Liam Mariadas
CHEMISTRY: UNIT 4 CONTENTSArea of Study 1: Chapter 15: Fast and
Slow Chemistry..2-12 Chapter 16: Controlling the Yield of
Reactions..12-21 Chapter 17: Equilibria Involving acids and
Bases..22-26 Detailed Study Covered: Sulfuric Acid:
Chapter 18: Production of Sulfuric Acid.27-33 Area of Study 2:
Chapter 23: Fossil Fuels33-37 Chapter 24: Alternative Energy
Sources37-42 Chapter 25: Energy from Chemical Reactions..43-46
Chapter 26: Electricity from Chemical Reactions46-53 Chapter 27:
Cells and Batteries.53-60 Chapter 28: Electrolysis..60-65
Additional Information about these notes: This is for the 2012
study design for VCE Chemistry. There is an additional Health and
Safety chapter not present in these notes. Images taken from
Heinemann Chemistry. Hopefully these notes will equip you with the
knowledge needed for the exam.
Area of Study 1: Industrial Chemistry:Chapter 15: Fast and Slow
Chemistry:Yes, this is the actual name of the chapter- which
examines the changes in energy during chemical reactions as well as
factors that influence the rate of these reactions. Keep in mind:
During chemical reactions, particles (atoms, molecules, ions)
collide and undergo change- where atoms rearrange to form new
particles. An example: Transfer of electrons (Redox) or transfer of
protons (Acid/Base.) However, collisions between particles do not
always result in a chemical change (i.e: when the use of a catalyst
is necessary.) Chemical Energy: What is Chemical Energy? All
substances have chemical energy. It is the sum of stored
(potential) energy and kinetic (movement) energy.
These energies may result from such things including:
Attractions between electrons and protons in the atoms Repulsion
between nuclei Repulsion between electrons Movement of electrons
Vibration and rotation of and around bonds. The chemical energy of
a substance is often called its heat content or enthalpy, denoted
by the symbol H. Energy Changes during Chemical Reactions:
When a chemical reaction does take place, atoms of the reactants
are rearranged to form products with different chemical energies.
Depending on the relative energies of the reactants and products,
two situations arise: Exothermic Reactions: Total chemical energy
of products is less than the energy of the reactants. The
difference in this energy is usually released as heat energy (not
lost.) Thus, exothermic reactions are the release of energy. (i.e:
combustion of petrol.)
Endothermic Reactions:Chemical energy of the products are
greater than the energy of the reactants. Meaning energy is
absorbed from a surrounding environment. This absorbance of energy
is an endothermic reaction.
Energy released/absorbed during a chemical reaction is called
the heat of reaction. The heat of the reaction is equal to the
difference in enthalpy of the reactants and products of a reaction,
this it is denoted as H .
H = H (Products) H (Reactants) H, for Psychology students
(simple terms), is the change in energy. For exothermic reactions,
the value of H(Products) is less than H(Reactants) so the values of
H are negative.
For endothermic reactions, the value of H(Products) is more than
H(Reactants) so the values of H are positive.
MEMORY NOTE: Exo- ex-exit sign-leave/release- release of energy.
Endo-end- end with more-end>start-products > reactants- must
absorbing energy. Most reactions we encounter are exothermic, such
as the burning of fossil fuels, wood, coal, and even the oxidation
of glucose in our bodies. Examples of endothermic reactions include
condensation polymerization of glucose to starch. Thermochemical
Equations:
Photosynthesis is another endothermic reaction, changing carbon
dioxide and water to glucose and oxygen. The thermochemical
equation for this is: 6CO2(g) + 6H2O(l) C6H12O6(aq)+6O2(g); H=
+2803kJmol-
In this reaction 2803kj per mol of energy is absorbed when 6
moles of CO2 reacts with 6 mol H2O to form 1 mol glucose and 6 mol
O2. Thermochemical equations show the energy released or absorbed
during a chemical reaction. (Will be + in endothermic, - in
exothermic.) Energy is measured in Joules (J) or kilojoules (kj),
yet the heat of reaction or H is in kj mol-, saying the energy
corresponds to the mol amounts in the equation. The thermochemical
reaction for the oxidation of glucose: (reverse of photosynthesis)
C6H12O6(aq)+6O2(g) 6CO2(g) + 6H2O(l); H= -2803kJmolNote the
negative H value indicating an exothermic reaction taking place
(energy released.) The thermochemical reaction for the combustion
of methanol:
CH3OH (l) + 3/2O2 (g) CO2 (g)+ 2H2O(g); H= -726kJmolNote: If we
double the number of moles of methanol, the energy released also
doubles: 2CH3OH (l) + 3O2 (g) 2CO2 (g)+ 4H2O(g); H= -1452kJmolKeep
in mind when writing, doubling the moles of methanol also doubled
the moles of every other substance in the reaction.
Keep in mind: Temperature of surroundings will decrease during
endothermic reactions as it absorbs energy, likewise during
exothermic reactions when energy is released, the temperature of
surroundings increases.
Activation Energies: Getting a Reaction Started: What causes
substances to not react with each other instantly? Recall what
happens to bonds during a chemical reaction: Bonds between atoms in
the reactants must be broken, requiring energy to be absorbed.
Bonds between atoms in the products must then be formed, requiring
a release of energy.
In the combustion of methane, the bonds between methane atoms
and oxygen atoms are first broken and this causes absorption of
energy, then energy is released in forming the bonds of the
products- carbon dioxide and water. As combusting methane is an
exothermic reaction, more energy is released in forming, than
absorbed in breaking.
The heat of reaction, H, is the net result of the energy
absorbed in breaking bonds of reactants, and the energy released in
forming them in products.
Energy changes during the course of the reaction are shown in
energy profiles. The top of the curve (activation or transition
complex) is the intermediate step in the reaction. This is where
bonds in the reactants are partially broken and bonds in the
products are partially formed.
The energy required to break the bonds of reactants so that the
reaction can successfully take place is the activation energy.
This is why natural gas does not spontaneously combust- it
cannot naturally meet the activation energy requirements, and must
be lit to do so.
Making Reactions go Faster: The rate at which reactions occur is
of course important in chemical industry. Some reactions are
extremely fast whilst others very slow. In industry reactions need
to occur rapidly so profits can be made- maximizing reaction rates
are a focus in industry. To understand how to speed up or slow down
a chemical process we should visualize what happens to particles
during a reaction and understand energy changes occurring.
Collision Theory: For a reaction to take place, particles must
collide with each other with sufficient energy to overcome
activation energy of the reaction. Greater the number of collisions
with reactant particles, greater rate of reaction.
Majority of collisions dont result in a reaction. Although the
frequency of collisions is very high, explosions do not occur, as
not all collisions result in a reaction.
For collisions not to cause a reaction, the energy involved will
be less than the energy needed to break the bonds in the reactant
(less than the activation energy.) Only successful collisions, when
the energy involved exceeds the activation energy, allows the
chemical reaction to take place. Thus the rate of the reaction
depends on the number of collisions taking place, as well,
primarily, the amount of successful collisions that take place.
Factors that Affect Rates: The following factors influence the
frequency of collisions and the proportion of successful
collisions. There are four main ways in which reaction rates can be
increased: Increasing the surface area of solids. Increasing the
concentration of reactants in solution (or increase pressure of
gaseous reactants which increases the concentration of the gas
particles.) Increasing the temperature. Adding a catalyst.
Increasing the Surface Area of Solids: In solids, only particles
at the surface can be involved in reactions. If you crush a solid
into smaller parts, there are more particles present at the surface
to react (by making them into smaller parts we increase the surface
area.) A greater number of exposed particles means a higher
frequency of collisions, meaning a higher proportion of successful
collisions, increasing the reaction rate. Note: Surface area
changes do not affect the energy present in the reaction.
Increasing the concentration of the Reactants: (Also increasing
Pressure of Gases): With more particles moving randomly in a given
volume of solution, the frequency of the collisions increase and so
more successful collisions occur.
With gases, increasing the pressure of gases raises the
concentration of the gas molecules, causing more frequent
collisions also.
Note: Changes in concentration does not affect the energy in the
reaction.
Increasing the Temperature: As temperature increases the average
speed and average kinetic energy of the particles increases also.
They have increased energy. More particles have enough energy to
overcome the activation energy, thus more particles can actually
react in successful collisions to form reactions. Particles speed
increases, which increases the frequency at which collisions occur.
Thus as temperature increases, so too does the rate of the
reaction.
Extending Collision Theory: This is to do with the point made
above with temperature. Particles have a wide spread of kinetic
energies, so increasing the temperature will increase the AVERAGE
kinetic energy of particles. Although there will still be many
particles with widespread energies, it increases the proportion of
particles with higher energies, as shown below.
Therefore, more particles at higher temperatures will also
exceed the activation energy, increasing the likelihood of
collisions forming reactions due to a higher proportion of
particles able to overcome the activation energy.
The shaded area under the graph to the right represents the
number of particles able to successfully react due to their ability
to overcome the activation energy. EA represents the activation
energy point. As temperature increases this shaded region becomes
larger. More particles are able to be involved in SUCCESSFUL
collisions.
The factor of increasing particle energy so more react (as act.
energy is met), has a greater impact on the reaction rate then the
simplicity of increasing the frequency of collisions. Also, the
orientation and angle at which particles collide have a factor in
whether successful reactions occur. Below shows an unfavourable and
favourable orientation.
Catalyst: Many reactions occur more rapidly in the presence of
particular elements or compounds. These substances are known as
catalysts. Not used up or consumed in the reaction, so is neither
reactant nor product.
Chemical industry uses catalysts extensively as without them,
many reactions will be too slow and products cannot be obtained in
an economical rate.
Many catalysts are found by trial and error, and when found
chemists may look for similar substances that may speed up the
reaction rate even more. Some are shown:
There are two types of catalyst: Homogenous Catalysts: Are in
the same state as reactants and products. (i.e: Chlorine gas atoms
are catalysts on forming ozone gas to oxygen gas.) Heterogenous
Catalysts: Are in different states from the reactants and products.
(These catalysts are preferred in industry as they can be easily
separated from the product.)
How do Catalysts work?: As previously studied, the process of
adsorption is the forming of bonds between one particle (usually
solid) and another particle. Catalyst work in the same way, where a
solid catalyst is used in industry. Generally they use the largest
possible surface area of the catalyst, as this will allow more
reactant molecules to adsorb onto the catalyst for a faster
reaction.
One type of reaction is the Haber process, where nitrogen (gas)
molecules undergo hydrogenation (react with H2) using an iron
catalyst.Nitrogen and hydrogen molecules both adsorb onto the iron
surface (by forming intermolecular bonds.)
As they do so, the bonds within the molecules of N2 and H2 break
(or weaken) as a result of bonds forming with the iron
catalyst.
This allows the normal reaction of H2 + N2 to occur, as they
combine to form NH3, at a faster rate, as usually this reaction
requires 3000 degrees temp. Note: This is an exothermic reaction
releasing energy.
How do catalysts increase the rate of reaction? Catalyst speed
up the rate of the reaction as they lower the activation energy
needed for the reaction to occur, by providing an alternative
reaction pathway. As activation energy is lowered, more particles
can overcome the activation energy to result in more successful
collisions, increasing the rate of reaction. Catalysts lower the
activation energy but dont change H.
Catalytic affect on the reaction rate and activation energy is
shown below:
What has this chapter covered? Chemical energy, energy changes
during reactions, H (enthalpy/heat of reaction), exothermic and
endothermic reactions, thermochemical equations, activation
energies, energy profiles, collision theory, factors affecting
reaction rates, surface area changes, temperature changes,
concentration changes, extended collision theory, catalysts.
Chapter 16: Controlling the Yield of Reactions:A Problem for
Industries: Incomplete Reactions: Some reactions occur when not all
the product is formed. This fact has consequences on industrial
companies as large amounts of unreacted materials are costly and
wasteful. Thus the profitability of the company depends on the
reaction yield- that is the extent of the conversion of reactions
to products (how far the reaction will go). Take this example
reaction: N2 + 3H2 2NH3 One would expect that 2 mol NH3 is formed
from 1 mol N2 and 3 mol H2, however, one might find much less than
2 mol of NH3 forming. The reaction has seemed to Stopbefore the
reactants are completely consumed- as it is incomplete, and remains
as such. The stage at which the quantities of reactants and
products in the reaction remain unchanged during a reaction is
called chemical equilibrium. Chemical Equilibrium: The point in a
chemical reaction where the rate of the forward reaction is the
same as the rate of the back reaction, so that the quantities of
reactants and products in the mixture remain unchanged. These
questions are vital to consider in an effort to maximize the yield
in a reaction, so that industrial processes are more efficient: Why
do some reactions reach equilibrium?
How can the amount of product from a reaction that reaches
equilibrium be increased?
Why are some reactions Incomplete?: Reversible Reactions: Some
physical and chemical changes can be reversed. Water (ice form)
melting if placed in a drink and then freezing when in the freezer
can be noted by this equation: (The double arrows indicate
reversible reaction) H2O (s) H2O (l) It is made from the forward
reaction of water melting: H2O (s) H2O (l) And the back reaction of
water freezing: H2O (l) H2O (s) Likewise, the reaction of hydrated
copper sulfate (CuSO4.5H2O) when heated is reversible, as it forms
a white precipitate (CuSO4) and water- But, when the water formed
reacts with the precipitate, it re-forms the original reactant
(hydrated copper sulfate): CuSO4.5H2O (s) CuSO4 (s) + 5H2O (l) In
the same way this reaction is reversible.
Equilibrium Explained: Chemists have shown that forward and
reverse reaction occur simultaneously. Take the above example with
nitrogen gas and hydrogen gas. The following occurs: 1. Nitrogen
gas and hydrogen gas will react immediately to form ammonia. N2 +
3H2 2NH3
2. As the forward reaction proceeds, the concentrations of
nitrogen and hydrogen decreases- so the rate of reaction decreases
(less reactants), so
the rate at which ammonia is produced decreases- the quantity of
ammonia present increases. 3. As more ammonia is formed, ammonia
will break down to reform N2 and H2, thus the rate of the back
reaction will increase (as the concentration of ammonia increases).
4. Eventually the forward and back reactions proceed at the same
rate. At this rate ammonia is formed at the exact same rate as it
breaks down. The concentrations of NH3, N2 and H2 will remain
constant. N2 + 3H2 2NH3
The reaction has reached equilibrium (note reversible arrows
now) and the forward and back reaction rate do not change. The
reaction is described as dynamic, as although the forward rate is
the same as the back rate, they both are still occurring
simultaneously. During dynamic equilibrium: (step 4): Amounts and
concentrations of substances remain constant. The total gas
pressure is constant (if gases are involved.) The temperature is
constant. The reaction is incomplete (All substances are present in
the equilibrium mixture.)
How far do Equilibrium reactions go?: Different reactions
proceed to different extents.
Heinemann uses the example of HCL and H2O proceeding more into
the reaction than ethanoic acid and H2O, as seen by the electrical
conductivity of the resultant solution, in which H+ and Cl+ ions
are formed far more from HCL+ H2O, than the ion CH3COO- which is
formed in CH3COOH + H2O. (HCL/H2O conducts more, proceeds more into
reaction.) The Equilibrium Law: Consider the reaction: N2(g) +
3H2(g) 2NH3(g)
Now consider the table of data and concentrations (the reaction
is at 400 degrees):
Note: [x] denotes the concentration of the substance. As can be
seen there is no relationship between the initial concentrations of
N2,H2 and NH3 in any trial mixture (A.B,C or D)- particularly, NH3
is will not be calculated with a moles formula (using equation), as
the reaction does not go to absolute completion. Also, the is not
consistent, as it does not reflect the mole ratio of the
reaction.
Disregard this, but notice
has indices corresponding to the mole ratio of the
equation. This, in the table, gives a constant value (almost,
anyway) for each mixture.
K=K is known as the equilibrium constant. A and B are the
concentrations of the products, and C and D are the concentrations
of the reactants. a,b,c,d are the mole ratios in front of each
substance-according to the balanced equation representing it. This
concentration fraction is known as the reaction quotient for the
reaction. Only when the reaction is at equilibrium does the
reaction quotient have a constant value, equal to K. Until this
point it will not be equal to K. This reaction quotient can be used
for all types of reaction at equilibrium.
Different types of chemical reactions have different values for
K. For any given type of reaction that is the same, the reaction
quotient will give about the same value for K, if at a fixed
temperature. (i.e: If HCL + NaOH are reacted, but at different
concentrations each time, the quotient will still give a relatively
consistent value for all HCL + NaOH reactions.)
From the above example, the equilibrium constant K for the
reaction: N2 + 3H2 2NH3 at 400 degrees is 0.052. The units for K
can vary according to the equation. It requires index laws. For
example: (Where M is the concentration in mol L of each
substance):
The denominator is virtually M^4, and with index laws, indices
in fraction subtract from the numerator leaving M^-2. This means
the unit for K is , or: .
For a reverse reaction the equilibrium constant will be inverse
of the forward. Doubling the coefficients of a reaction will mean
squaring the original K value.What does and Equilibrium Constant
tell us? Gives us an indication of the extent of the reaction- how
far the forward reaction proceeds before equilibrium is
established. For values of K between and , a large amount of both
reactants and
products will be present at equilibrium. For values of K that
are very large, above , the equilibrium mixture consists
mostly of products with relatively small amounts of reactants.
For values of K that are very small, below , the equilibrium
mixture consists
mainly of reactants and relatively small amounts of products. If
not many reactants are present in the reaction mixture it means the
forward reaction has proceeded relatively far, and vice versa if
there is more reactant in the equilibrium mixture.
Effect of Temperature on Equilibria: Only temperature effects
the value of the equilibrium constant, K. The affects of
temperature change on an equilibrium depends on whether the
reaction is exothermic or endothermic. As temperature increases:
For exothermic reactions, the amounts of products decreases and so
the value of K decreases. For endothermic reactions, the amount of
products increases and so the value of K increases. Since the value
of K depends on the temperature and the mol ratios, both the
temperature and the equation should be noted or stated when an
equilibrium constant is involved. This means, if the temperature is
increased on a reaction that is exothermic, the BACK reaction of
this will increase (as it will be endothermic), and vice versa.
Changing the Equilibrium Position of a Reaction: The composition
of the equilibrium mixtures is important to industrial chemists
(that is, the more product in the mixture the better.) Ways to
maximize the yield for the desired products is sought after. The
equilibrium position (amounts of reactants and products) of a
reaction may be changed by: Adding or removing a reactant or
product. Changing the pressure by changing the volume (if
equilibria involves gases.) Dilution (if equilibria is in
solution.) Changing the temperature.
We will now go through each of these in depth.
Adding an extra Reactant or Product: For example, take a vessel
containing an equilibrium mixture represented by: N2(g) + 3H2(g)
2NH3(g) If more nitrogen was added to the vessel without changing
anything else, the mixture that was once in equilibrium will
momentarily no longer be. The following occurs as the composition
of the mixture adjusts to return to equilibrium: Forward rate of
reaction increases due to higher concentration of nitrogen
(reactant), which means more ammonia is produced. As the
concentration of ammonia then increases, the rate of the back
reaction also increases to reform N2 and H2. Eventually the forward
& back rate become equal again- new equilibrium formed.
When the new equilibrium is formed the concentrations of all
substances have changed. Addition of a reactant: Increases the
concentration of the product (as more can be formed) as there is
more forward reaction. There will also be a higher concentration of
reactants at the new equilibrium than the old one even though
reactants get used up during the forward reaction. The other
reactant that wasnt added in addition should decrease in
concentration in the next equilibrium. Addition of a product:
Increases the concentration of the reactant (as more can be formed)
as there is more back reaction. There will also be a higher
concentration of products at the new equilibrium than the old one
even though products get used up during the back reaction. Removing
a product or reactant from the original equilibrium mixture will
have the opposite affect on the concentrations of the substances at
the new equilibrium point. In summary, this principal can be used:
If an equilibrium system is subjected to change, the system will
adjust itself to partially oppose the effect of the said
change.
Changing the Pressure: By changing the Volume: For gases, the
pressure increasing in an equilibrium mixture can be caused by
decreasing the volume of the container in which the gas is kept
(whilst maintaining a constant temperature.) So how does pressure
affect the equilibrium position? Take this example:
On the reactants side, there are 3 particles of gas, on the
product side there are 2 particles. This means in the forward
reaction there will be a reduction in pressure (as it turns 3
particles into 2 particles, decreases number of particles,
decreases pressure). This causes a net back reaction to occur (to
increase pressure, changes the 2 particles to 3) to adjust the
pressure to normal- this continues until adjustment has been made.
Therefore, the adjustment of pressure depends on: The number of
particles on each side of the reaction. If the reactant side has
more particles in it, the forward reaction will decrease the
pressure, the back reaction will increase the pressure. (see above
for explanation). If the container volume is decreased, the
pressure increases, so the equilibrium mixture will undergo more
back or forward reaction (whichever one decreases the pressure), to
adjust the system. This depends on the particle number on each
side.
Increasing the volume (decreasing pressure) goes more to side
with more particles. (I.e: For N2 + 3H2 2NH3, a net back reaction
will occur). Decreasing the volume (increasing pressure) goes more
to side with less particles. (I.e: For N2 + 3H2 2NH3, a net forward
reaction will occur.)
By Adding an Inert Gas: If an unreactive gas is added (such as
helium, neon or argon) the pressure increases because there is more
particles- but none of the original substances are affected since
the
gas is inert. The concentrations of the substances are not
affected. The system therefore stays at equilibrium with no net
forward or back reaction. Dilution: When you dilute a solution, you
decrease the number of particles PER VOLUME. Therefore, when you
dilute something, the reaction will favor the side which creates
more number of particles. This is similar to how changing the
volume works (it depends on the number of particles on each side of
the reaction.) Therefore for:
If we dilute the solution, a net back reaction will occur (this
creates more particles.)
Changing the temperature: As temperature Increases: For
exothermic reactions, the amounts of products decreases and so the
value of K decreases. For endothermic reactions, the amount of
products increases and so the value of K increases. As temperature
increases: A net back reaction (less products) for exothermic
reaction occurs. A net forward reaction (more products) for
endothermic reactions occurs.
The Influence of a Catalyst: They increase the rate of the
forward and back reactions equally.
Therefore, Catalysts dont change the position of an equilibrium,
and therefore have no effect on the equilibrium yields of a
reaction.
However, catalysts may greatly increase the rate at which
equilibrium is attained.
Do All Reactions Reach Equilibrium? The simple answer is:
No.
Reactions can sometimes continue until they are complete (not
plateau to equilibrium), If: Reactions that produce products (such
as gases) escape from the reaction mixture as they are formed.
Continual loss of the product drives the reaction forward. (If a
product escapes, the mixture adjusts to produce more (forward
reaction), which will continue and continue until no reaction is
left- completion.) Reactions that form equilibrium in which only
minute quantities of reactants are present. (i.e: HCL and
water.)
Chapter 17: Equilibria Involving Acids and Bases:Acidity of
Solutions: As we know, acids are proton (H+) donators, bases are
proton (H+) acceptors. Some substances may behave as both acids and
bases depending on the conditions, these substances are
amphiprotic. (i.e: Water.) Ionization constant of Water: Water can
undergo self ionization in this reaction:
This means at equilibrium:
However, in aqueous solutions waters concentration is usually
constant at 56M, and so far more abundant than any other substance-
we therefore dont include H2O in calculations. Thus:
Where Kw is the ionization constant specific to water. It is (K
* [H2O] where H2O is constant.) It (Kw) applies to both pure water
and all aqueous solutions. In pure water (at 25 degrees celcius)
chemists have found the concentration of both H3O+ and OH- to be
about the same at: M.
Thus, the value of Kw at 25 degrees celcius is: = * =
Note: The concentrations of ions in water is very, very small-
there is much more water than ions in pure water. However, the ions
are still present so water can conduct electricity (only very
slightly!) Acidic and Basic Solutions: In solutions of acidic
substances, any acid in a reaction with water produces H3O+ ions.
This also occurs when water self ionizes. Thus, in acidic
conditions, the concentration of H3O+ will be greater than
The concentration of OH- will also be less than because the Kw
([OH-]*[H3O+]) is constant.
This is
In basic solutions, this is the opposite. (There is more OH- and
less H3O+).
In Summary, at 25 degrees: In pure water and neutral solutions,
In acidic solutions: In basic solutions:
The higher the concentration of H3O+ in the solution, the more
acidic the solution is. In strong basic solutions the concentration
of H3O+ is low (around NaOH. In strongly acidic solutions, like
HCL, this can rise to 10M. Indicators that change colour at
specific levels of H3O+ concentrations can help measure and
determine the acidity of a solution. pH: A Convenient way to
Measure Acidity: A pH scale has been used to measure the range of
acidity in a solution. pH is defined as: ) for example in
Alternatively:
This means at 25 degrees pure water, as [H3O+]=
,the pH of the solution is 7.
For acidic solution, the pH is < 7, as the more acidic a
solution is, the lower the pH. For basic solutions, the pH is >
7. As the more basic a solution is, the higher the pH.
Keep in mind, pH is only a measure of the [H3O+] concentration,
so in finding the pH you must obtain the H3O+ concentration value
for this. How is pH affected by temperature? The pH of pure water
is only exactly 7 (other times it is very close), at 25 degrees
Celsius. But what if a solution is not at 25 degrees Celsius as we
have considered?
It works similarly to how equilibrium worked: An increase in
temperature favors the endothermic reaction. A decrease in
temperature favors the exothermic reaction.
Therefore, lets take:
It is an endothermic reaction, if we increase the temperature,
its forward reaction is favored. This means the concentrations of
H3O+ and OH- increase (as more product is formed). Thus, according
to previous formula, the Kw of the solution increases. This means
the pH of the solution decreases (more H3O+). Likewise, for
endothermic reactions, a decrease in temperature will cause an
increase in pH.
Acidity Constants: Most acid-base reactions in water can be
considered as equilibrium reactions. Let HCL and water be an
example:
The equilibrium expression may be written as:
From knowledge, we know that in the above reaction Cl- in the
conjugate base of HCL, as it has lost a proton. Likewise, HCL is
the conjugate acid of Cl-. (As a note, just reverse the reaction to
see the gaining or losing affect of H+ to work out conjugates.) The
acidity constant, , can be calculated as such: (Water is left out
as it is constant.)
The acidity constants of some acids are shown below: The table
means that as 25 degrees, the Ka value for Hydrochloric acid is .
This means most of
the HCL in solution has converted to H3O+ and Cl-. This high
value is why HCL is a strong acid. In contrast, ethanoic acid has a
very low Ka value at 25 degrees, . The equilibrium
position has/favours the reactants, so there is less product.
This is why CH3COOH is a weak acid. The acidity constant is thus
used as a measure of acids strength. Buffers: Using Equilibrium to
Resist Change: Solutions that can absorb the addition of acids and
bases with little change in pH. Made most of the time by mixing a
weak acid and a salt of its conjugate base.
For example, a buffer can contain ethanoic acid and sodium
ethanoate. The equilibrium mixture will contain CH3COOH, H3O+ and
CH3COO-.
The important aspect of this solution and of buffers is that it
contains significant amounts of BOTH the weak acid and its
conjugate base. If a strong acid was used- this wouldnt be the case
as it would ionize completely. So, if a strong acid is added into a
buffer, the pH will decrease but by much less than expected. In
this, equilibrium works the same as it usually does. If a strong
acid is added, more H3O+ is present so the equilibrium will try to
oppose it (use up H3O+)- in this case it favours the back reaction
to make more CH3COOH. Likewise, adding a base consumed H3O+, so the
equilibrium will try to oppose this if a base is added and form
more H3O+. Buffers are important for delicate environmental and
living systems where it is important to maintain stable conditions,
also in labs where surroundings of an experiment should be kept
constant. pH in the Body: Many reactions in the body are acid-base.
The pH must be controlled within a narrow range so the body can
survive. Acidosis- Lethal drop in pH in the body. Alkalosis- lethal
rise in pH in the body.
Natural buffers prevent this from happening and controls the pH
levels.
Chapter 18: Production of Sulfuric Acid:Uses of Sulfuric Acid:
Sulfuric acid is used extensively in the production of fertilizers
(such as superphosphate). It is also used in the manufacturing of
paper, dyes, drugs,
detergents, petroleum refining and is an electrolyte in car
batteries. Superphosphate: Phosphorus is needed for plant growth,
but must be added since Australian soils are deficient in this. In
manufacturing superphosphate, insoluble calcium phosphate
(Ca3(PO4)2) contained in rock phosphate is converted to a soluble
form that plants can absorb. Sulfuric acid is reacted with rock
phosphate, over several weeks superphosphate is formed.
Another fertilizer it also produces is ammonium sulfate:
Other Uses:
As a strong acid (as already learnt, it is diprotic.) As a
dehydrating agent (as already learnt, ie: as a catalyst in
esterification.) As a strong oxidant (i.e: Oxidizes Zinc, is
reduced itself.)The Contact Process: Raw Materials for Making
Sulfuric Acid: Sulfuric Acid can be manufactured in a process
beginning with the starting substance, Sulfur dioxide. The sulfur
dioxide is then oxidized to form sulfur trioxide, and then follows
the conversion to the acid- sulfuric acid.
A summary of sulfur to sulfuric acid (constact process) can be
summarized:
The sulfur dioxide used to produce sulfuric acid is obtained
from two sources: Combustion of sulfur recovered from natural gas
and crued oils. Sulfur dioxide formed during the smelting of sulfur
ores of copper, zinc or lead. Mining of underground deposits of
elemental sulfur by a method known as the Frasch process but this
isnt used much in Australia (other two are more available.) The
sources used in Australia are environmentally friendly as they use
by-products from other industries, limiting the amount of SO2
emitted into the atmosphere. Less fossil fuels are being used to
obtain sulfur for use in sulfuric acid, with most of the sulfur
dioxide recovered from smelters. STEP 1: Contact Process: BURNING
SULFUR: When sulfur is used as a raw material for making sulfuric
acid, the first stage of this process is spraying molten sulfur
(liquid) under pressure into a furnace.
This sulfur then is heated and burns to produce sulfur dioxide.
(Combustion) Combustion is rapid due to the sulfur spray being of
high surface area.
Temperatures up to 1000 degrees celcius may be reached. Sulfur
dioxide gas formed is then cooled for the next stage in the
process.
STEP 2: Catalytic Oxidation of Sulfur Dioxide: Sulfur Dioxide
gas is then oxidized in this next step to produce sulfur trioxide
gas by using oxygen, and using Vanadium (V) oxide (V2O5) as a
catalyst.
This step is performed in a reaction vessel (known as a
converter). Sulfur dioxide is mixed with air and passed through
trays containing loosely packed porous pellets of catalyst
(catalyst beds of Vanadium).
The air enters from the top (see diagram) supplying oxygen,
passing over each catalyst bed level in succession with the Sulfur
dioxide gas. As the reaction is exothermic, it is neccessary to
cool the gas mixture as it passes from one tray to another so a
desired reaction temperature is maintained. (Exothermic causes it
to be a hotter environment- not always ideal.)
The temperature is maintained at around 400-500 degrees, with a
pressure of 1 atmosphere. Almost all Sulfur dioxide is converted to
Sulfur trioxide in this process.
Note that knowledge of equilibrium and reaction rate play an
important role in this: The reaction yield of SO3 will increase as:
The temperature decreases. The reaction is exothermic, then the
temperature decreases the mixture will want to release more heat to
maintain the temperature- thus it will favour the forward
(exothermic) reaction.
As pressure increases. As the pressure increases the mixture
will favour the side that produces less particles (to maintain
pressure), this means the forward reaction is favoured (see
equation above, 3 -> 2). Any reactants are added. (SO2 or O2)
The rate of the reaction will be faster if: Temperature increases.
Pressure increases. A catalyst is used. Conflict arises- rate
increases with higher temperatures, but yield lowers with higher
temperatures. A catalyst is used so that at low temepratures there
is a higher yield, but the rate is still very fast. Vanadium Oxide
(V2O5) is used as a catalyst- (economically viable and not
poisoned- not neccessarily most efficient but best for these
reasons.) Temperatures are maintined by cooling the reactant
mixture as they pass from catalytic bed tray to another. Increasing
the yield can also be done by using atmospheric air in the form of
oxygen gas (this is also cheaper.) Also, as the ideal pressure is
atmospheric, high pressure systems usually are not needed. Step 3:
ABSORPTION OF SULFUR TRIOXIDE: Sulfur trioxide can react with water
to form sulfuric acid.
However, this method is not used as there is a lot of heat
involved- it creates a mist of the acid which is difficult to
collect. Instead: Sulfur trioxide is passed through concentrated
sulfuric acid in an absorption tower. The reaction is regarded as
occurring in two steps.
1. The SO3 gas dissolves almost completely in the H2SO4 to form
a liquid (oleum.)
2. Oleum obtained from the abruption tower is then mixed with
water to produce sulfuric acid. This is a dilution process.
Waste Management: Environmental Management: (SO2 is a pollutant)
Use/recycle SO2 from other industries. This solves a waste problem.
Double absorption. Unreacted SO2 recycled from absorption tower
back into converter to try to react. Increases conversion of SO2 to
SO3 from 98% to 99.6%. Improved catalyst. More expensive catalyst
helps conversion (less SO2 pollute)
Waste Heat Produced used to Reduce Energy Costs: Since all the
reactions are exothermic, this heat energy can be used to boil
water to produce electricity for the plant itself. Energy can be
sold to other industries, for use as heat or to generate
electricity.
Health and Safety: Respiratory Irritants: Cause: SO2, SO3, H2SO4
mist. Precautions: Maximise conversion of SO2 to SO3 or Ventilation
and air filtering.
Corrosiveness: Cause: H2SO4 and H2S2O7
Precautions: Storage, highly regulated handling and
transport.
Acid Rain: Causes: SO2, SO3 Precautions: Limit emissions.
Acid Spills: Precautions: Contain within earth, clay or sand.
Neutralize with base (CaCO3 limestone or sodium carbonate.)
Dilution of H2SO4 with Water: Only add acid to water- stir.
Reaction is exothermic, releases a lot of heat.
General: Wear protective clothing: Safety glasses, lab coat,
gloves. Use fume hood (for ventilation).
Area of Study 2: Supplying & Using Energy:Chapter 23: Fossil
Fuels:Energy Sources Today: How much energy do we use? Energy is
measured in units called the joule, J. Larger amounts usually
expressed as kilo jouled (kj, 1000 joules). On a national level,
petajoules may be used as a unit. (1 PJ= J)
An individual in a modern society uses approximately 1000 MJ a
day. This figure is about 100 times more than what the body
actually requires- the bulk of our energy is used for transport,
heating and domestic purposes. Globally, around 4 * 10 ^20 J is
used per day. Consumption for different sources:
Meeting our Energy Needs: Dominant in our society are the fossil
fuels:
Coal Natural Gas OilWe are seeking alternative fuel sources, due
to their negative effects on the environment: Release into
atmosphere (SO2 causing acid rain, CO2 causing greenhouse effect.)
Fossil fuels are non renewable, reserves will eventually run
out.
In such a high demand, but non renewable.
Now we look to alternate sources (solar, wind, hydro, nuclear)
to combat this problem. Energy Converters: Anything that transforms
energy from one type to another is an energy converter. For
Example, a car engine converting chemical to thermal. Others are
shown below
There are always losses in energy through transformations. This
is usually as heat energy. Therefore energy converters may not
always be 100% efficient. Efficiency is desirable. This would mean
2J of chemical energy is needed to create 1J electrical in a fuel
cell. (50%) Fossil Fuels: Sustainability is the need to meet short
term needs, considering long term impact. Fossil fuels are
unfortunately non renewable, they are used at a greater rate than
what it is replenished. They are from finite sources of energy.
Fossil fuel energy are essentially trapped solar energy, the stored
chemical energy in the bonds of these fuels were transformed from
photosynthesis, from energy from the sun. Coal:
Formed from decaying vegetation. Plants lose their hydrogen and
oxygen content, and so their carbon content increases over time.
Becomes peat, then brown coal, then black coal. Also contains small
amounts of nitrogen and sulphur.
When coal is burnt, energy is used to vaporize water (reduces
the amount of energy released upon combustion.) Thus mining black
coal is worthwhile economically despite being buried further
underground.
Coal is burnt in a coal fired power station. Chemical energy
from the coal is released as thermal energy upon combustion.
Thermal energy produced is used to heat water, making a steam
vapour. This thermal energy in this steam/vapour then turns a
turbine, forming mechanical energy. This then generates a current,
forming electrical energy. This process is about 35% efficient,
with 50% of the lost energy occurring in the cooling tower where
heat is lost in steam. Overall, coal is not a clean burning fuel. A
lot of ash, water vapour and sulphur dioxide are produced, as well
as the primary product of Carbon Dioxide. Oil and gas produce less
pollutants, however coal has the largest reserves remaining in the
world- (followed by oil, then natural gas.) Its use is estimated to
increase over time. It can be converted to oil via the process
flash pyrolysis, where it is pulverized, heated to 600 degrees and
then reacted with hydrogen. Advantages of Coal: Cheap (economically
viable), and can be used as a baseload power source. (Can provide
most of the energy demands for a large population.) Disadvantages
of Coal: Releases greenhouse gases into the air, non renewable,
inefficient (process if only 35% conversion from chemical to
electrical energy.) Coal: Energy Transformations Summary: Chemical
energy in bonding of hydrocarbons Coal is combusted Thermal Energy
in temperature of steam from combustion Steam passes turbine
Mechanical energy from the motion of the turbine Generates a
current
Electrical energy in the movement of electrons Electricity used
by public. Crude Oil: Also known as petrolium. Its simply a mixture
of alkanes. (the relative amounts of which vary.) Crude oil
undergoes fractional distillation (seperation of several components
of the mixture based on their boiling points. (See Unit 3) Oil is
heated to about 400 degrees, enters the fractional tower, and the
vapour begins to rise. The temperature in the tower decreases the
higher up in the tower. Horizontal trays with bubble caps impere
the rise of the vapour, forcing it to bubble through condensed
liquid in the trays. These vapours then condense into trays
containing condensed liquids with similar boiling points to their
own. These fractions are then collected. The higher up the alkanes,
the lower the boiling points- i.e lighter alkanes. Typically the
fractions are: (Where Cx denotes the number of carbons) Refinery
gas (LPG, feedstock) C1 to C4, Gasoline (petrol) C5 to C6, Naphtha
(cracking to petrol feedstock) C6 to C10, Gas oil (diesel, cracking
to petrol) C14 to C20 and Bitumen (lubricating oil, fuel oil)
larger than C20. Keep note that these will vary with location. The
heavier fractions may undergo several seperations: Catalytic
cracking is the process in which heavier alkanes are broken into
lighter alkanes, alkenes, and hydrogen. A Zeolite catalyst is used
for this process. This is done because the lighter alkanes are more
useful (but crude oil contains larger ones usually.) The main uses
of crude oil products are fuels such as petrol, keosene, diesel and
liquidified petroleum gas, and as raw materials for the
manufacturing of products such as plastics and pharmaceuticals.
Natural Gas: Mainly refers to methane, but also small amounts of
hydrocarbons such as ethane and propane. Popular fuel for heating
and cooking purposes, may replace petrol and diesel as oil prices
increase. In a gas-fired power station, thermal energy in hot gases
from combustion expand air to spin the turbine, forming mechanical
energy. The gas is simply burnt in air to produce CO2 and H2O, so
less pollutants than coal. Natural gas provides for about 20% of
the worlds energy needs.
Natural Gas: Energy Transformations Summary: Chemical energy in
bonds combusted, creates a vapour Thermal energy of vapour turns
turbine Mechanical energy of turbine creates a current Electrical
energy due to current (electrons moving) Electricity used by
public.
Chapter 24: Alternative Energy Sources:Nuclear Energy: Nuclear
Fission: A process where a fissile isotope (usually Uranium-235) is
bombarded by a neutron and then splits up into two smaller nuclei
(and releases a large amount of energy.) This is called a nuclear
reaction, and new elements are formed from the original. Neutrons
are also released from these reactions and go on to bombard other
nuclei, causing a chain reaction to occur (think of all that
energy!) The Uranium -235 to start with is taken from Uranium ores.
However, these ores contain 99.3% U238, and only 0.7% U235, so the
ores must be refined for 3% U235.
Uranium 238 can also be exploited by fast-breed reactors,
decaying into Plutonium 239 to generate further energy.
Uranium 235 nuclear reactions is the primary source of nucleur
energy however, and occur in nuclear reactor power plants. In a
nuclear reactor powerplant, the chain reaction occurs in a reaction
vessel. Control rods and heavy water (deuterium oxide, 2H2O)
moderate the rate of reaction in the vessel to a safe level. The
reaction is shown:
A small amount of the reactants nuclear energy in the bonds, are
converted to kinetic energy in the products, and this then becomes
thermal energy in the water.
This then transfers to the boiler, creating steam which turns a
turbine (mechanical energy) and this creates a current (electrical
energy.) 1kg Unranium produces the same amount of energy as 2500
tonnes of coal.
Now we look to the environmental impact involving nuclear
reactors: Produces no gaseous emissions, but products are
radioactive and pose waste problems that a long term. Uranium and
Plutonium may be recycled for further use. Non renewable source of
energy (Uranium is finite.) Nuclear waste is stored for 5 years so
short lived isotopes/waste can decay and cool down. Long lived
isotopes must be stored safely for thousands of years. This is done
by burying in sealed tanks or deep mines in geologically stable
regions, stockpiling in air conditioned warehouses, dumping in
ocean trenches or sealing in special types of glass. The other
concern with nuclear power is the risk of release of radioactive
substances (by accident or sabotage, terroist attack). Fears of
Uranium being stolen for weapons (unlikely). Power Plants take a
long time to build and have a reletively short working life.
Advantages of Nuclear Energy: Relatively cheap, can be used as a
baseload power source (power a whole population), high amount of
energy per unit mass of fuel, no greenhouse gases released.
Disadvantages of Nuclear Energy: Non renewable, waste is
extremely tocix, potential terroist threats, nuclear meltdown
potentially catastrophic.
Nuclear Energy: Energy Transformations Summary: Nuclear energy
in the nuclear bonds of protons and neutrons Fission & steam
Thermal energy in the temperature of steam passes through turbine
Mechanical energy in the motion of the turbine Generates a current
Electrical energy in the movement of electrons Electricity used by
public Nucleaur Fusion: The opposite to fission, two or more nuclei
are smashed together to form a larger one. This is the source of
energy in the sun (when hydrogen fuses with helium). Fusion of one
kilogram of fuel yields more than fission, may reach the same
energy as 10000 tonnes of coal. However, great difficulty in
sustaining a temperature in a reactor for fusion to occur.
Temperature of 100,000,000 degrees needed as well as a place to
contain. Bioethanol, Biodiesel and Biogas: The fermentation of
glucose (with a catalyst) from plant crops produces bioethanol,
which can be blended with fuel to lower the demand and use of
petroleum (less pollutants etc.)
Biodiesel is derived from vegetable oils and animal fats,
hydrolysed to fatty acids and esterified.
Waste plant and animal material can be converted into syngas
(carbon monoxide and hydrogen) or biogas (carbon dioxide and
methane) which can generate heat/electricity.
Solar: Solar energy can be used directly. Solar efficient
building designs reduce heating, cooling and lighting costs. Solar
water heating systems provide hot water, for homes and for pools.
However, solar cells are very expensive, and inability to be used
efficiently when in the dark- either at night or in cloudy weather.
There is also a need for large areas of land due to relatively poor
energy conversion efficiency. Solar energy is renewable. Utilizes
energy from the sun (which always provides energy.) Solar energy is
stored for use as electricity. Solar: Energy Transformations for
STORING solar energy: Radiant energy in light waves from sun
Current made in solar panel. Electrical energy (movement of
electrons) in solar panel Recharges batteries Chemical energy of
the reactants in the battery made. The chemical energy is stored,
now, more transformations occur for its use (below). Solar: Energy
Transformations for the USE of solar energy: Chemical energy of the
reactants in batteries Discharge of battery Electrical energy of
movement of electrons Electricity for the home
Advantages of solar: Renewable source of energy, no greenhouse
gas emissions. Disadvantages of solar: Expensive, inefficient (only
about 1% conversion), cant be used as a baseload power source (for
an entire population.)
Hydroelectricity: Obtained by using falling water to turn
turbines and generate electricity.
Firstly, Solar energy evaporates this water transferring it to
higher altitudes. The water thus obtains more Gravitational
potential energy.
Then, as the water falls, it is converted into mechanical energy
as it spins a turbine, which generates a current for electrical
energy. Hydroelectricity supplies about 25% of the worlds and 10%
of Australias electricity. Developement of hydroelectric stations
are restricted, due to the limited number of suitable sites and
concern about its environmental impact. This is because storage
dams are often requires to establish a good flow of the water at
all seasons, but lead to further environmental concerns (loss of
natural habitat.)
Hydroelectricity is renewable.
Hydroelectricity: Energy Transformations Summary: Gravitational
Potential energy in high altitude of water Water falls, turns
turbine Mechanical energy of turbines movement Generates a current
Electrical Energy from movement of electrons (Current) Electricity
used by public
Advantages of Hydroelectricity: No greenhouse gases, no toxic
waste, renewable. Disadvantages of Hydroelectricity: Building dams
results in destruction of natural habitat.
Wind Power: Can be used to generate electricity through wind
turbines. Average wind speeds exceeding 5ms are suitable, but the
turbines are only as reliable as the wind. The turbines are highly
visible and loud, a number of them are needed to match the energy
production of a coal-fired plant. (Wind turbines make less energy)
Tidal Power: Made in tidal power stations.
Uses the movement of water caused by the moon to generate
electicity. Sites are limited, as a large difference in water
height between tides are required.
Geothermal Power: In volcanic regions, heat from underground
rocks can reach underground water. This can cause steam to rise to
the surface, which can be harnessed to spin turbines to create
electricity. This is the principal of geothermal energy, however
Geothermal power plants are again restricted to suitable sites- but
are reliable.
Chapter 25: Energy from Chemical Reactions:Thermochemical
Equations: Knowledge of thermochemical equations (covered in
chapter 1) is needed for this section.
In addition to what was covered in chapter 1, is it important to
know that for example, the above reaction means 2 moles of C8H18
produces 10108kjmol-, 1 mol will produce half of this: 5054kjmol-.
The Connection between Energy and Temperature Change: The specific
heat capacity of a substance is how much energy it will take to
increase the temperature of the substance by 1 degree Celsius. The
higher the heat capacity, the more the energy will store heat as
thermal energy in its bonds, meaning it will heat up at a much
slower rate. The specific heat capacity of water is 4.18jg-c-
(relatively high).
Measuring the energy when a substance is heated:
Where E is energy, m is mass, c is specific heat capacity and T
is temperature. Measuring the Heat Released during a Reaction: Bomb
and Solution Calorimetry: Enthalpy is measured directly from an
instrument known as a calorimeter.
Bomb Calorimeter:
Used for reactions involving gaseous reactants or products.
(Cant be done underwater.) The vessel is made to withstand high
pressures during reaction. Insulated to reduce loss or gain of
heat/energy from or to the environment.
Note what each component is used for:
Thermometer: To measure the temperature before and after the
reaction. Heating Coil: Heats calorimeter with a known amount for
calibration purposes. Insulator: To reduce heat loss to the
environment, and gain from the environment. Stirrer: To spread the
heat of the mixture around the area.
Solution Calorimeter:
Used for aqueous solutions. Insulated to reduce heat loss or
gain, to or from the environment.
In reference to both Calorimeters: When a reaction takes place
in a calorimeter, the heat chance causes a rise or fall in the
temperature of the contents of the calorimeter. (Which is
measured.) Before the Calorimeter can be used however, we must
determine how much energy is required to change the temperature
within the calorimeter by 1 degree. This is known as the
calorimeters calibration factor.
The calibration factor can be found by: Supplying a known
current (I) (and a known voltage(V)) and measuring the time (t) in
which this occurs. Energy (E) is calculated from this as: (in
joules)
This is the total energy change due to the supplied current.
Also, measuring the temperature of the calorimeter, then heating
the calorimeter with a known amount of thermal energy and measuring
the temperature again.
Then, calculating the Calibration Factor (CF): (In Joules per
degrees Celcius)
We now get the energy change per 1 degree Celcius. (Calibration
Factor)
With the calibration factor, we can now measure the temperature
change of the actual reaction. It is used to determine what energy
is responsible for this temperature change. We can get the energy
released during the ACTUAL reaction by reusing the Calibration
factor (CF) formula, only with the temperature change of the actual
reaction- not due to heating the calorimeter. (CF * Heat of
Combustion: of reaction = E of reaction)
The energy released when a specified amount (1g, 1L, 1mol) of a
substance burnscompletely in oxygen (combusted.)
For pure fuels and substances, we can measure the energy
released per mol. Substances like wood, coal, kerosene, are
mixtures of chemicals (so no specificchemical formula). There the
energy released when they burn is measured per gram or per liter
rather than per mol.
Heat of Combustion is measured using a Calorimeter. (Formulae
can be used.) Heat of Cumbustion of several substances are shown
below/next page.
Heat of Combustion for mixed solids, mixed liquids, and pure
substances:
For the above table, volumes of gases are measured at SLC with
H2O and CO2 as products.
Chapter 26: Electricity from Chemical Reactions:Galvanic Cells:
Remember the Daniel Cell from Unit 2? This requires a Zinc anode(in
ZnNO3 solution) and Copper cathode (In Copper (II) solution). Refer
to the diagram below:
A current passes through the circuit to the globe. This part of
the cell is the external circuit. The globe converts electrical
energy to light and heat energy.
The current flows because of the chemical reaction taking place
in the cell (of which there is little indication of happening
initially.)
In the above example, if left long enough we observe the
following: The zinc metal corrodes. The copper metal becomes
covered with a furry brown-black deposit. The blue copper (II)
sulfate solution loses some of its colour.
These changes provide evidence of a chemical reaction. There is
also a current flowing from the zinc electrode, through the wire,
to the copper electrode. Current flows only if a salt bridge is
present. (Made of some type of salt).
These findings help us decide what is happening inside the cell.
The reaction in the cell is redox, electrons are being produced and
consumed. The zinc electrode is eaten away, forming zinc ions in
solution.
The oxidation of the zinc metal releases electrons, these flow
through the wire to the copper electrode. Electrons are accepted by
copper ions in the solution when ions collide with the copper
electrode.
Copper atoms are insoluble and deposit on the electrode
producing brown-black coating.
Purpose of the Salt Bridge: The salt bridge is an essential
component of the cell, It allows charges to balance. Without it,
the cell will be polarized and electrons will accumulate on one
half of
the cell. This would prevent any further current passing
through, however the salt bridge stops this. The salt bridge
contains ions that can migrate to either half cell so that a
buildup of electrons (and charge) is prevented. The cation of the
salt goes to the Cathode, and the anion goes to the anode. Below is
a summary of the processes occurring in the cell: The overall
reaction in the cell is found by adding the two half equations that
occur in each cell.
Half Cells: In a half cell, an electrode is in a solution. The
species present in each half cell (the elctrode and solution) form
a conjugate redox pair. Generally, the metal in the conjugate redox
pair is used as the electrode. The other is used in the solution.
If there isnt a metal, graphite or platinum is used as the
electrode. When a gas is involved as one of the conjugates, a
special gas electrode is used.
Sometimes spectator ions are present (not involved in the
reaction.) The oxidation reaction is always at the anode. (Copper
is the oxidant)
The reduction reaction is always at the cathode. (Zinc is the
reductant) Remember to combine the two half cell equations for the
overall reaction.
Summary: (Daniel Cell)- Salt Bridge is KNO3:
Why is electrical energy released? By separating the two half
cells, a current is allowed to produce between them. This allows
the chemical energy of the bonds in the substances, to convert to
electrical energy (for use in batteries, etc.) If the cells were
not separated then there is no current, and instead of the chemical
energy transforming to electrical energy, in forms into thermal
energy. Writing Half Cell Equations? Half cell equations involving
a redox pair contains an atom and a simple ion. It can also be
taken from the electrochemical series. (For example, If Zn is the
electrode, we know it forms Zn+2 as a redox pair- so this will be
in the solution.)
Where it may get tough is when we have to work it out for
polyatomic ions half cell equations. In which case, just adopt
KOHES (from Unit 3 Redox) and find the corresponding redox pair
molecule.
The Electrochemical Series: Even though substances involved in
redox reactions may both want to lose electrons, they will differ
in their tendencies to do so. This will mean one loses its
electrons (oxidized) while the other is reduced. For example, Zinc
loses its electrons more readily than copper (we say it is more
reactive.) The stronger reductant will be at the anode (it is in an
oxidation reaction.) The stronger oxidant will be at the cathode
(it is in a reduction reaction.) How do we know what substance
between the half cells will be stronger oxidants or reductants?
This is done using an electro chemical series. The left side of the
of the series increases upwards to the strongest oxidant. The right
side of the series increases downwards to the strongest
reductant.
This means that if we have to half cells and the substances
making them up, we can identify the reactions taking place. The
half cell with the stronger oxidant out of the two will undergo
reduction (at the cathode), the other has the strongest reductant
so will undergo oxidation (at the anode.) For a direct redox
reaction to occur in the cell, a chemical on the left must be
higher and react with a lower chemical on the right side. A Z shape
should be formed. Likewise, the reduction reaction (higher) occurs
forward ways, with the oxidation reaction occurring reverse ways as
read from the electro chemical series.
The electrochemical series is shown below. This is for
conditions at a temperature of 25 degrees Celcius, pressure of 1
atm and 1M concentration of solutions:
Order of equations can change under varying conditions-
Experimentation should occur.
Potential Difference: Also called the EMF. There is a potential
difference between the two half cells. (As one half cell has a
greater tendency to push electrons intro the external circuit than
the other half cell.) It is measured in Volts (V) using a
voltmeter. The Daniel Cell has a potential difference of about
1V.
Cell potentials are given in the electrochemical series for the
listed half cell equations. This is given in E values. The hydrogen
half cell is used as a standard (its cell potential is 0.00V). The
cell potential values may vary from the electrochemical series if
not under the same, standard conditions. The standard half cell
potential is a numerical indication of the tendency of the cells
reaction to occur as a reduction reaction. (Giving away electrons.)
The high E values (most of the time) mean it is a reduction
reaction. Lower E mainly indicate an oxidation reaction. (This is
consistent with the E.C series.) The potential difference of the
overall cell is given by its two half cells:
Note: The lower word says oxidant not oxidation. This means the
OXIDANT comes first (the reduction reaction.) The equation can
similarly be written as:
This is because the oxidant is usually the higher value, and the
reductant usually lower. Limitations of Predictions: If the cells
are not in standard conditions, the series does not hold as
true.
Electrochemical series gives no indication of the rates of
reaction. Some reactions may actually be too slow to occur- the
electrochemical series wont show this! It only gives indication of
extent of reaction (using E values).
Experimentation should be done always to ensure accurate and
reliable results.
Chapter 27: Cells and Batteries:There are 3 types of Galvanic
cells (which we learnt in the last chapter.) These are:
Primary Cells Secondary Cells Fuel CellsPrimary Cells: These
cells are not rechargeable. This is because the products migrate
away from the electrodes after the reaction. Examples of a primary
cell will be shown, these do not have to be memorized.
Primary Cell products migrate away from the electrodes after
reaction. This helps the forward reaction within the cell to go on,
discharging. Chemical to electrical energy conversion takes place.
These cells cannot be recharged as the products migrate away. They
will go flat when there is a buildupof product on the electrodes.
Examples of Primary Cells: The Zinc- Carbon Dry Cell:
Electrons will flow from the Zinc case (negative charge) through
the circuit, and into the metal cap (positive charge.) They will
then flow into the carbon rod, and then into the manganese
dioxide.
The Electrolyte (salt bridge) substances are NH4Cl and ZnCl2 (in
a thick, liquid slurry). Produces usually 1.5V, used in torches.
Anode Reaction: (-)
Cathode Reaction: (+)
Alkaline Cell: Optimized for longevity and performance. Similar
reactions to a Dry cell. Lasts longer, more expensive. They need
less electrolyte than Dry cells, so more reactants can be used.
Electron flow is from the Zinc powder, to the brass anode ( -ve
charge), through the circuit- into the metal cap (+ve charge). They
then flow into the metal casing, results in manganese oxide and
CO2. The electrolyte consists of 7M KOH paste, in a slurry. Anode
Reaction: (-)
The Zinc is oxidized as normal (normal redox equation.) However,
it is then immediately reacted with the OH- group from the
electrolyte, thus overall, at the anode:
Cathode Reaction: (+)
Button cells: Another type of primary cell, much more expensive
because of their small size. They include Lithium cells and
Silver-Zinc cells.
Secondary Cells: These are rechargeable because the products
remain in contact with the electrodes after the reaction. This is
done through electrolysis (explored in next chapter.) Lead Acid
Cell: Used in car batteries. Cheap, reliable, strong current
provider. Can be recharged. Usually consists of three positive
electrodes in between four negative electrodes. A porous separator
is used to avoid contact between each electrode. The positive
electrodes are made of PbO2, the negative electrodes Pb powder. A
solution of about 4M H2SO4 acts as
the electrolyte. This is shown on the next page.
Reaction at Anode:
Pb is oxidizes to Pb2+, which then reacts with SO4-2 in
electrolyte. Oxidation reaction.
Reaction at Cathode: PbO2 is this time oxidized to form Pb+2,
which is then reacted again with SO4-2 from electrolyte to form the
products listed:
Overall reaction:
Note that the reactants in the two half cells are Pb(s) and PbO2
(s). Note that the product of both half cell equations, PbSO4,
forms a precipitate on the surface of the electrodes- allowing them
to be recharged.
To recharge the battery, the electrode reactions are reversed.
This is done by the alternator, which also causes the reverse
reactions to occur. Instead of electrons flowing from anode to
cathode, they will flow from cathode to anode.
When the Battery Recharges the overall equation is:
Nickel Based Cells: Two important ones are the Nickel Cadmium
cell, and as it is superseded, the Nickel metal Hydride cell. Both
are quite similar.
Both consist of a coiled sandwich of anode, a porous separator
and a cathode immersed in a concentrated potassium hydroxide
electrolyte. When connected to an external appliance:
Anode Reaction: In a Nickel Cadmium cell, oxidation of cadmium
generates electrons and Cd+2 ions are formed, which react with OH-
electrolyte to form a precipitate of solid Cd(OH)2.
In a Nickel metal hydride cell, the negative electrode is made
up of a special metal allow (not Cd). At the electrodes surface,
absorbed H+ reacts with OH- from the electrolyte- this forms water
and releases electrons.
Cathode Reaction: Electrons are accepted by the Nickel ions from
the Nickel hydroxide, and are reduced from an oxidation state of 3+
to 2+. The equation is:
The electrode reactions are fully reversible. Enables reactants
to be regenerated when the cell is recharged from the products
precipitated on the electrodes. Nickel Cadmium Cells last up to
1000 recharges (but is toxic), Nickel Metal hydride for 500 (and is
lighter.) Fuel Cells: The reactants in a fuel cell are continuously
supplied.
This means a constant supply of chemical energy- constant
production of electrical energy, no need for constant discharging
and recharging.
These cells are not rechargeable (the products dont stay with
the electrodes for recharging- they do not need to.) They are 80%
efficient, much more efficient than the 30-40% of coal fired power
stations. They are also more efficient in that they produce steam,
for mechanical energy production again after use.
Environmentally efficient- only by products are H2O and heat (in
hydrogen-oxygen fuel cells- which produce about 1 V.) Size of the
current depends on the surface area. Also increases by connecting
several fuel cells together. Here is an example of a
Hydrogen-Oxygen Fuel cell: There is a constant supply of the
reactants here (O2 and H2). The electrolyte is in between the
electrodes (here it is H2PO4-). Anode Reaction: Can be seen on
electrochemical series and the diagram.
Cathode Reaction:
NOTE: If the electrolyte was alkaline instead of acidic, there
would be a different reaction. (From the electrochemical series, a
half equation involving OH- as well as the other substances such as
H2 and O2 would be more appropriate.) Overall, the equation should
be:
Obviously, the half equations must combine to result in this-
which offers hints on what the half equations must be in order to
obtain the overall-there may need some cancelling out in writing
the half equations too. Fuel Cells uses in: Fuel cells are being
investigated as an alternative to the internal combustion engine,
such as in buses and cars. To Generate electricity domestically,
commercially, or in industry. This would need 100kw fuel cell for
domestic use, and several MW fuel cells for industry. Used in
small, portable appliances such as laptops. To recharge these, you
can just replace the fuel cell. Advantages of Fuel Cells: Fuel
Cells convert chemical energy directly to electrical energy, rather
going through heat or mechanical conversions. Hydrogen Fuel cells
produce water as a byproduct, and no greenhouse gases (like other
fuels in power stations.) Generate electricity as long as the fuel
is supplied, rather than needed an entire battery replacement or
recharging. Fuel cells can use a variety of fuels. Electricity
generated on site, no need for connection to electricity grids.
Disadvantages of Fuel Cells: Require constant fuel supply.
Expensive, with technology still developing in limited numbers.
Expensive electrolytes and catalysts in some fuel cells. Fuel cells
generate DC current, but home appliances need AC current so an
inverter is needed for converting current types. May be hydrogen
fuel impurities affecting effectiveness of lower temperature fuel
cells. Hydrogen storage and distribution is limited- so use in
transport requires more work on hydrogen filling stations.
Chapter 28: Electrolysis: If we use electrical energy from a
power source, we can make chemical energy through a redox reaction-
this is electrolysis. This is the reverse of what took place in the
Galvanic cells. Electrolysis has many applications: Electroplating,
extraction of metals from ores, production of NaOH, Cu, Cl, and H,
recharging car batteries. Electrolysis takes place in electrolytic
cells. They are non spontaneous reactions, and require electrical
energy from a power source. Chemicals formed by electrolysis are
usually not easy to obtain.
The cathode in electrolysis is negative electrode. The anode in
electrolysis is positive. Oxidation still occurs at the anode,
reduction still at cathode. The electron flow is from anode (+) to
cathode(-).Electroplating: Electroplating: the deposition of layers
of metal on the surface of another metal. This electroplating is
done in electrolytic cells.
For example, tin cans are generally steel- only a slight layer
is on the surface of the steel is tin. This plating or layering is
applied by electrolysis.
Object to be plated is placed at the negative terminal of a
power supply. (The negative electrode.) It is immersed in solution,
such as tin nitrate (a salt) which acts as an electrolyte. The
electrolyte should contain the metal that is to form the plating.
(This one has tin.) A metal to be plated is used as the positive
electrode. It must be able to conduct electricity. The power supply
is used as a source of electrons, pushing them into the negative
electrode (cathode) and removing them from the positive electrode.
(anode) At the Anode: Electrons are being removed from here, thus
it is oxidation.
At the Cathode: Electrons are being pushed into here, thus it is
reduction.
Overall, the concentration of Tin will be constant. This is
because at one electrode it is consumed, and at the other it is
produced.
Faradays Laws: Even when electroplating some questions must be
asked: How can I determine how much metal is being plated? How long
should I leave the object being plated in the electrolytic cell?
What size electric current should be used?
Electric charge (Q) is measured using the unit Coulomb (C). The
electric charge passing through the cell is calculated using this
and the time (t) for which the current (I) flows: Faradays First
Law:
The more charge passing through the cell, the more metal formed
at the cathode. The mass of the metal produced at the cathode
(where electrons are pushed into) is directly proportional to the
electricity passing through the cell. (m Q) The Second Law of
Electrolysis: We will now be working with electron moles in
equations to work out how much of metal is deposited. This is just
like simple stoichiometry! For example: In order to deposit 1 mol
of Silver from a solution of Ag+ on the cathode, just 1 mol of
electrons is required.
The charge on one mol of electrons must be 96500 C. A Faraday is
the charge on 1 mol of electron, 1 Faraday is known as Faradays
constant.
C molThis value is always used in the second law equation even
if there are 2 (or more) moles of electrons for the reaction (this
is accounted for in n in the equation), shown below:
The equation tells us how much charge is needed to deposit a
metal on an electrode. Similarly, (Q= I * T), so (I *T = n *F).
Note: F is Faradays constant which is 96500 Cmoln is the moles of
ELECTRONS. Be careful to consider how many moles of electrons are
in your reaction! This can be converted using stoichiometry to make
it relevant to your metal. For example, Silver (Ag) has a charge of
+1, there is 1 mol electron for 1 mol of Ag in the reaction. (If
there were 0.5 mol Ag, 0.5 mol electrons.) Lead (Pb) has a charge
of +2, so 2 moles of electrons per 1 mol of Lead. Faradays law
states that for metals, a whole number of 1, 2 or 3 electrons are
consumed to produce 1 mol of a metal. Competition at Electrodes:
Since we know oxidation is at the anode and reduction at the
cathode, it may be possible to predict which reactions occur at the
electrodes. There are often several chemicals present, even the
metal used for the electrode may react. We must decide which
reactions have a greater tendency to occur (electrochemical series
can be used). Dont forget to consider water in these
predictions!
We will look at an example on the next page.
Q: A 1M solution of Nickel Sulfate is with Copper electrodes.
Predict the electrolysis products? Consider chemicals present- The
solution consists of SO4 -2, Ni+2, and H2O. The electrodes are made
of Copper (Cu). This gives us two reductants and two oxidants:
From the electrochemical series we can limit down the possible
reactions. (shown above). Therefore, at the cathode, the reduction
reactions that can occur are:
HOWEVER, the strongest reduction reaction will be the one to
occur (the higher on the electrochemical series). Thus it will be
the Nickel reaction. At the anode, the oxidation reactions can
be:
However, the strongest oxidation reaction will be the one to
occur (the lower on the electrochemical series). Thus it will be
the Copper Reaction. Therefore, the overall equation is:
Note that this must always be concluded by experiment. Factors
affect the electrochemical series order (concentrations of
electrolytes, pressure, current, voltage, temperature). Even some
electrodes permit some reactions to occur over others. Additional
Information on Electrolysis: The EMF (or cell potential) of
electrolytic cells should always be a negative E value, unlike
galvanic cells where it is positive.
Overall in electrolysis, we can utilize electrical energy to
synthesize the production of useful chemicals. Some of these
chemicals are highlighted in Heinemann Chemistry (pg 453) The
energy conversion is obviously electrical energy chemical
energy.
End of Unit 4 Chemistry Notes. Good Luck! Liam M.