CatalystsOxford AQA IGCSE Chemistry: Revision notes

A catalyst is a substance that increases the rate of a chemical reaction without being used up or permanently changed in the reaction itself.

• Catalysts are not included in the overall symbol equation, but are often written above the arrow
• Different reactions require different catalysts — a catalyst that works for one reaction will not necessarily work for another
• Because they are not used up, a small amount of catalyst can be reused repeatedly

A catalyst works by providing an alternative reaction pathway with a lower activation energy than the uncatalysed reaction.

• Activation energy is the minimum energy particles need to react when they collide
• Because the activation energy is lower, a greater proportion of particle collisions have enough energy to react
• This means more successful collisions happen in a given time, increasing the rate of reaction

On an energy level diagram, the overall energy change of the reaction (the difference in energy between reactants and products) stays exactly the same whether or not a catalyst is used — a catalyst does not change the overall energy change.

What changes is the height of the 'hump' between reactants and products, representing the activation energy:

• The uncatalysed pathway has a higher hump (higher activation energy)
• The catalysed pathway has a lower hump (lower activation energy), because it follows a different route between reactants and products

Catalysts are widely used in industrial processes (e.g. iron in the Haber process, vanadium(V) oxide in the Contact process) because:

• They increase the rate of reaction, so products form more quickly
• Reactions can often be carried out at lower temperatures and/or pressures than would otherwise be needed, which reduces energy costs
• This reduces the overall cost of manufacturing chemicals on a large scale