Catalysts (WJEC GCSE Chemistry: Combined Science)

• Catalysts are substances which speed up the rate of a reaction while remaining chemically unchanged 
• 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)

Catalysts in industry

• The same catalyst does not work for all reactions
• Like all catalysts, the ones used in any industrial reaction are not used up
  • However, catalysts are affected by impurities and become less active over time
  • This means that need to be replaced regularly

The Effect of a Catalyst

Graph showing the effect of a catalyst on rate of reaction

Adding a catalyst increases the 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

Higher Tier

• As previously mentioned, catalysts increase the rate of a reaction while remaining chemically unchanged 
• Different processes require different types of catalysts but they all work on the same principle of providing an alternative reaction pathway that has a lower activation energy
• This means a higher proportion of the reactant particles have energy greater than the activation energy
  • This results in more successful collisions per second

Comparing a chemical reaction to a journey

The catalysed journey requires less energy and is faster than the uncatalysed journey. A catalysed reaction has a lower activation energy which means that less energy is required for the reaction to occur, resulting in a faster reaction