Catalysts: Students should be able to explain catalytic action in terms of activation energy.

The idea

A catalyst speeds up a reaction without being used up. It works by providing an alternative reaction pathway with a lower activation energy, the minimum energy that colliding particles need to react. On an energy profile, the reactants sit at one level and the products at another; the peak between them is the activation energy. With a catalyst, that peak is lower, so a greater proportion of collisions have enough energy to succeed, increasing the rate. For example, adding a catalyst to the decomposition of hydrogen peroxide lowers the energy barrier, so oxygen is produced faster at the same temperature. The catalyst is not consumed, so it can be recovered chemically unchanged at the end.

What earns the marks

• A catalyst increases the rate of a reaction without being used up or chemically changed at the end.
• Activation energy is the minimum energy that particles must have when they collide in order to react.
• A catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalysed route.
• On an energy profile, the catalysed pathway has a lower peak between reactants and products than the uncatalysed pathway.
• Because the energy barrier is lower, a greater proportion of collisions possess at least the activation energy, so more successful collisions occur per unit time.
• The overall energy change between reactants and products is unchanged by a catalyst; only the activation energy is lowered.

Common mistakes and how to put them right

• Saying a catalyst lowers the energy of the reactants or products.

  Put it right: It does not change their energies; it lowers the activation energy peak of the pathway.

• Stating that a catalyst increases the energy of collisions or makes particles collide more often.

  Put it right: It lowers the energy barrier, allowing more existing collisions to be successful.

• Claiming a catalyst is used up or becomes part of the product.

  Put it right: It is recovered chemically unchanged, which is why only a small amount is needed.

Exam tips

• Sketch or annotate an energy profile with two curves and label the lower peak as the catalysed route with lower activation energy.
• Use the phrase 'alternative pathway with lower activation energy' rather than 'lowers the activation energy of the reaction' alone.
• Link the lower barrier to a greater proportion of successful collisions to explain the increased rate, not just to a lower number.