Stoichiometry: Chemical Calculations - University of … · · 2006-02-153 Stoichiometry: Chemical Calculations Chemistry 120 The molecular mass of a compound is the mass ... Chemical

1

Chemistry 120Stoichiometry: Chemical Calculations

Chemistry is concerned with the properties and the

interchange of matter by reaction i.e. structure and

change.

In order to do this, we need to be able to talk about

numbers of atoms.

The key concept is the mole and the relationship

between the mole and the mass of the atom.


Each element has a distinct atomic mass – based on

the natural abundances of the various isotopes

present.

Atoms combine to form molecules in fixed

proportions which are usually small integers for

simple molecular or ionic compounds


The molecular mass of a compound is the mass of

each atom multiplied by the number of times it

appears in the formula unit

2





CH4

SF6

NaCl

Na2S2O3





CH4 12.0115 + 4 x 1.0079

SF6

NaCl

Na2S2O3





CH4 12.0115 + 4 x 1.0079

SF6 32.066 + 6 x 18.9984

NaCl

Na2S2O3

3





CH4 12.0115 + 4 x 1.0079

SF6 32.066 + 6 x 18.9984

NaCl 22.9898 + 35.453

Na2S2O3 2 x 22.9898 + 2 x 32.066 + 3 x 15.9994





CH4 12.0115 + 4 x 1.0079

SF6 32.066 + 6 x 18.9984

NaCl 22.9898 + 35.453

Na2S2O3 2 x 22.9898 + 2 x 32.066 + 3 x 15.9994





CH4 12.0115 + 4 x 1.0079

SF6 32.066 + 6 x 18.9984

NaCl 22.9898 + 35.453

Na2S2O3 2 x 22.9898 + 2 x 32.066 + 3 x 15.9994

4


The mole and atomic mass

The mole is defined as

the number of elementary entities as are present in 12.00 g of 12C.

Numerically, this is equal to Avogadro’s Number

6.022 x 1023

Therefore, in 12.00 g of 12C there are 6.022 x 1023 ‘elementary entities’, in this case atoms.


The mole and atomic mass

Atomic masses, in atomic units, u, are defined relative to 12C.

Therefore,

The formula mass of an element or compound contains 1 mole, 6.022 x 1023, of particles


Examples

How many particles are there in 5 g of Na?

5


Examples


The particles are atoms – how many atomsare there in 5 g of Na?


Examples


The particles are atoms – how many atomsare there in 5 g of Na?Atomic mass of Na = 22.9898 u


Examples


The particles are atoms – how many atomsare there in 5 g of NaAtomic mass of Na = 22.9898 uAs

1 u = 1/12 x mass (12C) And

1 mole = 6.022 x 1023 particles = number of particles in 12 g 12C

6


Examples


The particles are atoms – how many atomsare there in 5 g of NaAtomic mass of Na = 22.9898 uMass of 1 mole of Na = 22.9898 g


Examples

How many particles are there in 5.0000 g of Na?

22.9898 g Na = 1 mole Na

Then 1 g Na = 1 mol Na22.9898

5 x 1 g Na = 5 x 1 mol Na22.9898

5 g Na = 0.2175 mol Na5 g Na = 0.2175 x (6.022 x 1023) particles Na


Examples

How many particles are there in 5.0000 g of Na?

1.310 x 1023 atoms

7


Examples

What is the mass of 0.23 mol of butane?


Examples


Molecular formula of butane: C4H10


Examples



Atomic mass of C = 12.011g

Atomic mass of H = 1.0079g

8


Examples





Molecular mass of C4H10 = (4x12.011)+(10x1.0079)u

= 58.123 u


Examples





Molecular mass of C4H10 = (4x12.011)+(10x1.0079)u

= 58.123 u

Relative Molecular Mass of Butane = 58.123 g


Examples



1 mole of butane = 58.123 g

0.23 x 1 mole of butane = 0.23 x 58.123 g

9


Examples



1 mole of butane = 58.123 g

0.23 x 1 mole of butane = 0.23 x 58.123 g

0.23 mole of butane = 13.368 g


Chemical Equations

These are formulæ which show the chemical change taking place in a reaction.

Sr(s) + Cl2(g) SrCl2(s)


Chemical Equations


Physical state


10


Chemical Equations


Physical state


Reactants Product


Chemical Equations

As matter cannot be created or destroyed in a chemical reaction, the total number of atoms on one side must be equal to the total number of atoms on the other.


Chemical Equations

Example

Cyclohexane burns in oxygen to give carbon dioxide and water

11


Chemical Equations

Example


Reactants: Cyclohexane, C6H12

Oxygen, O2


Chemical Equations

Example


Reactants: Cyclohexane, C6H12

Oxygen, O2

Products: Carbon Dioxide, CO2

Water, H2O


Chemical Equations

Example

Initially, we can write the reaction as

C6H12 + O2 CO2 + H2O

12


Chemical Equations

Example


C6H12 + O2 CO2 + H2O

This is NOT a correct equation – there are unequal numbers of atoms on both sides


Chemical Equations

Example


C6H12 + O2 CO2 + H2O

This is NOT a correct equation – there are unequal numbers of atoms on both sides

Reactants: 6 C, 12 H, 2 O

Products: 1 C, 2 H, 3 O


Balancing the equation

C6H12 + O2 CO2 + H2O

13

Chemistry 120Stoichiometry: Chemical CalculationsBalancing the equation

6 C, 12 H, 2 O 1 C, 2 H, 3 O6 C on LHS means there must be 6 C on the RHS

C6H12 + O2 CO2 + H2O

C6H12 + O2 6CO2 + H2O6 C, 12 H, 2 O 6 C, 2 H, 13 O

13 O on RHS means there must be 13 O on LHSC6H12 + 13/2 O2 6CO2 + H2O

6 C, 12 H, 13 O 6 C, 2 H, 13 O

Chemistry 120Stoichiometry: Chemical CalculationsBalancing the equation

C6H12 + 13/2 O2 6CO2 + H2O

6 C, 12 H, 13 O 6 C, 2 H, 13 O

12 H on RHS means there must be 12 H on LHS

C6H12 + 13/2 O2 6CO2 + 6H2O6 C, 12 H, 13 O 6 C, 12 H, 18 O

18 O on RHS means there must be 18 H on LHSC6H12 +9O2 6CO2 + 6H2O

6 C, 12 H, 18 O 6 C, 12 H, 18 O

Chemistry 120Stoichiometry: Chemical CalculationsThe final balanced equation is

and the coefficients are known as the

stoichiometric coefficients.

These coefficients give the molar ratios for reactants and products

This is a stoichiometric reaction – one where exactly the correct number of atoms is present in the reaction

C6H12 +9O2 6CO2 + 6H2O

14


If cyclohexane were burnt in an excess of oxygen,

the quantity of oxygen used would be the same

although O2 would be left over.

Chemistry 120Solutions and concentration

If cyclohexane were burnt in an excess of oxygen,

the quantity of oxygen used would be the same

although O2 would be left over.

Chemistry 120Solutions

A solution is a homogenous mixture which is composed of two or more components

the solvent

- the majority component

and

one or more solutes

- the minority components

15


Most common solutions are liquids where a solid, liquid or gas (the solute) is dissolved in the liquid solvent.

Some are solids where both the solvent and the solute are solids. Brass is an example







Cu

ZnHere copper is the solvent, zinc the solute.

NaCl(s) melts

16


Gas-Solid solution: Hydrogen in palladium

Steel


Common laboratory solvents are usually organic liquids such as acetone, hexane, benzene or ether or water.

Solutions in water are termed aqueous solutions and species are written as E(aq).

Water is the most important solvent. The oceans cover ~ ¾ of the surface of the planet and every cell is mainly composed of water.


Aqueous Solutions

Water is one of the best solvents as it can dissolve many molecular and ionic substances.

The properties of solutions which contain molecular and ionic solutes are very different and give insight into the nature of these substances and solutions.

17


Ionic Solutions

An ionic substance, such as NaClO4, contain ions –in this case Na+ and ClO4

-.

The solid is held together through electrostaticforces between the ions.

In water, the solid dissolves and the particles move away from each other and diffuse through the solvent. This process is termed

Ionic Dissociation


Ionic Solutions

In an ionic solution, there are therefore charged particles – the ions – and as the compound is electrically neutral, then the solution is neutral.

When a voltage is applied to the solution, the ions can move and a current flows through the solution.

The ions are called charge carriers and whenever electricity is conducted, charge carriers are present.


Molecular Solutions

A molecular solution does not conduct electricity as there are no charge carriers present.

The bonding in a molecule is covalent and involves the sharing of atoms and there is no charge separation.

18


Electrolytes

A solute that, when dissolved, produces a solution that conducts is termed an electrolyte, which may be strong or weak.

A strong electrolyte is one which is fully dissociated in solution into ions

A weak electrolyte is one which is only partially dissociated.


Moles and solutions

When a substance is dissolved in a solvent, we relate the quantity of material dissolved to the volume of the solution through the concentration of the solution.

The concentration is simply the number of moles of the material per unit volume:

C = nV

n = number of moles; V = volume of solvent


Moles and solutions

The units of concentration are:

C = n = molesV L3

and we define a molar solution as one which has 1 mole per liter.Alternatively,

Concentration = Molarity = number of molesvolume of solution

19


Example

4 g of Na2SO4(s) is dissolved in 500 ml of water. What is the concentration of the solution?


Example


Formula mass of Na2SO4(s):

Molar Atomic Mass of Na: 22.9898 gmol-1

Molar Atomic Mass of S: 32.064 gmol-1

Molar Atomic Mass of O: 15.9994 gmol-1


Example


Formula mass of Na2SO4(s):

(2 x 22.9898)+ 32.064+(4x15.9994)=142.041gmol-1

1 mole of Na2SO4(s) = 142.041g

1/142.041 mole of Na2SO4(s) = 1 g

20


Example


1/142.041 mole of Na2SO4(s) = 1 g

Therefore 4 g of Na2SO4(s) = 4/142.041 mole

= 2.82 x 10-2 mole


Example


2.82 x 10-2 mole is therefore dissolved in 500 ml of water;

So in 1 L, there are 2 x 2.82 x 10-2 mole

Molarity of solution = 5.64 x 10-2 molL-1


Example

The equation for the dissolution of Na2SO4(s) is

So if we have 5.64 x 10-2 molL-1 Na2SO4(s), we must

have 1.13 x 10-1 moles Na+(aq)

and 5.64 x 10-2 mol SO42-

(aq) as there are 2 Na cations for every sulfate ion

Na2SO4(s)H2O

2Na+(aq) + SO4

2-(aq)

21


If we change the volume of the solution then we change the concentration:

If the Na2SO4 solution is diluted with 500ml of water, the concentration or molarity would be halved:

2.82 x 10-2 mole is therefore dissolved in 1000 ml of water

Molarity = 2.82 x 10-2 molL-1


Dissolution on an atomic level.

Solids are held together by very strong forces.

NaCl(s) melts at 801oC and

boils at 1465 oC but it

dissolves in water at room

temperature.



When we dissolve NaCl(s) in water we break the bonds between ions but make bonds between the ions and the water

22



When we dissolve NaCl(s) in water we break the bonds between ions but make bonds between the ions and the water

The ions are hydrated or solvated in solution and these bonds between solvent and solute make the dissolution energetically possible

If something does not dissolve then the energetics are wrong for it do do so.


Solubility rules

All ammonium and Group I salts are soluble.

All Halides are soluble except those of silver, leadand mercury (I)

All Sulfates are soluble except those of barium and lead.

All nitrates are soluble.

Everything else is insoluble

Chemistry 120The Exam

23


• Solutions are homogenous mixtures in which the

majority component is the solvent

and the

minority component is the solute

• Solutions are normally liquid but solutions of gases in solids and solids in solids are known.

• Ionic compounds dissolve in water to give conducting solutions – they are electrolytes


• Electrolytes are either strong or weakdepending on the degree of dissociation in solution

• Molecular solutions do not conduct as molecules do not dissociate in solution

• The concentration or molarity of a solution is defined by

C = n = molesV L3

and the units are molL-1 or moldm-3


• When ionic substances dissolve,

bonds between particles in the solid break

and

bonds between the solvent and the ions are made

• There are general rules for the solubilities of ionic compounds

24

Chemistry 120Reactions in Solution

Reactions in solution include

• Acid – base reactions

• Precipitation reactions

• Oxidation- reduction reactions

Chemistry 120Reactions in Solution

Reactions and equilibria

Reactions are often written as proceeding in one

direction only – with an arrow to show the direction

of the chemical change, reactants to products.

Not all reactions behave in this manner and not all

reactions proceed to completion.

Even those that do are dynamic.

Chemistry 120

I-

I-

I-

I-

I-

I-

I-I-

I-

I-

I-

I-I-I-

I-

I- I-

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+ Na+

Reactions in Solution: Acid - Base

NaI*(s)

NaI(aq)

A saturated solution of NaI is placed in contact

with Na131I(s), which is radioactive.

25

Chemistry 120

I-

I-

I-

I-

I-

I-

I-I-

I-

I-

I-

I-I-I-

I-

I- I-

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+ Na+


NaI*(s)

NaI(aq)

A saturated solution of NaI is placed in contact

with Na131I(s), which is radioactive.

After time, the activity

in the solution is

measured and ..........

Chemistry 120

I-

I-

I-

I-

I-

I-

I-I-

I-

I-

I-

I-I-I-

I-

I- I-

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+ Na+

I-

I-

I-

I-

I-

I-

I-I-

I-

I-

I-

I-I-I-

I-

I- I-

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+ Na+

Reactions in Solution: Acid - BaseRadioactivity is found in the solution, even though

the concentration of I-(aq) has not changed.

Chemistry 120Reactions in Solution: Acid - Base

The equilibrium here is composed of two reactions:

So we write

Na131I(s)H2O

Na+(aq) + 131I-

(aq)

H2ONa+

(aq) + I-(aq) NaI(s)

H2ONa+

(aq) + I-(aq)NaI(s)

26


Such reactions are termed equilibria and all chemical reactions are equilibria.

The symbol for an equilibrium is a double-headed arrow

Forward reaction




+

Forward reaction




+

Forward reaction

Reverse reaction

27

Chemistry 120

+ =

Forward reaction

Reverse reaction





Equilibria are important in the chemistry of acids and bases

Strong acids and bases are completely ionized

But.....

Weak acids and bases are not.


The Arrhenius definition of acid and bases are:

an acid is a compound which dissolves in water or reacts with water to give hydronium ions, H3O+

(aq)

a base is a compound which dissolves in water or reacts with water to give hydroxide ions, OH-

(aq)

Svante Arrhenius (1859 – 1927)

28


A strong acid is a compound which dissolves and dissociates completely in water or reacts with water to give hydronium ions, H3O+

(aq)

- the double arrow implies that the reaction can go both ways – it is an equilibrium.As a strong acid, the reaction is completely on the RHS:

HCl(g)H2O

H3O+(aq) + Cl-(aq)

HCl(g)H2O

H3O+(aq) + Cl-(aq)

Chemistry 120Reactions in Solution: Acid - BaseA strong base is a compound which dissolves and dissociates completely in water or reacts with water to give hydroxide ions, OH-

(aq)

Again, we could write this reaction as an equilibrium with a double headed arrow, but the base is strong and the reaction is completely over to the right hand side.

NaOH(s)H2O

Na+(aq) + OH-

(aq)

Chemistry 120Reactions in Solution: Acid - BaseIn a reaction such as

we write the reaction as going from LHS to RHS.

Chemical reactions run both ways, so in this reaction, there are two reactions present:

Ionization

Recombination

H2OMeCO2H H3O+

(aq) + MeCO2-(aq)

H2OMeCO2H H3O+

(aq) + MeCO2-(aq)

H2OMeCO2HH3O+

(aq) + MeCO2-(aq)

29

Chemistry 120Reactions in Solution: Acid - BaseWe write the reaction for acetic acid, MeCO2H, as an equilibrium to include the ionization and recombination. Ionization

Recombination

As the amount of ionization and recombination are the same, the concentrations of the ions and the molecular form are constant

H2OMeCO2H H3O+

(aq) + MeCO2-(aq)

H2OMeCO2HH3O+

(aq) + MeCO2-(aq)

H2OMeCO2H H3O+

(aq) + MeCO2-(aq)

Chemistry 120Reactions in Solution: Acid - BaseIn solution, weak acids establish an equilibrium between the un-ionized or molecular form and the ionized form:

un-ionizedmolecular form

ionized

H2OMeCO2H H3O+

(aq) + MeCO2-(aq)

Chemistry 120Reactions in Solution: Acid - BaseIn solution, strong acids are completely ionized and even though there is an equilibrium, it lies entirely on the RHS and recombination is negligible:

un-ionizedmolecular form

ionized

H2OHBr(g) H3O+

(aq) + Br-(aq)

30

Chemistry 120Reactions in Solution: Acid - BaseAcids with more than one ionizable hydrogen are termed

PolyproticThe common polyprotic acids are

H3PO4 Phosphoric acid

H2SO4 Sulfuric acid

Chemistry 120Reactions in Solution: Acid - BasePolyprotic acids can ionize more than once

H3PO4

Each proton is ionizable and the anions, dihydrogen phosphate (H2PO4

-(aq))

and hydrogen phosphate (HPO42-

(aq)) both act as acids, though H3PO4 is a weak acid.

H2SO4(aq) H3O+(aq)

+ HSO4-(aq)H2O

HSO4-(aq) H3O+

(aq) + PO4

2-(aq)H2O

HPO42-

(aq) H3O+(aq)

+ PO42-

(aq)H2O

Chemistry 120Reactions in Solution: Acid - BasePolyprotic acids can ionize more than once

H3PO4

H2SO4

H3PO4(aq) H3O+(aq)

+ H2PO4-(aq)H2O

H2PO4-(aq) H3O+

(aq) + HPO4

2-(aq)H2O

HPO42-

(aq) H3O+(aq)

+ PO42-

(aq)H2O

H2SO4(aq) H3O+(aq)

+ HSO4-(aq)H2O

HSO4-(aq) H3O+

(aq) + PO4

2-(aq)H2O

31

Chemistry 120Reactions in Solution: Acid - BaseIn contrast, H2SO4 is a strong acid and hydrogen sulfate (HSO4

-(aq)) is also a strong acid.

H2SO4(aq) H3O+(aq)

+ HSO4-(aq)H2O

HSO4-(aq) H3O+

(aq) + PO4

2-(aq)H2O

Chemistry 120Reactions in Solution: Acid - BaseStrong or weak?

All acids can be assumed to be weak except the following:

HCl(aq) hydrochloric acid

HBr(aq) hydrobromic acid

HI(aq) hydriodic acid

HClO4(aq) perchloric acid

HNO3(aq) nitric acid H2SO4(aq) sulfuric acid

Chemistry 120Reactions in Solution: Acid - BaseHydrogens attached to carbon are not ionizable in water

Acetic acid, MeCO2H (or CH3CO2H) has the structure H

H

H

O

O H

32


Only the hydrogen attached to oxygen is ionized in aqueous solution

The methyl hydrogens are NOT ionizable in aqueous solution.

H

H

H

O

O H H2O

H

H

H

O

O

OH H

H+


Strong bases are those which ionize in solution of react to generate hydroxide ion. The common strong bases are those which already contain the OH- ion in the solid.

Sr38

Rb37

5

Ba56

Cs55

6

Ca20

K19

4

Mg12

Na113

Li3

2

Strong bases are therefore the hydroxides of the group I and II metals


Weak bases are the majority and are usually amines and ammonia. These react with water and deprotonate it, forming hydroxide ion and an ammonium ion:

Trimethylamine Trimethylammonium

N

H3C CH3

CH3H2O

N

H3C CH3

CH3

H

+ OH-

33

Chemistry 120Reactions in Solution: Acid - BaseReactions in Solution: Acid - BaseNeutralization reactions and titrations Hydroxide and hydronium ions will react to form water.

From the stoichiometry of the balanced equation, the hydroxide and hydronium react in a 1:1 ratio.

We can therefore neutralize a known concentration of base or acid with the same quantity of acid or base. This is an example of a titration.

2H2O(l)H3O+(aq) + OH-

(aq)

Chemistry 120Reactions in Solution: Acid - BaseReactions in Solution: Acid - BaseNeutralization reactions and titrations We use an indicator to determine the acidity or basicity of a solution:

An indicator is a compound which changes color strongly at a certain level of acidity.

Chemistry 120Reactions in Solution: Acid - BaseReactions in Solution: Acid - BaseNeutralization reactions and titrations

We add acid or base – the titrant - to a solution of unknown concentration containing a few drops of the indicator solution.

When the solution is still acid, no color change occurs; when the indicator changes color, we know the equivalence point – the point where the acidity or basicity has been neutralized.

By knowing the concentration and the volume of the titrant, we can calculate the concentration of the of the unknown solution.

34

Chemistry 120Reactions in Solution: Acid - BaseReactions in Solution: Acid - BaseDecompostion in acidA solid base, such as Ca(OH)2(s), will dissolve with reaction in an acid. The anion, hydroxide, reacts with the acid to form the calcium salt of the acid:

Ca(OH)2(s) + H2SO4(aq) Ca2SO4(s) + H2O(l)



Is this balanced?



Is this balanced? No

35


Is this balanced? No

Ca(OH)2(s) + H2SO4(aq) CaSO4(s) + H2O(l)

2Ca(OH)2(s) + H2SO4(aq) CaSO4(s) + 2H2O(l)

Chemistry 120Reactions in Solution: Acid - BaseReactions in Solution: Acid - BaseDecompostion in acidSome anions also decompose in acid. These are usually anions which are derived from gases which are not soluble in water:

CO32-

(aq) carbonate CO2(g)

HCO3-(aq) hydrogen carbonate CO2(g)

S2-(aq) sulfide H2S(g)

HS-(aq) hydrogen sulfide H2S(g)

SO32-

(aq) sulfite SO2(g)

HSO3-(aq) hydrogen sulfite SO2(g)

Chemistry 120Solution Reactions: Oxidation-Reduction

Reactions between elements and between

compounds often involves the exchange of electrons.

Mg(s) + Cl2(g) MgCl2(s)

• Magnesium and chlorine are in their elemental

states and react together so that magnesium forms

Mg2+ and chlorine forms Cl-

• The overall product is neutral - MgCl2(s) has no

net charge (even though it is ionic).

36


In this reaction, Mg has lost two electrons:

Mg(s) + Cl2(g) MgCl2(s)

Mg2+ + 2e-Mg

½Cl2 has gained an electron:

1/2Cl2 + e- Cl-


The oxidation state of magnesium has changed from

zero to +2

The oxidation state of ½Cl2 (Cl) has changed from

zero to -1

Mg Mg2+ + 2e-

1/2Cl2 + e- Cl-



zero to +2 Mg Mg2+ + 2e-

Oxidation state zero Oxidation state two

37



zero to +2

The oxidation state of ½Cl2 (Cl) has changed from

zero to -1

Mg Mg2+ + 2e-


1/2Cl2 + e- Cl-


Chemistry 120Solution Reactions: Oxidation-ReductionMg(s) + Cl2(g) MgCl2(s)

In this reaction, Mg has been oxidized

- the oxidation state of Mg has increased -

and Cl has been reduced

- the oxidation state of Cl has decreased -






Chlorine is an oxidant or an oxidizing agent

38






Chlorine is an oxidant or an oxidizing agent

Magnesium is a reductant or a reducing agent


Oxidation and reduction reactions are extremely

common and involves the formal interchange of

electrons between atoms.

The Oxidation State and Oxidation Number are key

concepts in the discussion of these reactions


In an oxidation-reduction reaction, or redox reaction,

there MUST be and oxidation part and a reduction

part.

39


In an oxidation-reduction reaction, or redox reaction,

there MUST be and oxidation part and a reduction

part.

WHY?


If the reduction and oxidation portions of the reaction

do not balance then

• Charges will not balance overall



do not balance then


• the Mass Balance - the number of atoms on

both sides - will not balance

40



do not balance then


• the Mass Balance - the number of atoms on

both sides - will not balance

• Mass and energy will therefore be created or

destroyed


Initially,

Oxidation implied reaction with oxygen

Reduction implied the liberation of a metal from it’s

ore - usually by reaction with carbon or air


The modern definition is based on the changes in

oxidation numbers and the actual charges or the

formal charges on atoms and ions (including

polyatomic ions)

(In organic chemistry, oxidation is still based on

reaction with oxygen and reduction in the addition of

hydrogen)

41


Some rules for redox reactions

• All elements have an oxidation state of zero

• The oxidation state of simple mono-atomic

cations is the charge on the ion:Element Ox. State

Group IA: Li+, Na+, K+, Rb+, Cs+ +1Group IIA: Be2+, Mg2+, Ca2+, Sr2+, Ba2+ +2Group IIIA: B3+, Al3+, Ga3+ +3


The mono-atomic anions can be treated in the same

way:Element Ox. State

Group VIIA: F-, Cl-, Br-, I- -1Group VIA: O2-, S2-, -2Group VA: N3-, P3-, -3


For an ion the oxidation state is the charge -

assignment of the charge requires some thought

Q: What is the oxidation state in MnO4-?

Q: What is the oxidation state in SO4-?

To answer these questions , we use some basic rules

which count the valence electrons in a species

42


All Group IA cations have an oxidation state of +1

Hydrogen has an oxidation state of +1, rarely -1

Fluorine always has an oxidation state of -1

The sum of the oxidation states must always equal

the charge on the species


Example 1:

Sodium fluoride, NaF


Example 1:


The formula unit is neutral, so the oxidation numbers

must sum to zero.

43


Example 1:



must sum to zero.

All group 1A cations have an oxidation number (or

are in oxidation state) +1


Example 1:



must sum to zero.

All group 1A cations have an oxidation number (or

are in oxidation state) +1

Fluoride must have an oxidation state of -1


Example 2:

What is the oxidation state of P in PO43-?

44


Example 2:


The charge on this ion is 3-


Example 2:



Oxygen has an oxidation number of -2


Example 2:




With four oxygens present, the total oxidation

number of the oxygens is 4 x -2 = -8

45


Example 2:



Oxygen has an oxidation number of -2With four oxygens present, the oxidation number of all the oxygens is 4 x -2 = -8 The balance of the oxidation states must equal -3


Example 2:




With four oxygens present, the oxidation number of

all the oxygens is 4 x -2 = -8

The balance of the oxidation states must equal -3

So, P has an oxidation state of +5 as (+5) + (-8) = -3


Example 2:


So, P has an oxidation state of +5 as (+5) + (-8) = -3

Remember that this does NOT imply that PO43- is

ionic, just that the oxidation state of P is +5

46


Example 3:

What is the oxidation state of Fe in Fe3O4?


Example 3:


The formula unit, Fe3O4, is neutral so all the

oxidation numbers must sum to zero


Example 3:




O has an oxidation state of -2, -2 x4 = 8

47


Example 3:




O has an oxidation state of -2, -2 x4 = 8

The oxidation states of Fe must balance this number,

so the oxidation state of Fe is 8/3 - a fractional

oxidation state.


Example 3:


The oxidation state of Fe is 8/3

This is the average over all Fe in the solid and does

not represent the charges on the ions.

Fe3O4 is actually Fe2O3.FeO, where the oxidation

states are + 3 and +2. The average is 1/3[(2 x 3) + 2]


Redox reactions

This reaction is the reduction of Copper (II) Oxide

with hydrogen gas to give copper metal and water.

CuO(s) + H2(g) Cu(s) + H2O(l)

48


Redox reactions

This reaction is the reduction of Copper (II) Oxide

with hydrogen gas to give copper metal and water.

What are the oxidation states of the reactants and

products?



Cu2+: oxidation state +2O2-: oxidation state -2


H2 is the elemental form, Oxidation state = 0


H: oxidation state +1O: oxidation state -2


Cu is the elemental form, Oxidation state = 0

49



Redox reactions

The oxidation state of Cu has changed from +2 to 0

- it has been reduced

The oxidation state of H2 has changed from 0 to +1

- it has been oxidized

Stoichiometry: Chemical Calculations - University of … · · 2006-02-153 Stoichiometry: Chemical Calculations Chemistry 120 The molecular mass of a compound is the mass ... Chemical

Documents