AQA A-Level Chemistry Paper 3, 2023: Question 12

Which statement explains why the rate of reaction is greater with a catalyst?

• A: The collision frequency increases because the catalysed reaction has a lower activation energy.
• B: The collision frequency increases because there is an increase in the average energy of the particles.
• C: The proportion of successful collisions increases because the catalysed reaction has a lower activation energy.
• D: The proportion of successful collisions increases because there is an increase in the average energy of the particles.

The correct explanation is that a catalyst lowers the activation energy required for the reaction. This means a greater proportion of particles have sufficient energy to overcome the activation barrier, leading to an increased rate of successful collisions.

• A: A catalyst does not increase collision frequency. Instead, it makes collisions more effective by lowering the activation energy.
• B: The average energy of particles is related to temperature, not the presence of a catalyst.
• D: While successful collisions do increase, this is not due to an increase in particle energy but because the activation energy is reduced.

A catalyst provides an alternative reaction pathway with a lower activation energy. It remains chemically unchanged at the end of the reaction. By lowering the energy threshold, more particles can successfully react even at the same temperature.

The correct explanation is:

C: The proportion of successful collisions increases because the catalysed reaction has a lower activation energy.