Catalysts (Edexcel IGCSE Science (Double Award))

The effect of a catalyst on rate of reaction

• Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction

  • Normally only small amounts of catalysts are needed to have an effect on a reaction

  • The mass of a catalyst at the beginning and end of a reaction is the same 

• Catalysts do not form part of the chemical equation but they are sometimes seen above or below the reaction arrow: 

N₂ (g) + 3H₂ (g)  2NH₃ (g)

SO₂ (g) + O₂ (g)  SO₃ (g)

• Different processes require different types of catalysts but they all work on the same principle:

  • They provide an alternative pathway for the reaction to occur

  • The alternative pathway has a lower activation energy

Catalysts work by attracting reactant molecules on to the surface and so providing an alternate reaction pathway of lower energy

• Catalysis is a very important  branch of chemistry in commercial terms as catalysts increase the rate of reaction (hence the production rate) and they reduce energy costs

• The transition metals are used widely as catalysts as they have variable oxidation states allowing them to readily donate and accept different numbers of electrons

  • This is key to their catalytic activity

• Enzymes act as catalysts in biological systems