How does a catalyst work?
3 marks · NormalA student says: 'When you add a catalyst, more particles in the reaction mixture have the required activation energy to react.'
Explain, with reference to reaction profile diagrams, whether this statement is correct.
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The statement is partially correct, but the reason is not clearly stated. A catalyst does not give particles more energy. Instead, it provides an alternative pathway for the reaction with a lower activation energy. On a reaction profile diagram, this is seen as a lower 'hump.' Because the activation energy is lower, more particles already have enough energy to react, so the reaction is faster.
Also accept: A catalyst provides an alternative route with lower activation energy; this makes it so a greater fraction of particles can react, but the actual energy of the particles does not increase.
Common mistakes: Saying particles have more energy after adding a catalyst; Not referring to the profile diagram or activation energy
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Marking points
M1 · 1 mark · States that a catalyst gives an alternative pathway with lower activation energy (seen as a lower peak on the reaction profile diagram).
M2 · 1 mark · Makes clear that the catalyst does not give the particles more energy.
A1 · 1 mark · Explains that, because the required activation energy is lower, a greater fraction of particles already have enough energy to react.
Worked solution
The student is not entirely correct. A catalyst does not change the energy of the particles; it does not make them move faster or have higher energy.
On a reaction profile diagram, adding a catalyst is shown as a lower peak, meaning the activation energy is lower.
Since more particles already have energy greater than or equal to this lower activation energy, more of them are able to react. This is why a reaction is faster with a catalyst.