Adding a catalyst
3 marks · NormalA reversible reaction is at dynamic equilibrium in a sealed container. The forward reaction is exothermic.
A catalyst is added that increases the rate of both the forward and reverse reactions equally.
Explain, using calculations or reasoning, why adding a catalyst does not change the position of equilibrium for this reaction.
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Adding a catalyst increases the rate at which equilibrium is reached by speeding up both the forward and reverse reactions by the same proportion. Therefore, the relative rates remain unchanged and the concentrations of reactants and products at equilibrium stay the same. The position of equilibrium is not altered by the presence of a catalyst.
Must show: states no change in equilibrium position, explains both forward and reverse rate are increased equally, explains that the ratio of products to reactants at equilibrium does not change
Also accept: A catalyst affects the speed but not the equilibrium concentrations because it lowers activation energy equally for both directions.
Common mistakes: Claiming that a catalyst increases yield at equilibrium; Saying that catalyst shifts equilibrium
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Marking points
M1 · 1 mark · States that catalyst speeds up (increases rate of) both forward and reverse reactions equally.
M2 · 1 mark · Explains that the relative rates are unchanged or that ratio of products to reactants does not change.
A1 · 1 mark · Concludes that the position of equilibrium does not change (supported by reasoning).
Worked solution
A catalyst works by lowering the activation energy for both the forward and reverse reactions.
This means both the forward and reverse reactions speed up by the same factor.
Let the uncatalysed rate constants be (forward) and (reverse). With a catalyst, they become and , where .
The equilibrium constant is given by . With a catalyst, .
Therefore, the concentrations of reactants and products at equilibrium do not change. The position of equilibrium is unaffected by a catalyst.