Adding a catalyst
5 marks · HardA student investigates the following reversible reaction at equilibrium: \[ \ce{CO(g) + H2O(g) \rightleftharpoons CO2(g) + H2(g)} \] The student adds a catalyst to the closed reaction vessel.
State what happens to: (i) the rate at which equilibrium is reached (ii) the position of equilibrium after the catalyst is added.
the rate at which equilibrium is reached
the position of equilibrium
Explain, with reasons, why a catalyst does or does not affect the position of equilibrium in a reversible reaction. Include reference to the rates of the forward and reverse reactions in your answer.
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(a(i)) The rate at which equilibrium is reached increases (or becomes faster).
(a(ii)) The position of equilibrium does not change.
(b) A catalyst does not affect the position of equilibrium because it increases the rate of both the forward and reverse reactions by the same amount. This means the ratio of products to reactants at equilibrium stays the same; only the time taken to reach equilibrium is reduced.
Common mistakes: Saying catalyst changes yield or position of equilibrium
Show mark scheme and worked solution
Marking points
B1 (a(i)) · 1 mark · States that the rate at which equilibrium is reached increases or equilibrium is reached faster.
B2 (a(ii)) · 1 mark · States that the position of equilibrium does not change.
M1 (b) · 1 mark · Mentions a catalyst affects rates of reaction or increases the speed of both directions.
M2 (b) · 1 mark · Explains that the catalyst increases both the forward and reverse reaction rates equally.
A1 (b) · 1 mark · Concludes that the position of equilibrium is unchanged so the ratio of products and reactants at equilibrium is the same.
Worked solution
a(i) The rate at which equilibrium is reached increases because the catalyst speeds up both reactions.
a(ii) The position of equilibrium does not change because the catalyst does not favour either side.
b) A catalyst works by increasing the rate of a chemical reaction. In a reversible reaction, a catalyst increases both the forward and reverse reaction rates by the same factor. Since both directions speed up equally, the composition of the mixture at equilibrium (the position of equilibrium) remains the same. Therefore, adding a catalyst makes the system reach equilibrium faster, but does not alter the final proportions of reactants and products.