Adding a catalyst
4 marks · NormalChemists compare the effect of adding a catalyst to two different reversible reactions, P and Q.
| Reaction | Forward rate without catalyst (mol/dm³/s) | Reverse rate without catalyst (mol/dm³/s) |
|---|---|---|
| P | 0.03 | 0.03 |
| Q | 0.07 | 0.02 |
For which reaction, P or Q, is equilibrium reached faster after adding a catalyst? Explain your answer.
State whether the position of equilibrium changes for either reaction when a catalyst is added.
Give a reason for your answer to part (b).
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(a) Both reactions, P and Q, will reach equilibrium faster once a catalyst is added, because a catalyst increases the rates of both reactions equally.
(b) No, the position of equilibrium does not change for either reaction.
(c) A catalyst increases the rate of both forward and reverse reactions by the same amount, so the ratio of products to reactants at equilibrium is unchanged. · must show Both directions sped up equally, No change to equilibrium composition
Common mistakes: Saying catalyst moves equilibrium; Focusing only on one direction
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States both P and Q reach equilibrium faster / increased reaction speed for both reactions.
M2 (b) · 1 mark · States equilibrium position does not change for either reaction.
A1 (c) · 2 marks · Explains that a catalyst speeds up both reactions equally so the relative concentrations at equilibrium are unchanged.
Worked solution
a) Both P and Q will reach equilibrium faster after a catalyst is added because the rates of both forward and reverse reactions increase.
b) The position of equilibrium does not change for either reaction.
c) This is because a catalyst increases the rate of both the forward and reverse reactions by the same amount, so the ratio of products to reactants at equilibrium is unchanged.