1. C5f Equilibria
In C1 you learnt that a chemical change is one in which a ________ substance is made and
that it is an _____________________ change.
That’s not quite true, some changes are reversible.
• In a reversible reaction there is a forward and a backward reaction.
e.g.
Learn
this • In some reversible reactions both forward and backward reactions are proceeding
definition
at the same time.
• If the forward reaction happens at the same __________ as the reverse reaction,
then we say the system has reached an _____________________.
(not all reversible reactions do this)
• When a system is in equilibrium the _____________________ of the reactants
and products does not change (because the forward and reverse reactions happen at the same
rate)
• Reactions that can reach an equilibrium are given a special sign:
• A system in equilibrium does not necessarily have 50% reactants and 50% products.
When the position of equilibrium is on the right the concentration of __________
is greater than the concentration of ______________.
When the position of equilibrium is on the left the concentration of ___________
is greater than the concentration of _______________.

2. NOTE: To get a high yield of product the equilibrium needs to be as far towards the
________________ (right) as possible.
Higher
Why do reversible reactions sometimes reach an equilibrium:
Fizzy drinks are made fizzy by the dissolved CO2 in them. Consider what happens when you
put the lid back on a bottle of drink after you have poured yourself a glass:
Very importantly the system is closed (nothing can get in or out).
1. The dissolved CO2 comes out of solution and into the air quickly at first but then the
rate _______________.
2. Some of the CO2 in the air starts to __________________ into the solution. The
rate _______________ as more CO2 is present in the air.
3. Eventually the forward reaction gets ___________ and the reverse gets _________
until the two reactions happen at the _________ rate.
Summary
• initially rate of forward reaction decreases
• initially rate of backward reaction increases
• eventually rate of forward equals rate of
backward reaction.

3. How do you increase the yield of the product?
• A closed system will remain in equilibrium as long as none of the conditions change.
• So, if you want to increase the yield of products you need to change the conditions in a
way that moves the equilibrium to the _____________ side of the equation (right).
• The conditions that could be changed are:
1.
2.
3.
NOTE: An equilibrium position will move to try and oppose/minimise any changes
to conditions
We are going to use the Haber process to consider how to increase the yield
of a reaction in equilibrium (also in C4)
1. Temperature
• If you increase the temperature, the equilibrium will try to decrease it by moving in the
______________________ direction
• If you decrease the temperature, the equilibrium will try to increase it by moving in the
______________________ direction.
So, for the Haber Process you need to ____________________ the temperature to
increase the yield.
• BUT they actually use a high temperature of 450oC, why?

4. 2. Pressure
• If you increase the pressure the equilibrium will try to reduce it by moving to the side with
the _______________ number of moles of gas molecules
• If you decrease the pressure the equilibrium will try to increase it by moving to the side with
the _______________ number of moles of gas molecules
For the Haber Process there are ____ moles of reactants altogether and only ___ moles
of products so it will move to the ______________ side and _____________ the yield of
the products.
3. Concentration
• Removing a product moves the position of equilibrium to the ____________
• Adding extra reactant moves the position of equilibrium to the ____________
How do they use this in the Haber Process?

5. The Contact Process
• The Contact Process is used to make ____________________
• The raw materials used to make sulphuric acid by the Contact Process:
1.
2.
3.
• The production of sulfuric acid by the Contact Process involves the reversible reaction
between sulfur dioxide and oxygen:
• the conditions used in the Contact Process:
1. V = vanadium (transition metal)
2.
3. (i.e. no change in pressure)
There are 3 stages in the manufacture of sulfuric acid by the Contact Process:
Stage one:
Stage two: > 95% yield
Stage three:
Explain the conditions used in the Contact Process:
1. high temperature ________________ the yield but ________________ the rate of reaction
so an optimum is used
2. catalyst increases the rate but it does not change position of equilibrium
3. position of equilibrium is already on right (> 95% yield) so a high pressure is not
needed as it is expensive to maintain.

6. Practical
Reversible reactions between acids and alkalis using an indicator.
Reversible reactions between chromate and dichromate.
Demonstration of the reaction of BiCl3 in concentrated HCl with water.
Fizzy pop
Show a video about Contact Process.