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Revision on Equilibrium 2
1. Chemical Equilibrium
Saturday, 16 May 2026Chemical Equilibrium
REVISION
2. Assessment Objectives
• 11.3.3.1 Recall and be able to use ideas from previous teaching onequilibria, including the concept of dynamic equilibrium
• 11.3.3.2 Be able to predict the effects change of temperature,
concentration and pressure on equilibria
• 11.3.3.3 Understand why a catalyst does not affect the position of
equilibrium
• 11.3.3.4 Understand the importance of equilibria in industrial chemistry
• 11.3.3.5 Understand (with examples) why reaction conditions for industrial
processes involving equilibria are often a compromise
• 11.3.3.6 Be able to write the equilibrium constant (Kc) for a given reaction
• 11.3.3.7 Be able to predict the effect of changing temperature on Kc and
understand that Kc is not affected by changes in concentration or the
addition of a catalyst
3. Reversible Reactions
Saturday, 16 May 2026Reversible Reactions
⇌
Generic Example -
A+B⇌C+D
At this point the rate of the
Don’t get confused! forwards reaction EQUALS the
rate of the backwards
This is NOT the
reaction. We call this a
same as the
DYNAMIC EQUILIBRIUM
amount of reactant
The concentration of each
and product is the
substance remains constant
same.
EQUILIBRIUM
REACHED
Reversible reactions are
represented by this arrow-
Concentration
Reversible reactions are where reactions can go forward and backward
Time (s)
Only
occurs in
closed
systems
KEY
Reactant
Product
Forward Reaction
Initially reactants are
used up quickly but then
slow as their
concentration drops.
Backward Reaction
Initially reactants are
reformed slowly but then
speed up as the
concentration of products
increases.
4. Le Chatelier’s Principle – Changing Concentration
Saturday, 16 May 2026Le Chatelier’s Principle – Changing Concentration
If we increase the concentration of a reactant OR product the
equilibrium will shift to try to reduce the concentration. (The
opposite will happen if concentration is decreased)
REMEMBER - Le
Chatelier’s Principle
If a reaction at
equilibrium is subjected
to a change in pressure,
temperature or
concentration, the
position of equilibrium
will move to counteract
the change.
If we increase the concentration of H2 equilibrium will shift to the right
to use it up and reduce the concentration. More NH3 will be produced
3H2(g) + N2(g) ⇌ 2NH3(g)
If we increase the concentration of NH3 equilibrium will shift to the left
to use it up and reduce the concentration. More H2 and N2 produced.
3H2(g) + N2(g) ⇌ 2NH3(g)
5. Le Chatelier’s Principle – Changing Pressure
Saturday, 16 May 2026Le Chatelier’s Principle – Changing Pressure
REMEMBER - Le Chatelier’s Principle
If a reaction at equilibrium is subjected to a change in pressure, temperature or
concentration, the position of equilibrium will move to counteract the change.
If we increase the pressure the equilibrium will shift to try to reduce
the pressure. (The opposite will happen if pressure is decreased)
If we increase the pressure equilibrium will shift to the side with the fewest
number of gas particles this will reduce the pressure. More NH3 will be produced
3H2(g) + N2(g) ⇌ 2NH3(g)
If we decrease the pressure equilibrium will shift to the side with the most
number of gas particles this will increase the pressure. More N2 and H2 will be
produced.
3H2(g) + N2(g) ⇌ 2NH3(g)
6. Le Chatelier’s Principle – Changing Temperature
Saturday, 16 May 2026Le Chatelier’s Principle – Changing Temperature
REMEMBER - Le Chatelier’s Principle
If a reaction at equilibrium is subjected to a change in pressure, temperature or
concentration, the position of equilibrium will move to counteract the change.
Forward
reaction is
exothermic. ΔH
is negative
If we increase the temperature the equilibrium will shift to try to reduce
the temperature. (The opposite will happen if temperature is decreased)
If we increase the temperature equilibrium will shift in the endothermic
direction this will reduce the temperature. More N2 and H2 will be produced
3H2(g) + N2(g) ⇌ 2NH3(g) ΔH = -46.2kJmol-1
If we decrease the temperature equilibrium will shift in the exothermic
direction this will increase the temperature. More NH3 will be produced.
3H2(g) + N2(g) ⇌ 2NH3(g) ΔH = -46.2kJmol-1
7. Le Chatelier’s Principle – Catalysts
Saturday, 16 May 2026Le Chatelier’s Principle – Catalysts
REMEMBER - Le Chatelier’s Principle
If a reaction at equilibrium is subjected to a change in pressure,
temperature or concentration, the position of equilibrium will move to
counteract the change.
Catalysts have no effect on the position of equilibrium.
A catalyst will speed up the rate of the forward and backward
reaction equally.
A catalyst will speed up the rate at which equilibrium is
reached but have no effect on yield.
8. Kc – The Equilibrium Constant
Saturday, 16 May 2026Kc – The Equilibrium Constant
Kc can be worked out from the stoichiometric coefficients in a reaction
The coefficients in front of each species are
powers in the Kc expression
2A + B ⇌ 2C + D
Square brackets
mean
concentration
of…
Write the Kc expression and calculate the value of
Kc of the following reaction [SO2] = 0.4moldm-3 ,
[O2] = 0.2moldm-3, [SO3] = 0.8moldm-3
2SO2(g) + O2(g) ⇌ 2SO3(g)
The expression
Kc = [SO3]2
[O2] [SO2]2
Write the units top and
bottom and cancel. Invert
the powers if there is more
units under the line. Do
nothing if it is on the top.
Kc = [C]2 [D]
[A]2 [B]
The calculation
Kc = [0.8]2
[0.2] [0.4]2
Products
Reactants
Kc = 20 mol-1dm3
The units
Kc = moldm-3 moldm-3
moldm-3 moldm-3 moldm-3
9.
10.
[CO]eq = 0.20 – 0.09534 = 0.105 M x 5 L= 0.525 mol
[H20]eq = 0.20 – 0.09534 = 0.105 M x 5 L
= 0.525 mol
[H2]eq = 0.0953 M x 5 L = 0.477 mol
[CO2]eq = 0.0953 M x 5 L = 0.477 mol