Catalysts (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Catalysts

• 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

• 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

• 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

• Enzymes act as catalysts in biological systems

The effect of a catalyst

Graph showing the effect of a catalyst on rate of reaction

Explanation

• Compared to a reaction without a catalyst, the line graph for the same reaction with a catalyst:

  • Has a steeper gradient at the start

  • Becomes horizontal sooner

  • Forms the same amount of product

• This shows that the addition of a catalyst increases the rate of reaction