How a catalyst works
A catalyst is a substance that increases the rate of a chemical reaction without undergoing an overall change in its chemical composition or amount.
A catalyst provides an alternative reaction route whose activation energy is lower than that of the uncatalysed reaction.
- The uncatalysed reaction follows a pathway with a higher activation-energy barrier.
- The catalysed route has a lower activation-energy barrier.
- More collisions can therefore satisfy the energy requirement for reaction.
- Successful collisions occur more frequently, so the reaction rate increases.
Comparing reaction pathways
Both pathways start with the same reactants and finish with the same products. The catalyst changes the route between them rather than the energies of the reactants or products.
The catalysed pathway therefore has a smaller , while the overall energy change, , is unchanged.
Key idea: a catalyst lowers the activation energy by supplying a different route; it does not lower the energies of the reactants or products.
Exam Tip: When explaining a catalyst, state that it provides an alternative reaction route with a lower activation energy. On a reaction profile, keep the reactant and product energy levels unchanged and draw a lower maximum for the catalysed route.