• Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction.
  • The mass of a catalyst at the beginning and end of a reaction is the same and they do not form part of the equation nor do they participate chemically in the formation of the products.
  • An industrially important 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.
  • Normally only small amounts of catalysts are needed to have an effect on a reaction.
  • Different processes require different types of catalysts but they all work on the same principle of providing an alternate route for the reaction to occur.
  • They do this by lowering the activation energy required, hence providing a reaction pathway of least resistance by way of requiring less energy.
  • Catalysis is a very important  branch of chemistry in commercial terms as catalysts increase the rate of reaction (hence the production rate) and reduce energy costs.
  • The transition metals are used extensively in this area as they readily donate and accept different numbers of electrons due to their having variable oxidation states.
  • This allows them to flood reaction mixtures with electrons and mop up any leftovers before the next batch of reactants is added to the reactor.

Hydrogenation of Ethylene, IGCSE & GCSE Chemistry revision notes

Nickel is used to catalyse the production of ethane from ethylene

Activation Energy

  • Catalysts work by decreasing the activation energy of a particular reaction by providing an alternative reaction pathway which requires less energy.
  • This means that a higher percentage of the particles will have the minimum energy required to react, hence there is a higher number of successful collisions.
  • Therefore more product molecules are produced in a shorter time, thus increasing the rate of reaction.
  • Reaction profile graphs show the relative energy levels of reactants and products on a graph.
  • The reaction profile graph below shows the effect of catalysts on reactions.


Effect of using catalyst, IGCSE & GCSE Chemistry revision notes

Graph showing the effect of the use of a catalyst on the rate of reaction


  • Catalysts reduce the activation energy as they create alternative pathways requiring lower activation energy, allowing more successful and frequent collisions.
  • This shows that when a catalyst is used, the rate of reaction will increase.
  • Enzymes are nature’s catalysts.
  • They are biological substances that catalyse reactions in living cells.
  • Important reactions that are biologically catalysed include respiration, photosynthesis and protein synthesis.
  • The production of alcohol by the fermentation of sugars occurs in the presence of a biological catalyst, yeast enzymes:

C6H12O6 + enzymes → 2CO2+ 2C2H5OH

AQA GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.