Reversible Reactions and Equilibria
6 marks · NormalThe following equation represents a reversible reaction carried out in a sealed container:
State what would happen to the position of equilibrium if the temperature of the system is increased.
Explain why a catalyst does not change the position of equilibrium in this reaction.
The reaction mixture is left for several hours at a constant temperature and with a catalyst present. Would changing the catalyst affect the final amount of SO₃ present at equilibrium? Explain your answer.
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(a) Equilibrium shifts to the left, favouring the reverse (endothermic) reaction.
(b) A catalyst increases the rate of both the forward and reverse reactions equally, so it does not change the position of equilibrium.
(c) Changing the catalyst does not affect the final amount of SO₃ at equilibrium, because catalysts only speed up how quickly equilibrium is reached but do not change the equilibrium position.
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Marking points
B1 (a) · 1 mark · Identifies direction of shift: left/reverse
B2 (a) · 1 mark · Recognises this is the endothermic direction
B3 (b) · 1 mark · States that catalyst increases rate of both forward and reverse reactions
B4 (b) · 1 mark · States that position of equilibrium is unchanged
B5 (c) · 1 mark · States that the final amount of SO₃ does not change
B6 (c) · 1 mark · Explains that catalyst affects only rate, not equilibrium position
Worked solution
(a) Increasing temperature favours the endothermic direction.
For this reaction, the forward reaction (to SO₃) is exothermic (ΔH negative), so the reverse is endothermic.
Therefore, the equilibrium shifts to the left (reverse reaction favoured).
(b) A catalyst increases the rate of both the forward and reverse reactions by the same amount.
This means that it does not change the position of equilibrium.
(c) Changing the catalyst may change how quickly equilibrium is reached, but it does not affect the final concentrations or amounts of the products or reactants.
Therefore, the final amount of SO₃ at equilibrium stays the same.