction of the cell stops when an exactly equal opposing emf is applied.

(ii) The chemical reaction of the cell is reversed and the current flows in opposite direction when the opposing emf is slightly greater than that of the cell.

Any other cell which does not obey the above two conditions is termed as irreversible. A cell consisting of zinc and copper electrodes dipped into the solution of sulphuric acid is irreversible. Similarly, the cell

Zn|H2S04(aq)|Ag

is also irreversible because when the external emf is greater than the emf of the cell, the cell reaction,

Zn + 2H+ --> Zn2+ + H2


is not reversed but the cell reaction becomes

2Ag + 2H+ --> 2Ag+ + H2

Cell notation in chemistry is a shorthand way of expressing a certain reaction in an electrochemical cell. The cell anode and cathode (half-cells) are separated by two bars or slashes representing a salt bridge, with the anode on the left and cathode on the right.[1][2] Individual solid, liquid or aqueous phases within each half-cell are separated by a single bar. Concentrations of dissolved species, in each phase written in parentheses and the state of each phase (usually s (solid), l (liquid), g (gas) or aq. (aqueous solution)) is included in a subscript after the species name.

Examples

This illustrates a Silver (Ag) - Cadmium (Cd) cell defined by the following half reactions, with 0.010M Cadmium chloride (CdCl2) as electrolyte. The Mercury (Hg) does nothing except act as an amalgam.

Cathode reaction

AgCl(s) + e- --> Ag(s) + Cl-(aq) E = 0.222V

Anode reaction

Cd(s) --> Cd2+(aq) + 2e- E = +0.403V

Cd(s), Hg(s) | CdCl2(aq)(0.010M) || AgCl(s) , Ag(s) E = 0.7585V

Note

This cell has a non-standard electric potential due to the concentration (0.010M)

and the mean activity coefficient (0.513).

Other Examples

The Zinc-Hydrogen Cell Zn | ZnSO4(aq) || HCl(aq) | H2(g, p=101.3 kPa) | Pt

Some gas electrodes Pt | Cl2, HCl (aq, 0.1 mol L-1)

Pt | H2, H3O+ (aq, 1 mol L-1)

Calculating Cell EMF (voltage)

The overall electrochemical cell reaction can be written as 2 half-equations: 1 equation for the reduction reaction and 1 equation for the oxidation reaction

The number of electrons gained in the reduction half reaction must equal the number of electrons lost in the oxidation half reaction

Eo is a type of energy per electron so it remains unchanged even if we double the numbers of reactants and products in the reaction

If an equation is reversed (so that the reactants become the products), the sign of Eo is also reversed

http://en.wikipedia.org/wiki/Chemistry

http://en.wikipedia.org/wiki/Electrochemical_cell

http://en.wikipedia.org/wiki/Anode

http://en.wikipedia.org/wiki/Cathode

http://en.wikipedia.org/wiki/Salt_bridge

http://en.wikipedia.org/wiki/Cell_notation#cite_note-0



http://en.wikipedia.org/wiki/Solid

http://en.wikipedia.org/wiki/Liquid

http://en.wikipedia.org/wiki/Gas

http://en.wikipedia.org/wiki/Aqueous_solution

http://en.wikipedia.org/wiki/Silver

http://en.wikipedia.org/wiki/Cadmium

http://en.wikipedia.org/wiki/Molar_concentration

http://en.wikipedia.org/wiki/Cadmium_chloride

http://en.wikipedia.org/wiki/Electrolyte

http://en.wikipedia.org/wiki/Mercury_(element)

http://en.wikipedia.org/wiki/Amalgam_(chemistry)

http://en.wikipedia.org/wiki/Electric_potential

http://en.wikipedia.org/wiki/Activity_coefficient


The cell's emf (electromotive force, often referred to as the cell voltage), is

calculated by adding together the Eo values for each half reaction: Eo

cell = Eoreduction + Eo

oxidation The reaction is spontaneous in the direction as written if

Eocell > 0

(Eocell positive)

The reaction is spontaneous in the reverse direction to that written if Eo

cell < 0 (Eo

cell negative) A galvanic cell (voltaic cell) produces electricity so the overall cell reaction must

have a positive Eocell value

(Eocell > 0)

In practice, cell emf depends on temperature and concentration of reactants and products. If the concentration of reactants increases relative to products, the cell reaction becomes more spontaneous and the emf increases. As the cell operates, the reactants are used up as more product is formed causing the emf to decrease.

An electrolytic cell requires an input of electricity so the overall cell reaction must have a negative Eo

cell value (Eo

cell < 0) If an electrolytic cell is operating in aqueous solution, then the water is also being

reduced at the cathode or formed at the anode, so these equations must also be incorporated into the calculation of Eo

cell

Calculating Cell EMF Examples

a. Calculate the emf (voltage) for the following reaction: Zn(s) + Fe2+ -----> Zn2+ + Fe(s)

Write the 2 half reactions: Zn(s) -----> Zn2+ + 2e Fe2+ +2e -----> Fe(s)

look up the standard electrode potentials in the table above Zn2+ + 2e -----> Zn Eo = -0.76V This equation needs to be reversed, so the sign of Eo will also be reversed. Zn(s) -----> Zn2+ + 2e Eo = +0.76V Fe2+ +2e -----> Fe(s) Eo = -0.41V

Add the two equations together:

Zn(s) -----> Zn2+ + 2e Eo = +0.76V

Fe2+ + 2e -----> Fe(s) Eo = -0.41V

Zn(s) + Fe2+ -----> Zn2+ + Fe(s) Eocell = +0.76 + (-0.41) = +0.35V


Eo

cell > 0 (Eocell positive) so the reaction is spontaneous as written

b. Calculate the cell emf (voltage) for the following reaction: Br2(aq) + 2Fe2+ -----> 2Br- + 2Fe3+

Write the two half equations: Br2(aq) + 2e -----> 2Br- 2Fe2+ -----> 2Fe3+ + 2e

Look up the standard electrode potentials in the table above ½Br2(aq) + e -----> Br- Eo = +1.09V This equation needs to be multiplied by 2, however, the value of Eo remains the same Br2(aq) + 2e -----> 2Br- Eo = +1.09V Fe3+ + e -----> Fe2+ Eo = +0.77V This equation needs to be reversed, the sign of Eo will also be reversed Fe2+ -----> Fe3+ + e Eo = +0.77V This equation needs to be multiplied by 2, however, the value of Eo remains the same 2Fe2+ -----> 2Fe3+ + 2e Eo = +0.77V

Add the two equations together:

Br2(l) + 2e -----> 2Br- Eo = +1.09V

2Fe2+ -----> 2Fe3+ + 2e Eo = -0.77V

Br2(aq) + 2Fe2+ -----> 2Br- + 2Fe3+ Eocell = +1.09 + (-0.77) = +0.32V

Eocell is positive (Eo

cell > 0) so the reaction is spontaneous in the direction as written


ELECTRODE POTENTIAL

When the metal (M) consists of metal ions (M n+) with valence electrons is

placed in a solution of its own salt, any one of the following reactions will occur.

1. Positive metal ions may pass into the solution.

M M n+ + ne - (Oxidation)

2. Positive metal ions from the solution may deposit over the metal.

M n+ + ne - M (Reduction)

Example 1


When Zn electrode is dipped in ZnSO4 solution, Zn goes into the solution as Zn

2+ .

Now the Zn electrode attains a negative charge,due to the accumulation of

valence electrons on the metal.

The negative charges developed on the electrode attract the positive ions from

solution.

Due to this attraction the positive ions remain close to the metal.


Example 2

\When Cu electrode is dipped in CuSO4 solution, Cu2+ ions from the solution

deposit over the metal..

Now the Cu electrode attains a positive charge, due to the accumulation of Cu2+

ions on the metal.


The positive charges developed on the electrode attract the negative ions from

solution.

Due to this attraction the negative ions remain close to the metal.

Thus a sort layer is formed around the metal.

This layer is called Helmholtz electrical double layer.

This prevents further passing of the positive ions from the or to the metal.

A difference of potential is consequently set up between the metal and the

solution.

At equilibrium , the potential difference becomes a constant value, which is

known as the electrode potential of metal.

Thus the tendency of an electrode to lose electrons is called the oxidation

potential and the tendency of an electrode to gain electrons is called the

reduction potential.

Single electrode potential(E)

It is the measure of tendency of a metallic electrode to lose or gain electrons,

when it is in contact with a solution of its own salt.

standard electrode potential(E0)

It is the measure of tendency of a metallic electrode to lose or gain electrons, when

it is in contact with a solution of its own salt of 1 molar concentration at 250c

MEASUREMENT OF EMF OF A CELL THEORY The electromotive force (emf) of a cell is its terminal voltage when no current is

flowing through it. The terminal voltage of a cell is the potential difference between its electrodes. A voltmeter cannot be used to measure the emf of a cell because a voltmeter draws some current from the cell. To measure a cell's emf a potentiometer is used since in a potentiometer measurement no current is flowing. It employs a null method of measuring potential difference, so that when a balance is reached and the reading is being taken, no current is drawn from the source to be measured.

Here s is the standard cell (emf = 1.0186 volts), and x is the unknown cell whose emf is to be measured. G is the galvanometer which has an internal resistor R1 in series with the meter to decrease its sensitivity. Once the


potentiometer is balanced by adjusting point C until there is no deflection of G, switch K1 (a pushbutton on top of the galvanometer) is closed to increase the sensitivity of G by shorting out R1. Point C is then further adjusted with K1 closed until there is no deflection of G.

Since the electromotive force of the standard cell is equal to the potential drop in the length of wire spanned (measured from A) for a condition of balance and the same is true for the unknown cell, the emf of each cell is proportional to the lengths of wire spanned. Thus

and the unknown emf is given by

where x is the unknown emf and, s is the emf of the standard cell, Lx is the

length of wire (AC) used for balancing the unknown cell, and Ls is the length of wire used for balancing with the standard cell.

If we have a test cell of emf, and internal resistance r supplying current to a variable load resistor R (see figure 4), then we will measure a terminal voltage V which is a function of the load resistance R.

Applications of emf Measurements:

1. Determination of standard free energy change and equilibrium constant. 2. Solubility of a sparingly soluble salts. 3. Determination of valency of ions 4. Determination of pH 5. Nernst equation


NERNST EQUATION

Consider the following redox reaction

M n+ + ne- M

For such a redox reversible reaction, the free energy change (G) and its equilibrium

constant (K) are inter related as

G = RT lnK + RT ln [Product]

[Reactant]

G = G0 + RT ln [Product] (1)

[Reactant]

Where,

G0 = Standard free energy change

The above equation (1) is known as Van‟t Hoff isotherm.

The decrease in free energy (G) in the above reaction will produce electrical

energy.

In the cell, if the reaction involves the transfer of „n‟ number of electrons, then „F‟

faraday of electricity will flow.

If E is the emf of the cell, then the total electrical enegy (nEF) produced in the cell is ,

G= nEF

(OR)

G0 = nFE0 (2)

Where,

G = decrease in free energy change

(OR)

G0 = decrease in standard free energy change

Comparing equation 1 and 2 , it becomes

nFE = nFE0 + RTln [M] (3)

[Mn+]

Dividing the above equation (3) by nF

Since the activity of solid metal [M] =1

E = E0RT ln 1

nF [Mn+]

In general, E =E0 RT ln [product]

nF [Reactant]

(OR)

E= E0+RT ln [M n+]

nF


E=E0+ 2.303 RT log [M n+] (4)

nF

When, R=8.314J/K/mole

F= 96500 Coulombs

T = 298K

So the above equation becomes

E= E0red + 0.0591 log [M n+] (5)

n

Similarly for oxidation potential

E= E0oxi 0.0591 log [M n+] (6)

n

The above equations 5&6 are knownas Nernst equation for single electrode potential.

Applications of Nernst Equations

1. Nernst equation is used to calculate electrode potential of unknown metal.

2. Corrosion tendency of metal can be predicted.

3. Applications of emf series also included.


REFERENCE ELECTRODES OR STANDARD ELECTRODES

The electrode potential is found out by coupling the electrode with another

electrode known as reference electrode.

The potential of reference electrode is known or arbitrarily fixed as zero.

The important primary reference electrode is standard hydrogenelectrode,

standard electrode potential of which is taken as zero.

It is very difficult to set up a hydrogen electrode . So other electrodes

called secondary reference electrodes like calomel electrodes are used.

PRIMARY REFERENCE ELECTRODE

EXAMPLE - STANDARD HYDROGEN ELECTRODE (SHE) Construction:

Hydrogen electrode consists of platinum foil, that is connected to a platinum wire

and sealed in a glass tube .

Hydrogen gas is passed through the side arm of the glass tubes.

This electrode when dipped in a 1N HCl and hydrogen gas at 1 atmospheric

pressure is passed forms a standard hydrogen electrode.

The electrode potential of SHE is zero at all temperatures.

It is represented as

Pt,H 2 (1atm)/H+ (1M); E0 = 0V


In a cell , when this electrode acts as anode, the electrode action can be written as

H2(g) 2H+ + 2e-

When this electrode acts as cathode the electrode reaction can be

written as

2H+ + 2e- H2(g)

LIMITATIONS

1. It requires hydrogen gas and is difficult to set up and transport.

2. It requires considerable volume of test solution.

3. The solution may poison the surface of the platinum electrode.

4. The potential of the electrode is altered by changes in barometric

pressure.

SECONDARY REFERENCE ELECTRODES


EXAMPLE - SATURATED CALOMEL ELECTRODE Construction:

Calomel electrodeconsists of a glass tube cont6aining mercury at the bottom

over which mercurous chloride isplaced.

The remaining portion of the tube is filled with a saturated solution of `KCl.

The bottom of the tube is sealed with a platinum wire.

The side tube is used for making electrical contact with a salt bridge.

The electrode potential of the calomel electrode is +0.2422V.

Hg, Hg2Cl2(s ), KCl (Sat. solution);Pt


If the electrode acts as anode the reaction is

2Hg (l) Hg2 2+ + 2e-

If the electrode acts as cathode the reaction is

Hg2 2+ + 2Cl- Hg2Cl2(s )

The electrode potential is given by (for example cathode)

E(calomel)=E0

(calomel)- RT lna cl-

2F


The electrode potential depends on the activity of the chloride ions and it decreases as

the activity of thee chloride ions increases.

The single electrode potential of the three calomel electrodes on the hydrogen

scale at 298K are given as

0.1NKCl= 0.3338V

1.0N KCl= 0.2800V

Saturated KCl = 0.2422V.


Glass electrode

It is used in PH measurement.

Construction;

It consists of a thin glass bulb with a long neck made up of a special type having low

melting point and high electrical conductivity. It is filled with 0.1N HCl. Ag wire

coated with AgCl is inserted to make the electrical contact.


Working

When two solutions of different pH values are separated by a thin glass membrane,

there developed a potential difference.


This potential difference is proportional to the difference in pH value.

Representation;

Ag,AgCl/HCl (0.1N) / glass

Advantages

1. It is simple and can easily be used.

2. Equilibrium is rapidly achieved.

3. The results are accurate.

4. can be used in any solutions.

5. A small quantity of given solutions is sufficient for the determination of pH.

6. It can be used even in the presence of metallic ions.

Limitations:

1. It cannot be used in strongly alkaline solution. In such cases special type of

glass must be used.

2. Electronic potentiometers are needed for the measurement since the resistance

of glass membrane used in the bulb is very high.


ELECTROCHEMICAL SERIES OR EMF SERIES

DEFINITION

When the various electrodes are arranged in the order of their increasing values of

standard reduction potential on the hydrogen scale, the then arrangement is called

electrochemical series or emf series.

ELECTROCHEMICAL SERIES

Electrode Electrode reaction E0, volts Nature

Li+/Li Li+ + e Li -3.01 Anodic

Mg2+/Mg Mg2+ +2e Mg -2.37

Pb2+/Pb Pb2++2e Pb -1.12

Zn2+/Zn Zn2++2e Zn -0.76

H+/H2 2H++2e H2 0.00 Pt - reference

Cu2+/Cu Cu2++2e Cu +0.34

½ F2/F- ½ F2+e F- +2.87 Cathodic

SIGNIFICANCE OR APPLICATIONS OF EMF SERIES

1. Calculation of standard emf of the cell

E 0Cell = E0 R.H.E – E0 L.H.E

2. Relative ease of oxidation or reduction

Higher the positive value of E0 greater is tendency to get reduced whereas higher is

negative value of E0 greater is tendency to get oxidized.

3. Displacement of one element by the other

Metal which lie higher in the series can displace those elements which lie below them in

the series.

4. Determination of equilibrium constant for the reaction

-∆G0 = RTlnK =2.303 RT logK

log K = -∆G0 / 2.303 RT

since -∆G0 = nFE0

log K = nFE0/2.303RT

5. Hydrogen displacement behavior

Metal placed above H2 in the emf series will displaced hydrogen from an acid solution.

Zn + H2SO4 ZnSO4 +H2 E0Zn= -0.76

Ag + H2SO4 no reaction E0Ag=+0.80V

6.Predicting spontaneity of a reaction

If E0 is positive the cell reaction is spontaneous & if E0 is negative the cell

reaction is not feasible


POTENTIOMETRIC TITRATION

It is a method of volumetric analysis based on the change in emf of the solution at the equivalent point during titration .

Both oxidation and reduction takes place in this titration. It is used to follow redox potential.

The electrode potential of electrode depends upon the concentration of its ions in solutions.

The determination of equivalent point of titration on the basis of potential measurements is called potentiometric titrations.

ESTIMATION OF FERROUS ION BY POTENTIOMETRIC TITRATION

When potassium dichromate is added to an acidified Fe2+ solution it oxidizes Fe2+ to Fe3+ and the redox couple Fe2+/Fe3+ is set up.

Addition of Cr2O42- solution , increases the Fe3+ and decreases the ratio

Fe2+/Fe3+ and increases the observed emf.

At the equivalent point (Fe2+) reduces to zero, the new redox couple Cr3+/Cr6+ isd just set up and hence a sudden jump in the emf is observed.

The equivalent point is indicated by fairly a large change of potential.

On plotting emf with volume of potassium dichromate added, obtain „S‟ shaped curve.

Advantages

These titrations are applicable to even coloured solutions.

These are very rapid as compared with the gravimetric methods.

These can be carried out on a micro-scale with little difficulty.

These titrations give highly accurate results.

Even weak acid – weak base titrations are possible by this method.

CONDUCTOMETRIC TITRATION

It is a method of volumetric analysis based on the change in conductance of the solution

at the equivalent point during titration.

The conductance of a solution depends upon

1. The number of free ions in the solution.

2. mobility of the ions

3. The charge of the free ions.

Acid- Base titration

1. Titration of strong acid Vs strong base (HCl Vs NaOH )


The conductance of HCl is due to H+ and Cl - ions.

If HCl is titrated against NaOH, the fast moving H+ ions are replaced by slow

moving Na+ ions.

Conductance decreases until the acid is completely neutralized.

Further addition of NaOH introduces the fast moving OH – ions.

The conductance is therefore increases after reaching certain minimum value.

On plotting ,the conductance against volume of NaOH added, the two straight

lines intersect at a point „O‟.

This corresponds to the volume of NaOH required for Neutralisation.

HCl + NaOH NaCl + H 2 O

2. Weak Acid Vs Strong Base (CH3 COOH Vs NaOH )


The initial conductance of CH3 COOH will be low on account of its poor

dissociation.

CH3 COOH CH3COO־ + H +

When it is titrated against NaOH, highly ionized CH3 COONa is formed.

CH3COOH + NaOH CH3 COO־ Na + + H2O

Therefore the conductance increases.

When the acid is neutralized, further addition of NaOH introduces excess

of fast moving OH- ions.

Now the conductance begins to increases more sharply than before.

3. Strong acid Vs Weak base (HCl Vs NH4OH )


An adding NH4OH solution to the HCl, the highly conducting H+ ion is replaced

by NH4+ ion. The conductance of the solution decreases.

HCl + NH4OH NH4Cl + H2O

When the acid is completely neutralized, the addition ofr NH4OH does not cause

any appreciable change in the conductance

4. Weak acid Vs weak base ( CH3COOH Vs NH4OH)


CH3COOH + NH4OH CH3COONH4 + H2O

Addition of NH4OH from the burette to the beaker, causes decrease in

conductance in the beginning because the common ions formed

depresses the dissociation.

Further addition of NH4OH increases the conductivity of the solution,

because of the conductance of highly ionized salt.

When the acid is completely neutralized, further addition of NH4OH does

not cause any appreciable change in the conductance.

5. Mixture of strong acid Vs strong base ( HCl +CH3COOH Vs NaOH)


Co

nd

uct

ance

( m

ho

)

Volume of NaOH (ml)

A

Mixture of acid ( HCl &CH3COOH ) Vs NaOH

As NaOH is added to the mixture of the acids, the strong acid is neutralized first.

Therefore conductance decreases.

Once all the HCl has been neutralized, that of CH3COOH begins. During the

neutralization of the weak acid there is a small increase in conductance.

Beyond this end point, the addition of excess NaOH, increases the conduction

steeply.

HCl + NaOH NaCl + H2O

CH3COOH + NaOH CH3COONa + H2O

Thus the two points on the graph corresponding to the two sudden changes in

slopes represent the two end points.

6. Precipitation titration (AgNO3 Vs KCl )


Co

nd

uct

ance

( m

ho

)

Volume of NaOH (ml)

PRECIPITATION TITRATION (AgNO3 Vs KCl )

KCl is titrated against AgNO3, Cl- ions are replaced by free NO3

-. And AgCl gets precipitated.

KCl+ AgNO3 AgCl ↓+ KNO3

Since the mobility of Cl- and NO3- ions are practically same, the conductance

will not change with the addition of AgNO3.

When the end point is reached, further addition of AgNO3 introduces fast moving Ag+ and NO3- ions and hence the conductance increases.

Advantages

They give more accurate end point.

Coloured colutions can be titrated.

Indicator is not required.

They are used in the case of very dilute solutions.


UNIT II CORROSION AND CORROSION CONTROL

DEFINITION

Corrosion is defined as the gradual destruction or deterioration of

metals or alloys by the chemical or electrochemical reaction with its environment.

CAUSES OF CORROSION

i.e. How & why corrosion occurs?

The metals are extracted from compounds(ores)

During extraction, these ores are reduced to their metallic states

In the pure metallic state ,the metals are unstable (i.e. in excited state or

higher energy state)

As soon as the metal is extracted from their ores, the reverse process begins &

form metal compounds, which are thermodynamically stable(i.e. in lower

energy state)

Hence, when metals are exposed to environment,(such as dry

gases,moisture,etc.,) the exposed metal surface begin to decay i.e.,

conversion into more stable compound.

This is the basic reason for metallic corrosion.

Due to corrosion, some useful properties of metals such as electrical

conductivity, ductility & malleability etc., are lost.


CONSEQUENCES OF CORROSION

1. Efficiency of the machine is lost due to corrosion products

2. Products get contaminated due to released toxic products

3. Corroded equipment must be replaced frequently

4. Failure of plants

5. Necessity of over designing

CLASSIFICATION OF CORROSION

DRY OR CHEMICAL CORROSION

Due to atmospheric gases such as oxygen, hydrogen sulphide,

sulphur oxide, nitrogen, etc.

MECHANISM OF DRY CORROSION

(i)Oxidation occurs first at the surface of the metal forming metal ions at the metal/oxide

interface

CORROSION

NNNN

DRY OR CHEMICAL

CORROSION WET OR

ELECTROCHEMICAL

CORROSION

OXIDATION

CORROSION

OR

CORROSION

BY OXYGEN

CORROSION

BY

HYDROGEN

LIQUID

METAL

CORROSION

GALVANIC

CORROSION

DIFFERENTIAL

AERATION OR

CONCENTRATION

CELL CORROSION

Eg: Pitting

corrosion


M M2+ +2e-

(ii)Oxygen changes into O2- due to the transfer of electrons from metal at oxide

film/environment interface.

1/2O2+2e- O2-

(iii)Oxide reacts with metal ion to form the metal-oxide film

M + 1/2O2 M2+ + O2- MO (Metal-oxide film)

NATURE OF OXIDE FILM

1. Stable oxide layer

It is fine grained in structure & tightly absorbed to the metal surface

Such layer is impervious in nature & stops further oxygen attack through

diffusion

Such film acts as a protective coating & no further corrosion develop

Eg:- Oxides of Al,Sn,Pb,Cu,etc are stable oxide layers

2. Unstable oxide layer


It is mainly produced on the surface of noble metals

It decomposes back into metal & oxygen

Metal oxide ↔ Metal + Oxygen

Eg:- Oxides of Pt,Ag,etc are unstable oxide layers

3. Volatile oxide layer

The oxide layer volatilizes as soon as it is formed, leaving the metal

surface for further corrosion

Eg:- Molybdenum oxide (MoO3) is volatile

4. Protective or Non-protective oxide film(Pilling-Bedworth rule)

The ratio of the volume of the oxide formed to the volume of the

metal consumed is called “Pilling-Bedworth rule” (or) “Pilling-

Bedworth ratio”

According to it, if the volume of the oxide layer formed is less than

the volume of the metal, the oxide layer is porous & non-protective.

Eg: - The volume of the oxides of alkali & alkaline earth metals such as

Na,Mg,Ca,etc., is less than the volume of the metal consumed. Hence

the oxide layer is porous & non-protective.

On the other hand,if the volume of the oxide layer formed is greater

than the volume of the metal, the oxide layer is non-porous &

protective.

Eg: - The volume of the oxides of heavy metals such as Pb,Sn,etc., is

greater than the volume of the metal . Hence the oxide layer is non-

porous & protective.

WET OR ELECTROCHEMICAL CORROSION

Conditions to occur

a) When 2 dissimilar metals or alloys are in contact with each other in

presence of aqueous solution or moisture

b) When a metal is exposed to varying concentration of oxygen or any

electrolyte

MECHANISM

1. In anodic part oxidation occurs

M----->M2+ + 2e-


2. In cathodic part reduction occurs depending on nature of corrosive

environment

a. If the corrosive environment is acidic hydrogen evolution

occurs

2H+ +2e- H2↑

b. If the corrosive environment is slightly alkaline or neutral

hydroxide ion is formed

1/2O2+2e-+H2O 2OH-

TYPES OF ELECTROCHEMICAL CORROSION

1. GALVANIC CORROSION

When two different metals are in contact with each other in presence

of aqueous solution or moisture galvanic corrosion occurs

More active metal (with more –ve electrode potential ) acts as anode

Less active metal (with more +ve electrode potential ) acts as cathode


Example

a. Steel screw in a brass marine hardware corrodes

Iron becomes anodic and it is corroded while brass acts as cathode and it is

not attacked

b. Bolt and nut made of the same metal is preferred

Galvanic corrosion is avoided due to homogenous metals. No anodic and

cathodic part is set up


2. DIFFERENTIAL AERATION (OR) CONCENTRATION CELL CORROSION

This type of corrosion occurs when a metal is exposed to varying

concentration of oxygen or any electrolyte on the surface of the base

metal.

E.g.(a) Pitting corrosion

Pitting is a localized attack resulting in the formation of hole

around which the metal is un attacked

For e.g. Metal area are covered by a drop of water, sand, dust,

scale etc

Area covered by a drop of water acts as anode due to less oxygen

concentration and suffer corrosion

The uncovered area freely exposed to air acts as cathode

The rate of corrosion will be more when the area of the cathode is

larger and the anodic area is smaller

Therefore more and more material is removed from the same spot

forming a small hole or pit

At anode:

Fe Fe2+ +2e-

At cathode:


1/2O2+2e-+H2O 2OH-

Net reaction: (O)

Fe2+ +2OH- - Fe (OH) 2 Fe (OH) 3

This type of intense corrosion is called pitting corrosion.

(b)Crevice corrosion

If a crevice between metallic objects or between metal & non-metallic

material is in contact with liquids, the crevice becomes the anodic

region & suffers corrosion

This is due to less oxygen with crevice area

Exposed areas act as the cathode

(c)Pipeline corrosion

Buried pipelines or cables passing from one type of soil to another

say, from clay(less aerated) to sand (more aerated) may get corroded

due to differential aeration.


(d)Corrosion on wire-fence

The areas where the wires cross are less aerated than the rest of the

fence & hence corrosion occurs at the wire crossings, which are

anodic.

Difference between chemical and electro chemical corrosion

S.no Chemical corrosion Electro chemical corrosion

1 Occurs in dry condition Occurs in prescience of

moisture

2 Due to direct chemical attack Due to set up of large


by environment number of cathodic and

anodic areas

3 Even a homogenous metal

surface get corroded

Heterogeneous or

bimetallic contact is the

condition

4 Corrosion product accumulate

in the place where corrosion

occurs

Corrosion occurs at anode

while products formed else

where

5 Self controlled Continuous process

6 Adsorption mechanism Electro chemical reaction

7 E.g. formation of mild scale on

iron surface

E.g. rusting of iron in moist

atmosphere

CORROSION CONTROL

The rate of corrosion can be controlled by either modifying the metal or the

environment

A.Controlling of corrosion by modifying metal

1. By proper designing

a. Avoid galvanic corrosion


Select metals as close as possible in the electro chemical series

E.g. Bolt and nut made of the same metal is preferred

Provide smaller area for cathode and larger area for anode

Insert insulating material between two metals

b. Improper draining of tanks and other containers causes corrosion


c. Avoid sharp corners and bends. Provide smooth corners or curved

pipe bends

d. Avoid crevices by filling them with the filler


E.g. riveted joints produce crevices corrosion so welded joints are

preferred.

2. By using pure metals

The presence of impurity in metals create heterogeneity and so galvanic cells are

set up with distinct anode and cathodic areas

Higher the % of impurity faster the rate of corrosion

%purity if Zn 99.999 99.99 99.95

Corrosion rate 1 2650 5000

Pure metals like Al, Mg, etc makes them corrosion resistant by forming

coherent and impervious protective oxide film

But Pure metals are costly and possess in adequate mechanical property

like softness and low strength and hence great thickness of pure metal is

required

3. By alloying

By alloying metals like Fe, Cu, etc with noble metals the anodic activity is

lowered

Alloy should be completely homogenous


E.g. stainless steel containing Cr produce coherent oxide film which protect

steel from further attack. The film is self healing

4. By cathodic protection

The principal involved is to force the metal to behave like a cathode

The important cathodic protection methods are:

CORROSION CONTROL

The rate of corrosion can be controlled by either modifying the metal or the

environment

A.Controlling of corrosion by modifying metal

2. By proper designing

a. Avoid galvanic corrosion

Select metals as close as possible in the electro chemical series

E.g. Bolt and nut made of the same metal is preferred

Provide smaller area for cathode and larger area for anode

Insert insulating material between two metals


b. Improper draining of tanks and other containers causes corrosion

c. Avoid sharp corners and bends. Provide smooth corners or curved

pipe bends


d. Avoid crevices by filling them with the filler

E.g. riveted joints produce crevices corrosion so welded joints are

preferred.

2. By using pure metals

The presence of impurity in metals create heterogeneity and so galvanic cells are

set up with distinct anode and cathodic areas

Higher the % of impurity faster the rate of corrosion

%purity if Zn 99.999 99.99 99.95

Corrosion rate 1 2650 5000

Pure metals like Al, Mg, etc makes them corrosion resistant by forming

coherent and impervious protective oxide film

But Pure metals are costly and possess in adequate mechanical property

like softness and low strength and hence great thickness of pure metal is

required


5. By alloying

By alloying metals like Fe, Cu, etc with noble metals the anodic activity is

lowered

Alloy should be completely homogenous

E.g. stainless steel containing Cr produce coherent oxide film which protect

steel from further attack. The film is self healing

6. By cathodic protection

The principal involved is to force the metal to behave like a cathode

The important cathodic protection methods are:

i. SACRIFICIAL ANODIC PROTECTION METHODS

The metallic structure to be protected is made cathode by

connecting it with more active metal(anodic metal) , so that the

corrosion concentrate only on the active metal

The artificially made anode gets corroded

Al, Zn, Mg are used as sacrificial anode

Application of Sacrificial anodic protection


a. Used for protection of ships and boats. Sheets of Mg or Zn (active

metals )

are hung around the hull of the ship (made of iron). The active metal

get sacrificed saving iron

b. Protection of under ground pipe lines, cables from soil corrosion

c. Insertion of Mg sheets in to domestic water boilers prevent rust

formation

d. Ca metal minimize engine corrosion

ii. IMPRESSED CURRENT CATHODIC PROTECTION METHOD


An impressed current is applied in the opposite direction of the

corrosion current to nullify it

The corroding metal is converted to anode to cathode by connecting

–ve terminal of the battery to the metallic structure to be protected

and the +ve terminal to an inert anode

Inert anode used are graphite, Platinized titanium

The anode is buried in a back fill (containing mixture of gypsum,

coke, breeze, Na2SO4 )

The back fill provide good electrical contact of anode with

surrounding soil

Application of impressed current protection

Structure like tanks, pipe lines, transmission line tower, underground water

pipe lines, oil pipe lines, ships etc can be protected by this method

Comparison of sacrificial anodic and impressed current methods


S.no sacrificial anodic methods impressed current methods

1 No external power supply external power supply

2 Periodical replacement of

sacrificial anode

Anode are stable

3 Low investment High investment

4 Soil and micro biological

corrosion effects are not

taken in account

Soil and micro biological

corrosion effects are taken

in account

5 Economical method when

short term protection is

required

Well suited for large

structure and long term

operation

6 Suitable when the current

& resistivity of electrolyte

are low

Practiced even if the

current & resistivity of

electrolyte are high

B.Control of corrosion by modifying the environment

A corrosion inhibitor is substance which reduces corrosion of metals when added to the

corrosive environment.

TYPES OF INHIBITORS

They are of 3 types

1. Anodic inhibitors

anodic inhibitors prevent corrosion occurring at anode by forming an

insoluble compound with the newly produced metal ions forming a

protective film at anode

it may be dangerous if some areas are uncovered as severe local attack

can occur

E.g. chromate, nitrates, phosphates, tungstates or other ions of transition

2. Cathodic inhibitors


Cathodic reactions are of 2 types depending on the environment

a. in a acidic solution

The cathodic reaction is hydrogen evolution

2H+ +2e- H2↑

It can be reduced by slowing down the diffusion of H+ ions to the cathode. This is

done by adding amines, pyridines.

E.g. organic inhibitors like amines, mercaptans, heterocyclic nitrogen

compounds, thioureas, substitute ureas, heavy metal soaps

b. In a neutral solution

The cathodic reaction is

1/2O2+2e-+H2O 2OH-

The corrosion can be reduced in 2 ways

i. By eliminating the oxygen from neutral solution their by

formation of OH-ions are inhibited. This can be done by adding

the reducing agents like Na2SO3, N2H4…..etc

ii. By eliminating the OH- ions from the neutral solution. this can

be done by adding Mg, Zn, or Ni salts which form insoluble

hydroxide with OH- ions deposited on the cathode forming

impermeable self barriers

E.g. Na2SO3, N2H4…..etc

3. Vapour phase inhibitors (VPI)

VPI readily vapourise and form aprotective layer on the

metal surface

Used in the protection of storage containers, packing

materials, sophisticated equipments ectc

E.g. Dicyclohexylammonium nitrate, benzotriazole etc

2. ELECTRO PLATING OR ELECTRO DEPOSITION

PRINCIPLE

It is the process in which coating metal is deposited on base metal by

passing direct metal through an electrolytic solution containing soluble of

salt of coating metal

Base metal to be plated is cathode


Anode is either made of coating metal itself or inert material of good

electrical conductivity

Objectives of electroplating

1. On metals

Increase corrosion resistance

Improve hardness

2. On Non metals

Increases strength

decorate surfaces

THEORY

E.g. if CuSO4 solution is used as an electrolyte it ionses as

CuSO4 Cu2+ + SO4-

On passing current Cu2+ ions go to the cathode and get deposited their

Cu2+ + 2e- Cu (at cathode)

The free sulphate ions migrate to the Cu anode and dissolve an equivalent

amount of Cu to form CuSO4

SO42- + Cu CuSO4 + 2e-

The CuSO4 formed to get dissolved in the electrolyte. Thus there is a continuous

replenishment of electrolyte during electrolyses

PROCESS

The article is first pickled with dil.H2SO4 to remove dust or rust present

The cleaned article is made cathode


Anode is either made of coating metal itself or inert material of good electrical

conductivity

When direct current is passed from battery coating metal ions migrate to cathode

and get deposited there.

A thin layer of coating metal is obtained on the article

Characteristics of various electroplating

1. Characteristic of Ni plating

Hard, adherent and good wear resistant surface

Undercoat for articles which are finally to be Cr plated

2. Characteristic of Cr plating

Porous and non adherent

Articles are first given an under coat of Cu or Ni before Cr plating

3. Characteristic of Cu plating

Under coat for Ni-Cr electrodeposit

Coated at the bottom of stainless steel cooking utensils for better heat

transfer

4. Characteristic of gold plating

Use for electrical and electronic application

For high quality decoration and high oxidation resistance coating

3. ELECTROLESS PLATING

Principle:

It is a technique of depositing noble metal (from its salt solution) on a

catalytically active surface of metal to be protected by using a suitable

reducing agents without using electrical energy

Reducing agents reduces metallic ions to metal

Metal ions + reducing agents ----------->metals (deposited) +oxidized

products

Various steps of electro less plating

Step I

Preparation of active surface of the object to be plated

This is achieved


I. Etching i.e., removal of unwanted particles by acid treatment

II. Electroplating i.e., a thin layer of the metal to be plated or any other suitable

metal is coated on the surface of the object

III. Treatment with stannous chloride followed by dipping in palladium chloride

This treatment yields a thin layer of Pd on the treated surface. This method is

applied only for plastic and printed circuit boards

Step II

Preparation of plating bath

The plating bath is composed of

i. Coating metal: soluble salt of the metal (like chloride or sulphate) to be plated

ii. Reducing agents like formaldehyde, hypophosphite etc.

iii. Exaltant like succinate, fluoride etc which enhances the plating rate

iv. Complexing agent like tartarate, citrate, succinate, etc. which improves the

quality of the deposit

v. Stabilizer like cations of Pb, Ca etc which prevent the decomposition of the

plating bath solution

vi. Buffer solution like sodium acetate, sodium hydroxide + Rochelle salt etc is

added to control the pH of the bath

Step III

Procedure

Various reactions

Some electro less plating

1. Electro less Ni plating

Step1: Pre treatment and activation of surface

Surface to be plated is degreased using organic solvents or alkali

followed by acid treatment

Step2: plating bath

Nature of the compound Name of the compound Quantity(g/l)

Coating solution NiCl2 20

Reducing agent Sodium hypophosphite 20


Complexing agent cum

exhaltant

Sodium succinate 15

Buffer Sodium acetate 10

Optimum pH 4.5

Optimum temp. 93⁰C

Step 3

Procedure

Various reactions

Application

Used in electronic application

Used in domestic as well as automotive fields

2. Electro less copper plating

Step1: pre treatment and activation of surface

Surface to be plated is degreased using organic solvents or alkali

followed by acid treatment

Step2: plating bath

Nature of the compound Name of the compound Quantity(g/l)

Coating solution CuSO4 12

Reducing agent formaldehyde 8

Complexing agent cum

exhaltant

EDTA 20

Buffer

NaOH + Rochelle salt

15+14

Optimum pH 11.0

Optimum temp. 25⁰C


Step 3

Procedure

E.g. preparation of PCB by substractive method

A thin layer Cu of (5-100μm) is first electroplated over PCB (Printed Circuit

Board) made of phenolic or epoxy polymer or glass reinforced rubber

Selected areas are protected using photo resist and remaining areas are etched

away

Now the required type of circuit pattern is obtained

Double sided track are prepared in order to pack more number of component in

small space

Finally connection between two sides of PCB are made by drilling holes followed

by electro less Cu plating through holes which provide electrical contact in both

sides

Application

Used in double or multilayered boards (PCB) in which plating through holes

are required. Such holes cannot be Cu electroplated

Advantages of electro less plating over electroplating

No electricity is required


Electroless plating on insulators(plastic, glass) and semi-conductors can be

easily carried out

Complicated parts can also be plated uniformly

Possess mechanical, chemical and magnetic properties

Difference b/w electroplating and electrolessplating

S.no electroplating electrolessplating

1 Carried by passing current Carried by auto catalytic redox reaction

2 Separate anode is provide Catalytic surface of substrate act as

anode

3 Anodic reaction Anodic reaction

4 Object to be coated is cathode Object to be coated after making its

surface catalytically cathode

5 Not satisfactory for the object

having irregular shape

satisfactory for all parts

6 Carried out on conducting

materials

Carried out on conducting, semi-

conducting (plastic) materials

7 Thickness of the plating is 1-

100μm

Thickness of the plating is 1-100μm


UNIT III FUELS AND COMBUSTION

DEFINITION

• Fuel is a combustible substance

• During combustion of it, the atoms of C,H, etc combine with oxygen with

simultaneous liberation of heat.

FOR EXAMPLE

C + O2 CO2 +94 K cals

2H2+O2 2H2O +68.5 K cals


REQUIREMENTS OF A GOOD FUEL A good fuel should have the following characteristic.

High calorific value.

Moderate ignition temperature.

Low moisture content.

Low contents of non-combustible matters.

Free from objectionable and harmful gases.

Moderate velocity of combustion.

Combustion should be controllable.

Easy to transport and readily available at low cost. ADVANTAGES OF SOLID FUELS:

Solid fuels are easily available and they are cheap.

Handling and transportation are easy.

They can be stored conveniently without any risk.

They have a moderate ignition temperature.

DISADVANTAGES OF SOLID FUELS:

They form large amount of ash during burning and its disposal is a big problem.

A large space is required for storage.

Combustion process cannot be easily controlled.

Since a lot of air is required for complete combustion, the thermal efficiency is not so high.

The calorific value is comparatively lower.

ADVANTAGES OFLIQUID FUELS:

They have higher calorific value than solid fuel.

They occupy less storage space than solid fuels.

Their combustion is uniform and easily controllable.

Liquid fuels do not yield any ash after burning. DISADVANTAGES OF LIQUID FUELS:


Liquid fuels are more costly than the solid fuels.

Liquid fuels give unpleasant odor during incomplete combustion.

Special type of burners is required for effective combustion.

Some amount of liquid fuels will escape due to evaporation during storage.

ADVANTAGES OF GASEOUS FUELS:

Gaseous fuels have high calorific value than solid fuels.

During burning they do not produce any ash or smoke.

Compared to solid and liquid fuels, they have high thermal efficiency.

They can be easily transported through the pipes.

DISADVANTAGES OF GASEOUS FUELS:

They are highly inflammable and hence the chances for fire hazards are high.

Since gases occupy a large volume, they require large storage tanks. SOLID FUELS

COAL

It is a carbonaceous material formed by coalification.

Analysis of coal

Proximate Analysis:

It involves the determination of percentage of following in coal

Moisture content

Volatile matter

Ash content

Fixed carbon

Moisture content

1gm of powdered and air-dried coal sample in crucible is heated at

100-150⁰C in an electric air oven for 1 hour.

The loss in weight of the sample is found out and percentage of

moisture is calculated as


Percentage of moisture = Loss in weight of coal ×100

Weight of air-dried coal

Volatile matter:

In this the crucible with residual coal sample is covered with a lid and

heated at 950± 20⁰C for 7mins in a muffle furnace. The loss in weight is found out.

Percentage of volatile matter = Loss in weight of coal ×100

Weight of moisture free coal

Ash Content:

The crucible with residual coal sample is heated without lid at

700 ± 50 ⁰C for ½hour in a muffle furnace. The loss in weight is found out.

Percentage of ash content = weight of ash formed x100

Weight of dried coal

Fixed Carbon:

Percentage of fixed carbon = percentage of (MC+VM+AC)

Importance or significance of proximate analysis:

1. High percentage of moisture content is undesirable because

It decreases Calorific value of coal.

Consume more heat in the form of latent heat of evaporation.

Transport cost increases.

2. High percentage of volatile matter is undesirable because


Large proportion of fuel escapes out unburnt as vapour.

Burns with long flame and high smoke.


3. High percentage of ash content is undesirable because


High ash content causes hindrance to heat flow and produces clinkers which

blocks air supply through fuels.

It increases transporting, handling, storage cost and involves additional cost in

ash disposal.

4.High percentage of fixed carbon content is desirable because

It increases calorific value of coal.

It helps in designing the furnace and shape of fire box.

CARBONISATION OFMETALLURGICAL COKE

When coal is heated strongly in the absence of air (called

destructive distillation) it is converted into lustrous, dense, porous and coherent

mass known as coke. This process of converting coal is known as carbonization.

METALLURGICAL COKE

When bituminous coal is heated strongly in the absence of air, the volatile matter

escapes out and the mass becomes hard, porous and coherent which is called

Metallurgical coke.

Requisites (or) Characteristics of good metallurgical coke

(i) Purity

The moisture, ash, sulphur and phosphorous contents in metallurgical coke should be

low.

(ii) Porosity


Coke should be highly porous so that oxygen will have intimate contact with carbon and

combustion will be complete and uniform.

(iii) Strength

The coke should have very high mechanical strength in order to withstand high pressure

of the overlying material in the furnace.

(iv) Combustibility

The coke should burn easily.

(v)Calorific value

The calorific value of coke should be very high.

(vi) Reactivity

The reactivity of the coke should be low because reactive cokes produce high

temperature on combustion.

(vii) Cost

It should be cheap and readily available.

MANUFACTURE OF METALLURGICAL COKE

There are two important methods used for the manufacture of metallurgical coke.

1)Otto-Hoffman‟s by product oven

In order to

(i) Increases the thermal efficiency of the carbonization process and,

(ii) Recover the valuable by products (like coal gas, ammonia, benzyl oil, etc.)

Otto-Hoffman developed modern by product coke oven.


The oven consists of a number of silica chambers. Each chamber is about 10-

12m long, 3-4m height and 0.4-0.45m wide.

Each chamber is provided with a charging hole at the top, it is also provided with

a gas off take valve and iron door at each end for discharging coke.

Coal is introduced into the silica chamber and the chambers are closed.

The chambers are heated to 1200ċ by burning the preheated air and the

producer gas mixture in the interspaces between the chambers.

The air and gas are preheated by sending them through 2nd and 3rd hot

regenerator.

Hot flue gases produced during carbonization are allowed to pass through 1st

and 4th regenerators until the temperature has been raised to 1000ċ.


While 1st and 4th regenerated are heated by hot flue gases, the 2nd and 3rd

regenerators are used for heating the incoming air and gas mixture.

For economical heating, the direction of inlet gases and flue gases are changed

frequently.

The above system of recycling the flue gases to produce heat energy is known

as the regenerative system of heat economy.

When the process is complete, the coke is removed and quenched with water.

Time taken for complete carbonization is about 12-20 hours.

The yield of coke is about 70%.

The valuable by products like coal gas, tar, ammonia, H2S and benzyl, etc. can

be recovered from flue gas.

Recovery of by-products

(i) Tar

The flue gases are first passed through a tower in which liquor ammonia is

sprayed. Tar and dust get dissolved and collected in a tank below, which is heated by

steam coils to recover back the ammonia sprayed.

(ii) Ammonia

The gases are then passed through another tower in which water is sprayed.

Here ammonia gets converted to NH4OH.

(iii) Naphthalene

The gases are again passed through a tower, in which cooled water is sprayed.

Here naphthalene gets condensed.

(iv)Benzene

The gases are passed through another tower, where petroleum is sprayed.

Here benzene gets condensed to liquid.

(v)Hydrogen Sulphide

The remaining gases are then passed through a purifier packed with moist

Fe2O3. Here H2S is retained.


The final gas left out is called coal gas which is used as a gaseous fuel.

Advantages of Otto Hoffman‟s process

1. Valuable by products like ammonia, coal gas, Naphthalene etc. are recovered.

2. The carbonization time is less.

3. Heating is done externally by producer gas.

BEEHIVE OVEN METHOD

Description of the oven

The oven is made of bricks and it is dome shaped structure. It is about 4m

diameter and 2.5m height.

The oven consists of two doors. One at the top onto charge the coal called coal

charging door. The other at the side helps to remove the coke formed, and it

also help as the air inlet. These two doors can be operated as desired.

Process


The top door is opened and coal is charged through it. The coal is the spread

uniformly to give a layer of about 0.75m thickness.

Through the side door minimum amount of air is supplied and the coal is ignited.

The volatile matters escape and burn inside the door. Combustion is allowed

to take place slowly from top to bottom with minimum supply of air.

The carbonization completes within 3-4 days. Then the hot coke is treated

immediately (quenching) with water. It is then taken out through the side door.

Yield -80% of the coal charged.

Advantage

Cheapest process for the manufacture of metallurgical coke.

Disadvantage

Most of the volatile matter which is the potential source for many chemical is

allowed to escape into atmosphere as waste.

It is a time consuming process.

LIQUID FUELS

PETROLEUM (OR) CRUDE OIL

It is naturally occurring liquid fuel.

It is dark brown or black coloured viscous oil

The oil is usually floating over a brine solution and above the oil, natural gas is

present.

Crude oil is a mixture of paraffinic, olefinic and aromatic hydrocarbons with small

amounts of organic compounds like N, O and S.

The average composition of crude oil is as follows

C = 80-87%

H = 11-15%


S = 0.1-3.5%

N+O = 0.1-0.5%

CLASSIFICATION OF PETROLEUM

It is classified into three types

1. Paraffinic-Base type crude oil

It contains saturated hydrocarbons from CH4 to C35H72 with a smaller amount of

naphthenes and aromatics.

2. Naphthenic or Asphaltic Base type crude oil

It contains Cycloparaffins or naphthenes with a smaller amount of paraffin and

aromatics

3. Mixed base type crude oil

It contains both paraffinic and asphaltic hydrocarbons.

REFINING OF PETROLEUM OR CRUDE OIL

The crude oil obtained from the earth is a mixture of oil, water, unwanted

impurities.

After the removal of water and other impurities, the crude oil is subjected to

fractional distillation.

During fractional distillation, the crude oil is separated into various fractions.

Thus, the process of removing impurities and separating the crude oil into

various fractions having different boiling points is called Refining of Petroleum.

The process of refining involves the following steps.

Step 1: Separation of water (Cottrell‟s process)

The crude oil well is an extremely stable emulsion of oil and salt water. The crude oil is

allowed to flow between two highly charged electrodes, where colloidal water droplets

combine to form large drops, which is then separated out form the oil.

Step 2: Removal of harmful sulphur compounds


Sulphur compounds are removed by treating the crude oil with copper oxide. The

copper sulphide formed is separated out by filtration.

Step: 3 Fractional distillation

The purified crude oil is then heated to about 400ċ in an iron retort, where the oil

gets vaporized.

The hot vapors are then passed into the bottom of a “fractionating column”.

The fractionating column is a tall cylindrical tower containing a number of

horizontal stainless steel trays at short distances.

Each tray is provided with small chimney covered with a loose cap.

When the vapors of the oil go up in the fractionating column, they become and

get condensed at different trays.

The fractions having higher boiling points condense at lower trays whereas

the fractions having lower boiling points condense at higher trays.

The gasoline obtained by this fractional distillation is called straight-run

gasoline.

Various fractions obtained at different trays are given in table.

TABLE- Various fractions, compositions and their uses

S.

No

Name of the fraction Boiling

Range ċ

Range of C-

Atoms

Uses

1. Uncondensed gases Below 30 C1-C4 As a fuel under

the name of

LPG

2. Petroleum ether 30-70 C5-C7 As a solvent

3. Gasoline or petrol 40-120 C5-C9 Fuel for IC

engines.


4. Naphtha or solvent spirit 120-180 C9-C10 As a solvent in

paints and in

dry cleaning.

5. Kerosene oil 180-250 C10-C16 Fuel for stoves

and jet engines.

6. Diesel oil 250-320 C15-C18 Diesel engine

fuel.

7. Heavy oil 320-400 C17-C30 Fuel for ships

and for

production of

gasoline by

cracking.

Heavy oils on refractionation gives

S.No Name of the Fraction Uses

1. Lubricating oils As lubricants.

2. Petroleum jelly or Vaseline Used in medicines and cosmetics.

3. Grease Used as lubricant.

4. Paraffin wax Used in candles, boot polishes etc.

5. Pitch at above 400ċ Used for making roads, water proof

roofing etc.

CRACKING( OR) THERMAL DECOMPOSITION

Cracking is nothing but breaking the higher molecular weight hydrocarbons in to

simpler low molecular weight and low boiling hydrocarbons


C10 H22 cracking C5H12 + C5 H10

The two types of cracking methods are

1. Thermal cracking

2. Catalytic cracking

1. Thermal cracking

When subjected to high temperature and pressure high molecular weight

hydrocarbons are decomposed to smaller ones. There are two types of thermal

cracking.

a) Liquid-phase thermal cracking

The heavy oils of any type (residue, fuel oil or gasoline‟s) are subjected to

high pressure (15-100kg/cm2) and temperature (420-550ċ) to give smaller

ones.

Yield 50-60%.

b) Vapour phase thermal cracking

This method is suitable only for more volatile liquids.

The temperature and pressure used for this method is 600-650ċ and 10-

20 kg/cm2 respectively.

Only less time is required for this cracking.

The yield is found to be 70%.

2. Catalytic cracking

In this method cracking is done in the presence of catalyst.

Hence the temperature (300-500ċ) and pressure (1-5kg/cm2) are lower than

used in thermal cracking.

There are two methods of catalytic cracking.


1) Fixed bed catalytic cracking

Description of the apparatus

The setup used for the cracking the heavy oil consist s of

(i) preheater maintained at about 400-500ċ

(ii) Catalytic tower is filled with catalyst such as silica, alumina gel (SiO2, Al2O3) or

bauxite with clay and zirconium oxide maintained at the temperature of 400-500ċ and at

the pressure of the1-5 kg/cm2.

(iii) There is a fractionating column to separate heavy oil from gasoline and other

gases.

(iv) The cooler is used to condense gasoline and other gases.

(v) Stabilizer is to separate gasoline from other condensed gases.


Process

The heavy oil is charged to the catalytic tower through the preheater

maintained at 400-500ċ.

In the preheater heavy oil gets vaporized and passes over the clays.

Catalytic splitting of higher hydrocarbons takes place.

The carbon formed during the process gets deposited on the catalyst bed.

The cracked vapors are now allowed to enter the fractionating column.

The gasoline and other gases are now passes through the cooler, where the

gasoline and other gases get condensed.

The condensed liquid is now sent to the stabilizers, to separate out gasoline.

Note

(1) The light gases produced during cracking are no longer allowed to escape, but

are converted into raw materials used for the synthesis of new types of fuels, plastics,

synthetic rubber etc.

(2)The carbon deposited over the catalyst make the catalyst bed inactive. The

catalyst bed can be regenerated by passing the steam of hot air over the bed. The

carbon burns off. The vapor is diverted through a stand-by catalyst chamber. Thus the

reaction can proceed nonstop.

2) Fluid – bed catalytic cracking (or) moving bed catalytic cracking


Heavy oil is heated in the heater and sent to the reaction in the form of gas

along with the finely powdered catalyst.

The vapors are cracked into smaller one on the surface of the catalyst in the

reactor maintained at 530ċ and at the pressure of 3-5 kg/cm2.

Near the top of the reactor there is a separator called cyclone helps in

sending only the cracked vapors into the fractionating column, and retains, all

the catalyst powder in the reactor itself.

The heavy fuel oil is collected at the bottom of the fractionating column.

The gasoline and other lighter cracked gases are removed from the top and

sent to the gasoline is then sent to the stabilizers to get the pure form of

gasoline.


The catalyst powder settle at the bottom of the reactor due to deposition of

carbon on them are then forced into the regenerator maintained at 600ċ.

Where the carbon burnt off, the regenerated catalyst is then reused for the

cracking of fresh batch of the oil.

The separator (called cyclone) near the top of the regenerator allows only the

flue gas to escape.

KNOCKING

Knocking is a kind of explosion due to rapid pressure rise occurring in an IC engine.

Causes of knocking in S.I Engine [Petrol engines]

In a petrol engine, a mixture of gasoline vapor and air at 1:17 ratio is used as fuel

The mixture is compressed and ignited by an electric spark.

The products of oxidation reaction (combustion) increase the pressure and push

the piston down the cylinder.

If the combustion proceeds in a regular way, there is no problem in knocking.

But in some cases, the rate of combustion (oxidation) will not be uniform due to

unwanted chemical constituents of gasoline.

The rate of ignition of the fuel gradually increases and the final portion of

the fuel-air mixture gets ignited instantaneously producing an explosive

sound known as “Knocking”.

Knocking property of the fuel reduces the efficiency of engine. So a good

gasoline should resist knocking.

Chemical structure and Knocking

The knocking tendency of fuel hydrocarbons mainly depends on their chemical

structures. The knocking tendency decreases in the following order.

Straight chain paraffin‟s > Branched chain paraffin‟s > Cycloparaffins> Olefins >

Aromatics.


Thus olefins of the same carbon-chain length possess better anti-knocking properties

than the corresponding paraffin‟s.

Improvement of anti knock characteristics

The octane number of fuel can be improved by

(i) Blending petrol of high octane number with petrol of low octane number, so that the

octane number of the latter can be improved.

(ii) The addition of anti-knock agents like Tetra-Ethyl Lead (TEL).

OCTANE NUMBER OR OCTANE RATING

Octane number is introduced to express the knocking characteristics of petrol. It

has been found that n-heptanes knock very badly hence, its anti-knock value

has been given zero.

On the other hand, iso-octane gives very little knocking and so, its anti-knock

value has been given 100.

Thus octane number is defined as „the percentage of iso-octane present in

a mixture of iso-octane and n-heptanes.‟

CH3 CH3

-CH-CH2-C-CH3

CH3

CH3

Iso-octane

(Octane no = 100)

CH3-CH2-CH2-CH2-CH2-CH2-CH3

N-heptanes (Octane no = 0)

LEADED PETROL (ANTI-KNOCK AGENT)

Tetraethyl lead (TEL) (C2H5)4 Pb is an important additive added to petrol. Thus

the petrol containing tetra ethyl lead is called leaded petrol.

Mechanism of Knocking


TEL reduces the knocking tendency of hydrocarbon.

Knocking follows a free radical mechanism, leading to a chain growth which

results in an explosion.

If the chains are terminated before their growth, knocking will cease.

TEL decomposes thermally to form ethyl free radicals which combine with the

growing free radicals of knocking process and thus the chain growth is stopped.

Disadvantages of using TEL

When the leaded petrol is used as a fuel, the TEL is converted to lead oxide and metallic lead.

This lead deposits on the spark plug and on cylinder walls which is harmful to the engine life.

To avoid this, small amount of ethylene dibromide is added along with TEL.

This ethylene dibromide reacts with Pb and PbO to give volatile lead bromide, which goes out along with exhaust gases.

CH2-Br

Pb+ PbBr2 +CH2= CH2

CH2-Br

But this creates atmospheric pollution. So now day‟s aromatic phosphates are used instead of TEL.

DIESEL OIL

It is a fraction obtained between 250-320ċ during fractional distillation of petroleum.

It is a mixture of C15H32 to C18H38 hydrocarbons.

Its calorific value is about 11000 kcal/kg.

It is used as a very good diesel engine fuel. Causes of knocking in CI (Diesel engines)

In a diesel engine, first air is alone compressed.

This compression raises the temperature of the cylinder to about 500ċ.


Then the oil is sprayed into the heated air. This further raises the temperature as

well as pressure.

The expanding gases push the piston and power stroke begins.

The combustion of a fuel in a diesel engine is not instantaneous and the time between injection of the fuel and its ignition is called Ignition lag or Ignition delay.

This delay is due to the time for the vaporization of oil droplets and raising the temperature of vapour to its ignition temperature.

Long ignition lags to accumulation of more vapors in the cylinder, which undergo explosion during ignition.

This is responsible for diesel knock.

If the ignition lags is short, diesel knock will not occur.

CETANE NUMBER OR CETANE RATING

Cetane number is introduced to express the knocking characteristics of diesel.

Cetane (C16H34) has a very short ignition lag and hence its cetane number is taken as 100.

On the other hand 2-methyl naphthalene has a long ignition lag and hence its cetane number is taken zero.

Thus the cetane number is defined as “the percentage of cetane present in a mixture of cetane and 2-methyl naphthalene which has the same ignition lag as the fuel under test”.

CH3

CH3-(CH2)14-CH3

n-cetane

2-methyl naphthalene

(cetane no = 100) (cetane no = 0)

The cetane number decreases in the following order.

Straight chain paraffin‟s >Cycloparaffins >Olefins >Branched paraffin‟s >Aromatics.

The cetane number of diesel oil can be increased by adding additives called dopes.

Example: Ethyl nitrate, Iso-amyl nitrate.


DIESEL INDEX

The quality of diesel oil is indicated by diesel index number using the following formula.

Diesel index number =Specific gravity (API) x Aniline in . F

100

Aniline point and specific gravity is noted from API (American Petroleum Institute)

Comparison of gasoline oil and diesel oil

S.No Gasoline oil Diesel oil

1. Low boiling fraction of petroleum

contains C5-C9 hydrocarbons.

High boiling fraction of petroleum

contains C15-C18 hydrocarbons.

2. Fuel for SI engine Fuel for Cl engine.

3. Knocking tendency is measured

in octane rating

Knocking tendency is measured in

cetin rating.

4. Knocking is due to premature

ignition

Knocking is due to ignition lag.

5. Antiknocking is improved by the

addition of TEL

Anti knocking is improved by doping

with ethyl nitrate.

6. Its exhaust gases contain higher

amount of pollutants

Its exhaust gases contain lesser

amount of pollutants.

7. More consumption, lower thermal

efficiency

Less consumption, higher thermal

efficiency.

2.Hydrogenation of coal

Coal contains about 4.5% of hydrogen compared to about 18% of in petroleum.

So, coal is a hydrogen deficient compound.

If coal is heated with hydrogen to high temperature under high pressure, it is

converted to gasoline.


The preparation of liquid fuels from solid coal is called Hydrogenation of

coal.

There are two methods available for the hydrogenation of coal.

(a) Bergius process (or direct method)

(b) Fischer- Tropsch Process (or indirect method)

(a) Bergius process (or indirect method)

Finely powdered coal + heavy oil+ catalyst powder (tin or nickel) is made

into a paste

The paste is pumped along with hydrogen gas into the converter, where the

paste is heated to 400-450ċ under a pressure of 200-250atm.


During this process hydrogen combine with coal to form saturated higher

hydrocarbons, which undergo further decomposition at higher temperature

to yield mixture of lower hydrocarbons.

The mixture is led to a condenser, where the crude oil is obtained.

The crude oil is then fractionated to yield.

(i) Gasoline (ii) Middle oil (iii) heavy oil

The middle oil is further hydrogenated in vapour phase to yield more

gasoline.

The heavy oil is recycled for making paste with fresh coal dust.

The yield of gasoline is about 60% of the coal.

(b) Fischer-tropics process (or indirect method)


In this process coal is first converted into coke. Then water gas is produced by

passing steam over red hot coke

C + H2O 1200ċ CO + H2

(Water gas)

The water gas is mixed with hydrogen and the mixture is purified by passing

through Fe 2O3 +Na2CO 3 (to remove sulphur compounds).

The purified gas is compressed to 5 to 25 atm and then led through a

converter, which is maintained at a temperature of 200-300⁰C.

The converter is provided with a catalyst bed consisting of a mixture of 100

parts cobalt, 5 parts thoria, 8 parts magnesia and 200 parts kieselgurh

earth.


A mixture of saturated and unsaturated hydrocarbon is produced as a result

polymerization.

nCO+2nH2 CnH2n +nH2O

nCO+(2n+1)H2 CnH2n +2+nH2O

The out coming gaseous mixture is led to condenser, where the liquid crude

oil is obtained.

The crude oil is fractionated to yield (i) Gasoline and (ii) Heavy oil.

The heavy oil is used for cracking to get more gasoline.

2.PRODUCER GAS

It is a mixture of CO&N2 with small amount of H2. Its average composition is as follows.

Constituents Percentage (%)

CO 30

N2 51-56

H2 10-15

CO2+CH4 rest

It is calorific value is about 1300 kcal/m3.

Manufacture

The reactor used for the manufacture of producer gas is known as gas

producer.

It consists of a tall steel vessel inside of which is lined with refractory bricks.


It is provided with cup and cone feeder at the top and a side opening for

producer gas exists.

At the bottom, it is provided with an inlet pipe for passing air and steam.

When a mixture of air and steam is passes over a red hot coke maintained at

about 1100ċ in a reactor, the producer gas is produced.

Reactions

The reactions of producer gas production can be divided into four zones as follows.

1. Ash Zone

This is the lowest zone consists mainly of ash. The incoming air and steam mixture is

preheated in this zone.


2. Combustion or oxidation zone

This is the zone next to ash zone. Both the reactions are exothermic. Hence, the

temperature of the bed reaches around 1,100ċ.

C+1/2O2 CO exothermic

C+O2 CO2 exothermic

3. Reduction Zone

This is the middle zone. Here both CO2 and steam are reduced.

C+CO2 2CO endothermic

C+H2O CO+H2O endothermic

The above reactions are endothermic. Hence the temperature of the coke bed falls to

1000ċ.

4. Distillation or Drying Zone

This is the upper most of the coke bed. In this zone (400-800ċ) the incoming coke is

heated by the outgoing gases.

Uses

1. It is used as a reducing agent in metallurgical operations.

2. It is also used for heating muffle furnaces, open-hearth furnaces etc.

3. WATER GAS

It is mixture of CO and H2 with small amount of N2. The average composition of

water gas is as follows.

Constituents Percentage

CO 41

H2 51

N2 4

CO2+CH4 rest


Its calorific value is about 2800kcal/m3

Manufacture

The water gas producer consists of a tall steel vessel, lined inside with

refractory bricks.

It is provide with cup and cone feeder at the top and a side opening of water

gas exist.

At the bottom on it is provide with two inlet pipes for passing air and steam.

When steam and little air is passed alternatively over a red hot coke

maintained at about 900-1000ċ in a reactor, water gas is produced.

Reactions

The reactions of water gas production involve the following two steps.

Step-I


In the first stage, steam is passed through the red hot coke, where CO &H2 are

produced. The reaction is endothermic. Hence, the temperature of the coke bed falls.

C+ H2O CO+H2 endothermic

Step-II

In the second stage, in order to raise the temperature of the coke bed to 1000ċ, the

steam supply is temporarily cut off and air blown in; the reaction is exothermic.

C+O2 CO2 exothermic

Thus the steam-run and air blow are repeated alternatively to maintain proper

temperature.

Uses

1. It is used for the production of H2 and in the synthesis of ammonia.

2. It is used to synthesis gasoline in Fischer-Tropics process.

3. It is also used in the manufacture of power alcohol and carbureted water gas

(water gas + oil gas).

5. LPG- Liquefied Petroleum gas

It is also known as bottled gas or refinery gas.

It is obtained by a by-product during the fractional distillation of heavy

oil or cracking of higher hydrocarbons.

It can easily be liquefied under pressure, but exist as gas at

atmospheric pressure.

LPG consists of the following hydrocarbons containing carbons atoms

up to 4(C4).

The average composition of LPG is

Propane - 24.7%

Butane - 38.5%

Isobutene - 37.7%

LPG has the calorific value of about 2500 Kcals/m3.


Uses

1. LPG is supplied with the trade name like indene bharath gas etc. It is mainly used

as domestic and industrial fuel.

2. It is also used as motor fuel, because it easily mixes with air and burns without any

pollution creating residue.

Advantages

1. It possesses high efficiency and heating rate.

2. Burns completely without smoke.

3. Needs only little care in maintenance.

4. Easily transported using steel cylinder to any places.

5. It is very cheaper than gasoline.

Disadvantages

1. Handling should be only under pressure.

2. User of LPG in engines is possible only if it works under high compression ratio.

3. Its response to blending is poor and so its uses are selective.

1. COMPRESSED NATURAL GAS (CNG)

When the natural is compressed, it is called compresses natural gas (CNG).

The primary component present in CNG is methane. It is mainly derived from

natural gas.

Properties

1. CNG is the cheapest, cleanest and least environmentally vehicle impacting

alternative fuel.

2. Vehicles powered by CNG produce less carbon monoxide and hydrocarbon (HC)

emission.

3. It is less expensive than and diesel.

4. The ignition temperature of CNG is about 55ċ.

5. CNG requires more air for ignition.


Uses:

CNG is used to run an automotive vehicle just like LPG.

Comparison of emission levels between CNG- driven vehicles and petrol

driven vehicles

Pollutants

Emission Levels

Petrol Driven Vehicle CNG Driven Vehicle

CO (gm/km) 0.92 0.05

HC (gm/km) 0.36 0.24

Calorific value:

.

It is defined as the amount of heat liberated by the complete combustion of a unit mass of the fuel.

Calorie;

The amount of heat required to raise the temperature of 1g of water

through 1⁰C.

Gross or high calorific value (GCV)

It is defined as the total heat generated when a unit quantity of fuel is completely burnt and the products of combustion are cooled to room temperature. Net or Lower Calorific Value(NCV) It is defined as the “net heat produced when a unit quantity o f fuel is completely burnt and the products of combustion are allowed to escape. NCV = GCV- latent heat of condensation of steam produced.

Theoretical calculation of calorific value by Dulong‟s formula:


According to Dulong, the calorific value of a fuel is the sum of the calorific

values of its constituent elements.

The calorific values of C, H&S are found to be 8080, 34500 and 2240kcals when 1kg of the fuel is burnt completely.

Thus,Dulong‟s formulae for GCV is written as GCV=1/100 [8080(C)+34500(H-O/8)+2240(S)]Cals/kg. NCV =[ GCV-9/100( H)x 587] Kcals/kg.

The latent heat of steam is 587Kcals/kg.

In Dulong‟s formula C, H, O&S represent the percentage of the corresponding elements.


UNIT IV PHASE RULE AND ALLOYS

Chemical reactions are of two types

1. Irreversible reaction

2. Reversible reaction – it‟s of two types

A. homogeneous reversible reaction

B.heterogeneous reversible reaction --- its behaviour can be studied by PHASE

RULE given by Willard Gibbs (1874).

Phase rule

The number of degree of freedom (F) of the system is related to number of components

(C) and number of phases (P) by the following phase rule equation.

F = C-P+2

Explanation or meaning of terms

1. Phase (P)

Any homogeneous physically distinct and mechanically separable portion of a system

which is separated from other parts of the system by definite boundaries.

a. Gaseous phase

All gases are completely miscible and there is no boundary between one gas and

the other.

For example: air – single phase

b.Liquid phase

It depends on the number of liquids present and their miscibilities.

i. If two liquids are immiscible, they will form three separate phases two liquid

phase and one vapour phase.

For example: benzene-water.

ii. If tow liquids are miscible, they will form one liquid phase and one vapour

phase.

For example: alcohol – water.

C .Solid phase


Every solid constitutes a separate phase

For example:

(i) Water system ------- three phases

(ii) Rhombic sulphur (s) monoclinic sulphur (s) ----- two phase

iii) Sugar solution in water ----- one phase

iv) CuSO4.5H2O(s) CuSO4.3H2O(s) + 2H2O(g) ---- three

phases.

2. Component (C)

“The smallest number of independently variable constituents, by means of which

the composition of each phase can be expressed in the form of a chemical

equation”.

For example:

i) Water system ---- one component ( H2O )

ii) An aqueous system of NaCl --- two component ( NaCl , H2O )

iii) PCl5(s) PCl3 (l) + Cl2 (g) --- two component ,three phases

iv) CuSO4.5H2O(s) CuSO4.3H2O(s) + 2H2O(g) ---- three

phases,two component

3. Degree of freedom(F)

“The minimum number of independent variable factors such as temperature,

pressure and concentration, which much be fixed in order to define the system

completely”.

i) Water system

Ice (s) water (l) vapour (g)

F = Non variant (or) zero variant

ii) Ice (s) water (l)

F = univariant (one)


iii) For a gaseous mixture of N2 and H2, we must state both the pressure and

temperature. Hence,the system is bivariant.

PHASE DIAGRAM:

Phase diagram is a graph obtained by plotting one degree of freedom against

another.

Types of phase diagrams

(i)P-T Diagram : used for one component system

(ii) T-C Diagram : used for two component system

APPLICATIONS OF PHASE RULE TO ONE COMPONENT SYSTEM

The water system:

Water exists in three possible phases namely solid, liquid and vapour. Hence

there can be three forms of equilibria.

Solid Liquid

Liquid Vapour

Solid Vapour

Each of the above equilibrium involves two phases. The phase diagram for the water

system is shown in the figure.

This phase diagram contains curves, areas, and triple.


(i)Curve OA

The curve OA is called vaporisation curve, it represents the equilibrium between

water and vapour. At any point on the curve the following equilibrium will exist.

Water Water vapour

The degree of freedom of the system is one, i.e, univariant.

This is predicted by the phase rule.


F=C-P+2; F=1-2+2; F=1

This equilibrium (i.e. Line OA) will extend up to the critical temperature (347o C).

Beyond the critical temperature the equilibrium will disappear only water vapour will

exist.

(ii) Curve OB

The curve OB is called sublimation curve of ice, it represents the equilibrium

between ice and vapour. At any point on the curve the following equilibrium will exist.

ICE VAPOUR

The degree of freedom of the system is one, i.e. univariant. This is predicted by

the phase rule.

F = C – P + 2; F = 1-2=2 ; F=1

This equilibrium (line OB) will extend up to the absolute zero (-273o C), where no vapour

can be present and only ice will exist.

iii) Curve OC

The curve OC is called melting point curve of ice, it represents the equilibrium between

the ice and water. At any point on the curve the following equilibrium will exist.

Ice water

The curve OC is slightly inclined towards pressure axis. This shows that melting point of

ice decreases with increase of pressure.

The degree of freedom of the system is one i.e., univariant.

iv) point O (triple point)

The three curves OA ,OB ,OC meet at a point “O” ,where three phases namely solid

,liquid and vapour are simultaneously at equilibrium .

This point is called triple point, at this point the following equilibrium will exist.

Ice water vapour

The degree of freedom of the system is zero i.e., nonvariant.This is predicted by the

phase rule.

F=C-P+2; F=1-3+2=0


Temperature and pressure at the point “O” are 0.0075 oC and 4.58 mm respectively.

(v) Curve OB‟: Metastable equilibrium

The curve OB‟ is called vapour pressure curve of the super-cool water or metastable

equilibrium where the following equilibrium will exist.

Super-cool water ---------- vapour

Sometimes water can be cooled below OoC without the formation of ice, this water is

called super –cooled water. Super cooled water is unstable and it can be converted in to

solid by seeding or by slight disturbance.

vi) Areas

Area AOC, BOC, AOB represents water, ice and vapour respectively .The degree of the

freedom of the system is two.i.e. Bivariant.

This is predicted by the phase rule

F=C-P=2; F=1-1+2; F=2

Two component alloy system or multi component equilibria

Reduced phase rule or condensed system

The system in which only the solid and liquid are considered and the gas

phase is ignored is called a condensed system.since pressure kept constant, the phase

rule becomes

F‟ = C – P + 2

This equation is called reduced phase rule.

Classification of two component system

Based on the solubility and reactive ablity, the two component systems are classified in

to three types.

1. Simple eutectic formation - A binary system consisting of two substances, which

are completely miscible in the liquid state, but completely immiscible in the solid state, is

known as eutectic (easy melt) system. They do not react chemically. Of the different


mixtures of the two substances, the mixture having the lowest melting point is known as

the eutectic mixture.

2. a) formation of compound with congruent melting point

b) Formation of compound with incongruent melting point

3. Formation of solid solution

Thermal analysis or cooling curve

Thermal analysis is a method involving a study of the cooling curves of various

compositions of a system during solidification. The form of the cooling curve indicates

the composition of the solid.

Ex: 1. Cooling curve of a pure solid.

Ex: 2. Cooling curve of a mixture A + B.

A cooling curve is a line graph that represents the change of phase of matter, typically from a gas to a solid or a liquid to a solid.

The independent variable (X-axis) is time and the dependent variable (Y-axis) is temperature. Below is an example of a cooling curve.

http://en.wikipedia.org/wiki/Graphics

http://en.wikipedia.org/wiki/Phase_(matter)

http://en.wikipedia.org/wiki/Matter

http://en.wikipedia.org/wiki/Independent_variable

http://en.wikipedia.org/wiki/Dependent_variable


The initial point of the graph is the starting temperature of the matter, here noted as the "pouring temperature". When the phase change occurs there is a "thermal arrest", that is the temperature stays constant. This is because the matter has more internal energy as a liquid or gas than in the state that it is cooling to. The amount of energy required for a phase change is known as latent heat. The "cooling rate" is the slope of the cooling curve at any point.

A Pure substance in the fused or liquid state is allowed to cool slowly.The

temperature is noted at different times.when represented graphically the rate of cooling

will be a continuous from „a‟ to „b‟.

When the freezing point is reached and solid making its appearance there will be a

break in the continuity of the cooling curve.The temperature will thereafter remain

constant until the liquid is completely solidified.Thereafter the fall in temperature wil

again become continuous.

a. Cooling curve of a pure substances b. Cooling curve of a

mixture

http://en.wikipedia.org/wiki/Internal_energy

http://en.wikipedia.org/wiki/Latent_heat


If a mixture of two solids in the fused state is cooled slowly we get

a cooling curve .Here also first a continuous coling curve will be obtained as long as the

mixture is in the liquid state .

When a solid phase begins to form there will be a break in the cooling curve .But the

temperature will not remain constant unlike in the case of cooling of a purified

substance.The temperature will decrease continuously but at a different rate.The fall of

temperature will continue till the mixture forms a eutectic and the eutectic point is

reached.

The temperature will thereafter remain constant until solidification is complete .

Thereafter the fall of temperature will become uniform ,but the rate of fall will be different

from that for a pure substance.

Uses of cooling curves

i) Percentage purity of the compounds can be noted from the cooling curve.

ii) The behaviour of the compounds can be clearly understood from the

cooling curve.

iii) The procedure of thermal analysis can be used to derive the phase diagram

of any two component system.

BINARY ALLOY SYSTEM OR THE SIMPLE EUTECTIC SYSTEM

The Lead – Sliver system

Since the system is studied at constant pressure,the vapour phase is ignorned

and the condensed phase rule is rule is used.

F „= C-P+1

The phase diagram of lead –sliver system is shown in the figure

It contains lines,areas and the eutectic point.


i) curve AO

The curve AO is known as freezing point curve of sliver.

Along the curve AO, solid Ag and the melt are in equilibrium.

Solid Ag melt

According to reduced phase rule

F‟=C-P+1

C=2

P=2

F‟=1

The system is univariant


ii) curve BO

The curve BO is known as freezing point curve of lead .

Along the curve BO, solid Pb and the melt are in equilibrium.

Solid Pb melt


F‟=C-P+1

C=2

P=2

F‟=1

The system is univariant.

iii) Point “ O ” (eutectic point)

The curves AO and BO meet at point „ O „ at a temperature of 303 o C

,where the three phases are in equilibrium.

Solid Pb + soild Ag melt


F‟=C-P+1

C=2

P=3

F‟=1

The system is non-variant.

The point “ O “ is called eutectic point or eutectic temperature and is

corresponding composition,97.4 % Pb and 2.6 % Ag ,is called eutectic

composition.below this point the eutectic compound and the metal

solidfy.

iv) Areas

The area above the line AOB has a single phase( molten Pb + Ag ).


F‟=C-P+1

C=2

P=1

F‟=2

The system is bi-variant.


The area below the line AO ,OB and point “O” have two phases and

hence the system is univariant.


F‟=C-P+1

C=2

P=2

F‟=1

The system is uni-variant.

The process of raising the relative proportion of Ag in the alloy is known as

pattinson‟s process.


The Pattinson process was patented in 1833. It depended on well-known

material properties; essentially that lead and silver melt at different temperatures. The

equipment consisted of a row of about 8-9 iron pots, which could be heated from below.

Agentiferous lead was charged to the central pot and melted. This was then allowed to

cool, as the lead solidified, it was skimmed off and moved to the next pot in one

direction, and the remaining metal was then transferred to the next pot in the opposite

direction. The process was repeated in the pots successively, and resulted in lead

accumulating in the pot at one end and silver in that at the other. The process was

economic for lead containing at least 250 grams of silver per ton.

Uses of eutectic system

1.suitable alloy composition can be predicted with the help of eutectic

systems.

2.eutectic systems are used in preparing solders ,used for joining two

metal pieces together.

Melting point

It is the temperature at which the solid and liquid phases, having the same composition

,are in equilibrium.

Solid A solid B

Eutectic point

It is the temperature at which two solids and a liquid phase are in equilibrium .

Solid A + solid B Liquid

Triple point

It is the temperature at which three phases are in equilibrium.

Solid liquid vapour

By definition ,

http://en.wikipedia.org/wiki/Patent


All the eutectic points are melting points, but all the melting points need not be eutectic

points.

ll ly , all the eutectic points are triple points ,but all the triple points need not be eutectic

points.

Uses (or) merits of phase rule

1. It is a convenient method of classifying the equilibrium states in terms of phases

,components and degree of freedom.

2. It helps in deciding whether the given number of substances remain in equilibrium or

not.

Limitations of phase rule

1.phase rule can be applied for the systems in equilibrium.

2.only three variables like P,T & C are considered ,but not electrical, magnetic and

gravitational forces.

ALLOYS

Definition

An alloy is defined as “homogeneous solid solution of two or more

different element one of which at least is essentially a metal”. Alloy containing Hg

as a constituent element are called amalgams.

Properties of alloys

1) Alloy are harder less malleable and possess lower melting point than their

component metals

2) Alloys possess low electrical conductivity

3) Alloys resist corrosion and the action of acids


Importance or need of making alloys

1. To increase the hardness of the metal

Example

Gold and silver are soft metal they are alloyed with copper to make

them hard

2. To lower the melting points of the metal

Example

Wood metal (an alloy of lead, bismuth, tin and cadmium) melts at

60.5⁰c which is far below the melting points of any of these constituent

metals

3. To resist the corrosion of the metal

Example

Pure iron rested but when it is alloyed with carbon chromium

(stainless steel) which resists corrosion

4. To modify chemical activity of the metal

Example

Sodium amalgam is less active than sodium but aluminium

amalgam is more active than aluminium

5. To modify the colour of the metal

Example

Brass an alloy of copper (red) and size (silver-white) is white colour.

6. To get good casting of metal

Example

An alloy of lead with 5% tin and 2% antimony is used fro casting

printing type due toits good casting property

Functions (or) effects of alloying elements

Addition of small amount of certain metals such as Ni, Cr, Mo, Mn, Si, v

and Al impart special properties like hardness, tensile strength, resistance to

corrosion and coefficient of expansion on steel. Such products are known as

special steel or alloy steels


Some important alloying element and their functions are given in table

CLASSIFICATION (OR) TYPES OF ALLOYS

Example

Example

(i)Nichrome

(i) Solder

(ii) Alnico

(ii) Brass

(iii)Stainless steel

(iii) Bronze

FERROUS ALLOYS OR ALLOY STEELS

Ferrous alloys are the type of steels in which the

elements like Al,B,Cr,Co,Cu,Mn are present in sufficient quantities, in

addition to carbon & iron.

PROPERTIES

1. High yield point & strength

2. Sufficient formability,ductility & weldability

3. Corrosion & abrasion resistant

4. Less distortion & cracking

5. High temperature strength

ALLOYS

FERROUS ALLOYS NON-FERROUS ALLOYS


IMPORTANT FERROUS ALLOYS

(i)NICHROME

Nichrome is an alloy of nickel & chromium

COMPOSITION

Nickel – 60%

Chromium – 12%

Iron – 26%

Manganese – 2%

PROPERTIES

1. Good resistance to oxidation & heat

2. High melting point & electrical resistance

3. Withstand heat up to 1000-1100⁰C

USES

1. Used for making resistance coils,heting elements in stoves &

electric irons

2. Used in making parts of boilers,steam lines stills,gas turbines,aero

engine valves,retorts,annealing boxes.

(ii)ALNICO

Alnico is an alloy of aluminium-nickel-cobalt .

COMPOSITION

Aluminium – 8-12%

Nickel – 14-28%


Cobalt – 5-35%

PROPERTIES

1. Excellent magnetic properties & high melting point

2. Magnetized to produce strong magnetic fields as high as 1500 gauss

TYPES OF ALNICO ALLOYS

Alnico alloys are of two types

1. ISOTROPIC ALNICO

It is effectively magnetized in any direction

2.ANISOTROPIC ALNICO

It possess preffered direction of magnetization.

Anisotropic alnico possesses greater magnetic capacity in their

preffered orientation than isotropic alnico.

USES

1. Used as permanent magnets in motors,generators,radio speakers

microphones,telephone receivers & galvanometers.

(iii)STAINLESS STEELS (or)CORROSIOPN RESISTANT STEELS

These are alloy steels containing chromium together with other

elements such as nickel,molybdenum,etc.

Chromium-16% or more

Carbon-0.3-1.5%


PROPERTIES

1. Resist corrosion by atmospheric gases & also by other chemicals.

2. Protection against corrosion is due to the formation of dense, non-

porous,tough film of chromium oxide at the metal surface. If the film

cracks, it gets automatically healed up by atmospheric oxygen

TYPES OF STAINLESS STEEL

1. HEAT TREATABLE STAINLESS STEEL

COMPOSITION

Carbon-1.2%

Chromium-less than 12-16%

PROPERTIES

Magnetic,tough & can be worked in cold condition

STAINLESS STEEL

HEAT TREATABLE

STAINLESS STEEL

NON HEAT TREATABLE

STAINLESS STEEL

MAGNETIC TYPE NON MAGNETIC

TYPE


USES

1. Can be used up to 800⁰C

2. Good resistant towards weather & water

3. In making surgical instruments,scissors,blades,etc.

2.HEAT TREATABLE STAINLESS STEEL

PROPERTIES

Possess less strength at high temperature

Resistant to corrosion

TYPES OF NON HEAT TREATABLE STAINLESS STEEL

(a)MAGNETIC TYPE

COMPOSITION

Chromium-12-22%

Carbon-0.35%

PROPERTIES

1. Can be forged,rolled & machined

2. Resist corrosion

USES

Used in making chemical equipments& automobile parts.

(b)NON MAGNETIC TYPE

COMPOSITION

Chromium-18-26%

Nickel-8-21%


Carbon-0.15%

Total % of Cr & Ni is more than 23%.

EXAMPLE:18/8 STAINLESS STEEL

COMPOSITION

Chromium-18%

Nickel-8%

PROPERTIES

1. Resistance to corrosion.

2. Corrosion resistance is increased by adding molybdenum

USES

In making household utensils,sinks,dental & surgical instruments.

NON FERROUS ALLOYS

Do not contain iron as one of the main constituent.

Main constituents are copper,aluminium,lead,tin,etc.

PROPERTIES

1. Softness & good formability

2. Attractive (or) very good colours

3. Good electrical & magnetic properties

4. Low density & coefficient of friction

5. Corrosion resistance

IMPORTANT NON FERROUS ALLOYS

1. COPPER ALLOYS (BRASS)

Brass contains mainly copper & zinc

PROPERTIES

Greater strength, durability & machinability

Lower melting points than Cu & Zn

Good corrosion resistance & water resistance property


2.BRONZE(COPPER ALLOY)

Bronze contains copper & tin

PROPERTIES

Lower melting point

Better heat & electrical conducting property

Non-oxidizing,corrosion resistance & water resistance property.

3.SOLDERS

Solders are low- melting alloys of tin & lead

PROPERTIES

Solder is melted to join metallic surfaces ,especially in the fields of

electronic & plumbing

USES

1. Used in electrical industry

2. Alloy with 50% tin is general-purpose solder

3. For sealing automotive radiator cores.

4. As fuses for fire-extinguishing equipments,boiler plugs,etc.

Heat treatment of alloys (steel)

Heat treatment is defined as” the process of heating and cooling of solid

steel article under carefully controlled condition”. During heat treatment certain

physical properties are altered without altering its chemical composition

Objectives (or) purpose of heat treatment

Heat treatment causes

i. Improvement in magnetic and electrical properties


ii. Refinement of grain structure

iii. Removal of the imprisoned trapped gases

iv. Removal of internal stress

v. Improves fatique and corrosion resistance

Types of heat treatment of alloys (steel)

1. Annealing

Annealing means softening. This is done by heating the metal to high

temperature followed by slow cooling in a furnace.

Purpose of annealing

i. It increases the machinability

ii. It also removes the imprisoned gases

Types of annealing

Annealing can be done in two types

i. Low temperature annealing (or) process annealing

ii. High temperature annealing 9or) full annealing

(i) Low temperature annealing (or)process annealing

It involves in heating steel to a temperature below the lower critical

point followed by slow cooling

Purpose

1. It improves mashinability by reliving the internal stress or internal strain

2. It increases ductility and shock resistance

3. It reduce hardness


(ii) High temperature annealing (or) fault annealing

It involves in heating to a temperature about 30 to 50⁰C above the

higher critical temperature and holding it at that temperature for

sufficient time to allow the internal changes to take place and

then cooled room temperature

The approximate annealing temperature of various grades of

carbon steel are

1. Mild steel=840-870⁰c

2. Medium carbon steel=780-840⁰c

3. High carbon steel=760-780⁰c

Purpose

1. It increases the ductility and machinability

2. It makes the steel softer, together with an appreciable increases in its

toughness

2.Hardening (or) quenching

It is the process of heating steel beyond the critical temperature

and then suddenly cooling it either in oil or brine water or some

other fluid.

The faster the rate of cooling harder will be the steel produced.

Medium and high carbon steel can be hardened but low carbon

steel cannot hardened

Purpose

1. It increases its resistance to wear ability ,to cut other metal and strength .

2. It increases abrasion resistance.

3. Used for making cutting tools.

3. TEMPERING

It is the process of heating the already hardened steel to a

temperature lower than its own hardening temperature & then

cooling it slowly.


The reheating controls the development of the final properties

Thus,

(a)For retaining strength & hardness, reheating temperature

should not exceed 400⁰C.

(b) For developing better ductility & toughness, reheating

temperature should be within 400-600⁰C.

PURPOSE

1. It removes stress &strains that might have developed during

quenching.

2. Increased toughness & ductility.

3. Used for cutting tools like blade,cutters etc.

4. NORMALISING

It is the purpose of heating steel to a definite

temperature (above its higher critical temperature) & allowing it to

cool gradually in air.

PURPOSE

1. Recovers homogeneity

2. Refines grains.

3. Removes internal stresses

4. Increases toughness

5. Used in engineering works

NOTE: The difference between normalised & annealed steel are

1. A normaled steel will not be as soft as annealed steel.

2. Also normalizing takes much lesser time than annealing.

5.CARBURIZING

The mild steel article is taken in a cast iron box within containing

small pieces of charcoal(carbon material).

It is heated to about 900 to 950⁰C & allow it for sufficient time,so

that the carbon is absorbed to required depth .

The article is then allowed to cool slowly within the box itself.


The outer skin of the article is converted into high carbon steel

containing about 0.8 to 1.2% carbon.

PURPOSE

To produce hard surface on steel article

6. NITRIDING

Nitriding is the process of heating the metal alloy in

presence of ammonia to about 550⁰C.

The nitrogen (obtained by the dissociation of ammonia)

combines with the surface of the alloy to form hard

nitride.

PURPOSE

To get super-hard surface.


UNIT-5 ANALYTICAL TECHNIQUES

8.1 INTRODUCTION

Analytical technique (or) Spectroscopy is one of the most powerful tool available

for the study of atomic and molecular structure, and is used in the analysis of a most of

the samples.

Spectroscopy deals with the study of interaction of electromagnetic radiation with

the matter. During the interaction, the energy is absorbed or emitted by the matter. The

measurement of this radiation frequency (absorbed or emitted) are made using

spectroscopy.

8.2 TYPES OF SPECTROSCOPY

The study of spectroscopy can be carried out under the following headings

1. Atomic spectroscopy.

2. Molecular spectroscopy.

1. Atomic spectroscopy

It deals with the interaction of the electro magnetic radiation with atoms. During

which the atoms absorb radiation and gets excited from the ground state electronic

energy level to another.

2. Molecular spectroscopy

It deals with the interaction of electromagnetic radiation with molecules. This results in

transi tion between rotational, vibrational and electronic energy levels.

Differences between molecular

spectra and atomic spectra


8.3 SPECTRUM

How does a Spectrum arise?

1. Absorption spectrum

Consider a molecule having only two energy levels

E1 and E2 as shown in the figure. 8.1.

(a) Absorption spectrum (b) Emission spectrum

Fig. 8.1

• When a beam of electromagnetic radiation is allowed to fall on a

molecule in the ground state, the molecule absorbs photon of energy hv

and undergoes a transition from the lower energy level to the higher

energy level.


• The measurement of this decrease in the intensity of radiation is the basis

of absorption spectro scopy. The spectrum thus obtained is called the

absorption spectrum. (Fig 8.1. a)

• 2. Emission spectrum

• If the molecule comes down from the excited state to the ground state with

the emission of photons of energy hv, the spectrum obtained is called emission

spectrum, (Fig. 8.1.b).

• 8.4 PHOTOPHYSICAL LAW

• The absorption of light in the visible and near UV region by a solution is

governed by a photophysical law known as Beer-Lambert‟s law.

• 8.4.1 Lambert’s Law

•

• Lambert‟s law states that, “when a beam of monochromatic radiation is

passed through a homogeneous absorbing medium the rate of decrease of

intensity of the radiation „dI‟ with thickness of absorbing medium „dx‟ is

Proportional to the intensity of the incident radiation „I‟.


8.4.2 Beer’s law (or) Beer-Lambert’s Law

Beer extended the above equation (2) to solutions of compound in transparent

solvent.

According to this law, “when a beam of monochromatic radiation is passed

through a solution of an absorbing substance, the rate of decrease of intensity of

radiation „dI‟ with thickness of the absorbing solution „dx‟ is proportional to the intensity

of incident radiation „I‟ as well as the concentration of the solution „C‟.”


The equation (4) is called Beer-Lambert‟s law. Thus, the absorbance (A) is directly

proportional to molar concentration (C) and thickness (or) path length (x).

8.4.3 Application of Beer-Lambert’s law

Determination of unknown concentration

First absorbance „As‟ of a standard solution of known concentration „Cs‟ is

measured, then according to Beer-Lambert‟s law



8.4.4 Limitations of Beer-Lambert’s law

1. Beer-Lambert‟s law is not obeyed if the radiation used is not monochromatic.

2. It is applicable only for dilute solutions.

3. The temperature of the system should not be allowed to vary to a large

extent.

4. It is not applied to suspensions.

5. Deviation may occur, if the solution contains impurities.

6. Deviation also occurs if the solution undergoes polymerization (or)

dissociation.

8.7 VISIBLE AND ULTRAVIOLET

(UV) SPECTROSCOPY

8.7.1 Principle


Visible and Ultraviolet (UV) spectra arises from the transition of valency electrons

within a molecule or ion from a lower electronic energy level (ground state E0) to higher

electronic energy level (excited state E1).

This transition occurs due to the absorption of UV (wavelength 100-400 nm) or

visible (wave length 400-750 nm) region of the electronic spectrum by a molecule (or)

ion.

The actual amount of energy required depends on the difference in energy

between the ground state and the excited state of the electrons.

E1 − Eo = hν.

8.7.2 Types of electrons involved in organic

molecule

The energy absorbed by a organic molecule involves transition of valency

electrons. The following three types of electrons are involved in the transition.


8.7.3 Energy level diagram (or)

Electronic transitions

Energy absorbed in the visible and UV region by a molecule causes transitions of

valence electrons in the molecule. These transition are

σ → σ∗, n → σ∗, n → π∗ & π → π∗

The energy level diagram for a molecule is shown in the fig 8.8. The energy

values for different transitions are in the following order.


8.7.4 Types of electronic transitions involved

in organic molecules

1. n → π∗ transitions

n → π∗ transitions are shown by unsaturated molecules containing hetero atoms

like N, O & S. It occurs due to the transition of non-bonding lone pair of electrons to the

antibonding orbitals. This transition shows a weak band, and occurs in longer

wavelength with low intensity.







8.7.6 INSTRUMENTATION


I. Components

The various components of a visible UV spectrometer are as follows.

1. Radiations source

In visible – UV spectrometers, the most commonly used radiation sources are

hydrogen (or) deuterium lamps.

2. Monochromators

The monochromator is used to disperse the radiation according to the

wavelength. The essential elements of a monochromator are an entrance slit, a

dispersing element and an exit slit. The dispersing element may be a prism or grating

(or) a filter.

3. Cells (sample cell and reference cell)

The cells, containing samples or reference for analysis, should fulfil the

following conditions.

(i) They must be uniform in construction.

(ii) The material of construction should be inert to

solvents.

(iii) They must transmit the light of the wavelength

used.

4. Detectors

There are three common types of detectors used in visible UV

spectrophotometers. They are Barrier layer cell, Photomultiplier tube, Photocell.

The detector converts the radiation, falling on which, into current. The

current is directly proportional to the concentration of the solution.

5. Recording system

The signal from the detector is finally received by the recording system. The

recording is done by recorder pen.

II Working of visible and UV spectrophotometer


The radiation from the source is allowed to pass through the monochromator

unit.

The monochromator allows a narrow range of wavelength to pass through an exit

slit.

The beam of radiation coming out of the monochromator is split into two equal

beams.

One-half of the beam (the sample beam) is directed to pass through a

transparent cell containing a solution of the compound to be analysed.

The another half (the reference beam) is directed to pass through an identical

cell that contains only the solvent. The instrument is designed in such a way that it can

compare the inten sities of the two beams.

If the compound absorbs light at a particular wavelength, then intensity of the

sample beam (I) will be less than that of the reference beam (Io).The instru ment gives

output graph, which is a plot of wave length Vs absorbance of the light. This graph is

known as an absorption spectrum.

8.7.7 Applications

1. Predicting relationship between different groups

UV spectroscopy is not useful in the detection of individual functional groups, but

it is used in predicting the relationship between different groups.

i.e.,

(i) between two or more C–C multiple bonds


(= (or) ≡ bonds).

(ii) between C – C & C – O double bonds.

(iii) between C – C double bonds and aromatic

benzene ring.

Thus the structure of several vitamins and steric

hindrance of the molecule can be determined using UV spectroscopy.

2. Qualitative analysis

UV absorption spectroscopy is used for charac terizing and identification of

aromatic compounds and conjugated olefins by comparing the UV absor ption spectrum

of the sample with the same of known compounds available in reference books.

3. Detection of impurities

UV absorption spectroscopy is the best method for detecting impurities in organic

compounds, because

(i) The bands due to impurities are very intense.

(ii) Saturated compounds have little absorption band

and unsaturated compounds have strong absor ption band.

4. Quantitative analysis

Determination of substances: UV absorption

spectroscopy is used for the quantitative determination of compounds, which absorbs

UV light. This deter mination is based on Beer‟s law


Then absorbance of unknown solution is measured. From the graph the

concentration of unknown substance is found out.

5. Determination of molecular weight

Molecular weight of a compound can be determined if it can be converted into a

suitable derivative, which gives an absorption band.



9. Determination of calcium in blood serum

Calcium in the blood can be determined by converting the „Ca‟ present in 1 ml of

the serum as its oxalate and redissolving it in H2SO4 and treating it with dilute ceric

sulphate solution. The absorption of the solution is measured at 315 nm. Thus the

amount of „Ca‟ in the blood serum can be calculated.

8.7.8 Problems based on visible–UV

spectroscopy









8.8.1 Principle

IR spectra is produced by the absorption of energy by a molecule in the infrared

region and the transitions occur between vibrational levels. So, IR spectroscopy is also

known as vibrational spectro scopy.

Range of Infrared Radiation

The range in the electromagnetic spectrum extending from 12500 to 50 cm− 1

(0.8 to 200 μ) is commonly referred to as the infrared. This region is further divided into

three sub regions.


8.8.2 Molecular Vibrations and Origin of IR

Spectrum

Since atoms in a molecule are continuously vibrating, molecules are also

vibrating. There are two kinds of fundamental vibrations in the mole cule.

1. Stretching vibrations: During stretching the distance between two

atoms decreases or increases, but bond angle remains unaltered.

2. Bending (or) deformation vibrations: During bending bond angle

increases and decreases but bond distance remains unaltered.

Vibrational changes depend on the masses of the atoms and their spatial

arrangement in the molecule.

When IR light of the same frequency is incident on the molecule, energy is

absorbed resulting in increase of amplitude of vibration.

When the molecule returns from the excited state to the original ground state, the

absorbed energy is released as heat.

Thus every compound shows characteristic absorption bands in the IR region of

the spectrum.


Different functional groups produce easily recognisable band at definite positions

in the IR spectral range (12500 to 50 cm− 1).

Finger Print Region

The vibrational spectral (IR spectra) region at

1400 − 700 cm− 1 gives very rich and intense absorption bands. This region is termed

as fingerprint region. The region 4000 − 1430 cm− 1 is known as Group frequency

region.

Uses of fingerprint region

1. IR spectra are often characterized as molecular finger prints, which detect

the presence of functional groups.

2. Fingerprint region is also used to identify and

characterize the molecule just as a fingerprint can be used to identify a person.

8.8.3 Types of stretching and bending vibrations

The number of fundamental (or) normal vibrational modes of a molecule can be

calculated as follows.

1. For Non-linear molecule

A non-linear molecule containing „n‟ atoms has

(3n − 6) fundamental vibrational modes.


Illustrations


vibration) in the IR region and the IR spectrum of water exhibits 3 absorption bands at

1596, 3652 and 3756 cm− 1 corresponding to the bending, symmetric stretching and

the asymmetric stretching vibrations respectively.

Thus, for a vibration to be active in IR, the

dipolemoment of the molecule must change.

2. Carbondioxide


Carbondioxide is a linear triatomic molecule, and has 3n − 5 (3 × 3 − 5) = 4

fundamental vibrational modes. These modes and corresponding frequencies are

shown in Fig. 8.12.

Of the four normal modes of vibration of CO2, only the asymmetric stretching and

bending vibrations ie., (ii), (iii) and (iv) involve change in dipolemoment (all are IR -

active).

(Note: + and − signs indicate the motion of the


corresponding atom, above and below the plane of the paper respectively).

Thus, though there are three active vibrations, two of them ((iii) and (iv)) have the

same frequency, so the IR spectrum of CO2 exhibits only two bands ie., one at 666 cm−

1 and another at 2350 cm− 1.


When they are heated electrically at 1200 to 2000°C, they glow and produce IR

radiation.

2. Monochromator

It allows the light of the required wave length to pass through, but absorbs the

light of other wave length.

3. Sample Cell

The cell, holding the test sample, must be transparent to IR radiation.

4. Detector

IR detectors generally convert thermal radiant energy into electrical energy.

There are so many detectors, of which the followings are important.


(a) Photoconductivity cell.

(b) Thermocouple.

(c) Pyroelectric detectors.

5. Recorder

The recorder records the signal coming out from the detector.

II. Working of IR Spectrophotometer

The radiation emitted by the source is split into two identical beams having equal

intensity. One of the beams passes through the sample and the other through the

reference sample.

When the sample cell contains the sample, the half-beam travelling through it

becomes less intense. When the two half beams (one coming from the reference and

the other from the sample) recombine, they produce an oscillating signal, which is

measured by the detector. The signal from the detector is passed to the recording unit

and recorded.

8.8.5 Applications of IR spectroscopy

1. Identity of the compound can be established

The IR spectrum of the compound is compared with that of known

compounds. From the resemblance of the two spectra, the nature of the

compound can be established. This is because a particular group of atoms

gives a characteristic

absorption band in the IR spectrum.



IR spectra of pure compound, presence of impurity can be detected.

4. Study of progress of a chemical reaction

The progress of a chemical reaction can be easily

followed by examining the IR spectrum of test solution at different time intervals.

Example

(i) Progress of oxidation of secondary alcohol to ketone is studied by getting IR

spectra of test solution at different time intervals.

The secondary alcohol absorbs at 2.8 μ (~ 3570 cm− 1) due to O − H stretching.

As the reaction proceeds this band slowly disappears and a new band near 5.8 μ (~

1725 cm− 1), due to C = O stretching appears.

(ii) Similarly, the progress of any chromatographic

separations can be readily monitored by examining the IR spectra of the

selected fractions.

5. Determination of shape (or) Symmetry of a molecule


Whether the molecule is linear (or) non-linear (bend molecule) can be found out

by IR spectra.

Example

IR spectra of NO2 gives three peaks at 750, 1323, and 1616 cm− 1.

According to the following calculations,

(i) For non-linear molecule = (3n − 6) = 3 peaks.

(ii) For linear molecule = (3n − 5) = 4 peaks.

Since the spectra shows only 3 peaks, it is confirmed that NO2 molecule is a non

linear (bend) molecule.

6. To study tautomerism

Tautomeric equilibria can be studied with the help of IR spectroscopy.


(b) Determination of molecular weight

Molecular weight, of a compound can be determined by measuring end group

concentrations, using IR spectroscopy.

(c) Crystallinity

The physical structure like crystallinity can be studied through changes in IR

spectra.














8.5 COLORIMETRY

Colorimetry is concerned with the visible region

(400-750 nm) of the spectrum. The instrument, used for measuring absorption of radiant

energy in the visible region from the substances is called colorimeter.

8.5.1 Principle

This method is convenient for the coloured substances or coloured solutions. The

intensity of colour can be easily measured by using a photo electric colorimeter, from

which the concentration of coloured solution can be obtained by using

Beer-Lambert‟s law.

If the substance is colourless, then a suitable complexing agent is added to the

solution so that a coloured complex is obtained, which can absorb the light.

Example


For the estimation of cuprous ions, complexing agent, ammonium hydroxide, is added

to get blue coloured solution.

8.5.2 Instrumentation

1. components.

All the colorimeters have the following components.

1. Radiation sources

The wavelength range of visible light lies between 400-750 nm. In this region, a

tungsten-filament lamp is most widely used.

2. Filter (or) monochromator

It is a instrument, which allows the light of the required wavelength to pass

through, but absorbs the light of other wavelengths.

3. Slits

(a) Entrance slit: It provides a narrow source of the light.

(b) Exit slit: It selects a narrow band of dispersed

spectrum for observation by the detector.

4. Cell

The cell, holding the test sample (usually a solution), should be transparent. For

visible region the cell is made of colour-corrected fused glass.

5. Detector

It is used for measuring the radiant energy transmitted through the sample.

Photosensitive devices are used to detect radiations. These detectors produce current,

which is directly proportional to the intensity of the incident radiation.

6. Meter

It is used to measure directly the fraction of light absorbed.

II. Working of Colorimeter

In a colorimeter, a narrow beam of light is passed from radiation source through

the test solution (cell) towards a sensitive detector (photocell).


Usually colorimeter is provided with the arrangement of filter and slits, which

select the light of required wave length.

The detector (photocell) generates the current, which is proportional to

the amount of light transmitted by the solution.

The amount of light transmitted depends on the depth of colour of

the test solution. Thus, the current from the

The transmitted light is now a days allowed to send through a meter, which is

calibrated to show not the fraction of light transmitted but the fraction of light absorbed.

The light absorbed is proportional to the concentration of the test

solution.

8.5.3 Estimation of iron by colorimetry

Principle


Reagents required

(a) Standard iron solution: 0.865 gms of FAS is dissolved in distilled water 5-10

ml of con. HCl is added and the solution is diluted to 1 litre. 1 ml of this solution

contains 0.1 mg of Fe.

(b) Potassium thiocyanate solution: 20 gms of KCNS is dissolved in 100 ml of

water.

(c) 1:1 con HCl: 50 ml of con. HCl is added to 50 ml of distilled water.

Procedure

A series of standard solution of Fe3+ (ferric ammonium sulphate) are prepared

by adding KCNS with small amount of 1:1 con. HCl.

Then the colorimeter is set at zero absor bance using a blank solution, with a

proper filter. Now absorbance of each standard solution is then measured using the

same filter.

A graph is plotted between absorbance vs concentration. This plot is called

calibration curve and will be the straight line passing through origin. This is according to

Beer-Lambert‟s law


Similarly, the absorbance of the test solution (unknown Fe3+ iron solution) is

measured using the same colorimeter. From the calibration curve, the concentration of

the unknown ferric iron solution can be evaluated.

8.5.4 Applications of colorimetry

1. Molar compositions of complexes can be determined.

2. The instability constants of metal complexes are also determined.

3. Dissociation constants (Pk) of an indicator can be determined.

4. Structure of inorganic compounds, complexes (cis & trans isomer) can be

determined.

5. Molecular weight of a compound can also be determined using colorimetric

measurements.


8.6 FLAME PHOTOMETRY OR FLAME EMISSION

SPECTROSCOPY

Flame photometry is a method in which, the intensity of the emitted light is

measured, when a atomised metal is introduced into a flame. The wavelength of the

colour tells us what the element is, and the intensity of the colour tells us How much

of the element is present.

8.6.1 Theory (or) Principle

When a metallic salt solution is introduced into a flame, the following processes

will occur.

(i) The solvent is evaporated leaving behind the solid salt particle.

(ii) The salt is vapourised into the gaseous state and dissociated into atoms.

(iii) Some of the atoms from the ground state are

excited to higher energy state by absorbing thermal energy from the flame.

The excited atoms, which are unstable, quickly emit photons of different wave

lengths and return to the lower energy state. Then the emitted radiation is passed

through the filter, which permits the characteristic wavelength of the metal under

examination. It is then passed into the detector, and

finally into the recorder.


The various components of the flame photometer are described as follows.

1. Burner

The flame must possess the following characteristics.

(i) It should evaporate the solvent from the sample solution.

(ii) It should decompose the solid into atoms.

(iii) It should excite the atoms and cause them to emit radiant energy.

2. Mirror

The radiation from the flame is emitted in all directions in space. In order to

increase the amount of radiation reaching the detector, a convex mirror is used which is

set behind the burner.

3. Slits


Entrance slits: It is kept between the flame and

monochromator. It permits only the radiation coming from the flame and mirror.

Exit slit: It is kept between the monochromator and

detector. It prevents the entry of interfering lines.

4. Monochromator (or) Prism (or)

Grating (or) Filter

It allows the light of the required wave length to pass through, but absorbs the

light of other wavelengths.

5. Detector

The radiation coming out from the filter is allowed to fall on the detector, which

measures the intensity of the radiation falling on it. Photo multiplier (or) photocell is used

as detector, which converts the radiation into an electrical current.


6. Amplifier & Recorder

The current coming out from the detector is weak, so it is amplified and recorded.

II. Working of Flame photometer

Air, at a given pressure, is passed into an atomiser. The suction so-produced

draws some solution of the sample into the atomiser.

Air + sample solution is then mixed with fuel gas in the mixing chamber. The Air

+ sample solution + fuel gas mixture is then burnt in the burner.

The radiation, emitted by burner flame, is passed successively through the lens,

filter, detector, amplifier and finally into a recorder.


The above experiment is first carried out using a series of standard solution, and

the reading for each solution is noted. Now the graph, called calibration curve, is drawn

between concentration vs intensity of emitted light (or) photometric reading.

Now the test solution (unknown) is taken and similar experiment is carried out.

From the graph the concentration of the unknown sample can be determined.

8.6.3 Applications of flame photometry

1. Estimation of sodium by flame photometry

The instrument is switched on. Air supply and gas supply are regulated. First

distilled water is sent and ignition is started.

After the instrument is warmed up for 10 min, the instrument is adjusted for zero

reading in the display.

Since sodium produces a characteristic yellow emission at 589 nm, the

instrument is set at λ = 589 nm and the readings are noted.


A series of standard NaCl solution (1, 2, 3, 4, 5 ... 10 ppm) is prepared and is

sent one by one and the readings (intensity of emitted light) are noted. The calibration

graph is drawn between the concentration Vs intensity of the emitted light. A straight

line is obtained.

Now the unknown sodium solution is sent and the reading (intensity of the

emitted light) is noted.

Then the concentration of sodium in the water sample is determined from the

calibration curve.

2. Qualitative Analysis

(a) The elements of group I & II. (K, Na, Li, Ca, Mg, etc) can be detected visually

from the colour of the flame.


(b) Non-radiating elements such as carbon, hydrogen and halides cannot be

detected using this method.

3. Quantitative analysis

(a) The amount of the elements in group I & II

(alkali & alkaline-earth metals) can be determined from the sample.

(b) Certain transition elements, such as

Cu, Fe & Mn can also be determined using flame photometry.

4. Other applications

1. The measurement of these elements is very useful in medicine,

agriculture and plant science.

2. Flame photometry is extensively used in the

analysis of biological fluids and tissues.

3. In soil analysis the elements like, Na, K, Al, Ca, Fe, etc., are determined.


4. Industrial and natural waters, petroleum products, cement, glass, and

metallurgical products can also be analysed by this method.

8.6.4 Interference of flame photometry

1. Spectral interference: It arises when two

elements exhibit different spectra but both emit

at particular wavelength.

2. Ionic interference: It arises due to the ionisation

of some metal atoms at high-temperature.

Ionisation decreases the atomic emission.

3. Cation-anion interference: It is due to presence

of certain anions like oxalate, phosphate, sulphate, which form stable

compounds with cations.

4. Cation-cation interference: It decreases the

signal intensity.

8.6.5 Limitations of flame photometry

1. It cannot be used for the determination of all metal atoms and inert gases.

2. Only liquid samples must be used.

3. It does not provide information about the molecular form of the metal present in

the original sample.

Atomic absorption spectroscopy is based on the

atomization of the sample followed by absorption of

characteristic radiation by the ground state gaseous atoms.

When the light of the required wavelength is allowed to pass through a flame,

having atoms of the metallic species, part of that light will be absorbed and the

absorption will be proportional to the concen tration of the atom in the flame.


Thus, in atomic absorption spectroscopy, the amount of light absorbed is

determined.

8.9.2 Instrumentation

I. Various components

1. Radiation source

The radiation source should emit, stable, intense,

characteristic radiation of the element to be determined. The hollow

cathode lamp, which consists of a glass tube containing noble gases like a

argon (anode) and hollow cathode, made of the analyte metal, is generally used.

2. Chopper

A rotating wheel is interposed between the hollow cathode lamp and the flame. It

breaks the steady light, from the lamp, into an pulsating light (because the recorder will

record only the pulsating (alternating) current).

3. Burner (or) Flame

The flame is used for converting the liquid sample into the gaseous state. It

converts the molecule into atomic vapour. Two types of burners are used

1. Total consumption burner.

2. Premixed burner.

4. Nebulisation of the liquid sample

Before the liquid sample enters the burner, it is first of all converted into small

droplets. This method of formation of small droplets from the liquid sample is called

nebulisation.

5. Monochromators

The monochromators select a given absorbing line from the spectral lines

emitted from the hollow cathode. The most common monochromators are

(i) Prisms.

(ii) Gratings.

6. Detectors


The photomultiplier tube is a most suitable detector. When the photon strikes the

photomultiplier tube, an electric current (emf) is produced.

7. Amplifier

The electric current, from the photomultiplier detector, is fed into the amplifier,

which amplifies the electric current many times.

8. Read-Out Device (or) Recorder

The signal coming out from the amplifier is recorded using chart recorder (or)

digital read-out devices.

The characteristic radiation, obtained from the hollow cathode lamp, is passed

through a flame into which the sample is aspirated. The metallic compounds are

decomposed into atoms of the element to be measured.

The atoms absorb a fraction of radiation in the flame. The unabsorbed radiation

from the flame is allowed to pass through a mono chromator.

From the monochromator the unabsorbed radiation is led into the detector. From

the detector, the output is amplified and measured on a recorder.


CY2161 - Engineering Chemistry – II

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