Catalysts (AQA GCSE Chemistry: Combined Science)

• 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

A reaction profile showing the effect of using a catalyst on the activation energy of a reaction

• This means a higher proportion of the reactant particles have energy greater than the activation energy and will result in more successful collisions per second

• An important industrial example is iron, which is used to catalyse the Haber Process for the production of ammonia
  • Iron beads are used to increase the surface area available for catalysis

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