How does a catalyst work?
4 marks · NormalThe diagram below shows a reaction profile for an exothermic reaction, both with and without a catalyst.
Identify which curve on the diagram represents the reaction with a catalyst.
On the diagram, mark with a label 'Ea (with catalyst)' the activation energy for the catalysed reaction.
State what happens to the enthalpy change (ΔH) of the reaction when a catalyst is added.
Explain why the reaction happens faster when a catalyst is used.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) The lower curve is with the catalyst (the catalysed route).
(b) A vertical arrow/label from the reactants to the catalysed transition state ('TS_C'), showing the lower activation energy, labelled 'Ea (with catalyst)'. · must show Arrow from reactants level to the catalysed route's highest point, labelled 'Ea (with catalyst)'
(c) The enthalpy change (ΔH) stays the same when a catalyst is added.
(d) The reaction is faster because more particles have enough energy to react as the activation energy is lower with a catalyst.
Must show: 'Ea (with catalyst)' correctly placed (vertical distance from reactants to catalysed TS)
Common mistakes: Saying ΔH changes with a catalyst; Labelling activation energy from products instead of reactants
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly identifies the lower curve as the catalysed reaction.
B2 (b) · 1 mark · Draws and labels the activation energy for the catalysed reaction as a vertical arrow from Reactants to catalysed transition state.
B3 (c) · 1 mark · States that the enthalpy change (ΔH) does not change when a catalyst is added.
B4 (d) · 1 mark · Explains that more particles can react because there is a lower activation energy with a catalyst.
Worked solution
a) The curve with the lower peak (the lower energy route) shows the catalysed reaction.
b) 'Ea (with catalyst)' is marked as a vertical distance from reactants to the catalysed transition state.
c) The enthalpy change (ΔH) does not change when a catalyst is added.
d) A catalyst increases the reaction speed because more particles have energy greater than or equal to the lower activation energy, so more successful collisions occur.