Specification Point 7.6:
  • Describe a catalyst as a substance that speeds up the rate of a reaction without altering the products of the reaction, being itself unchanged chemically and in mass at the end of the reaction.
  • 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.
  • An example we have seen already is iron, which is used to catalyse the Haber Process.
  • 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.
  • Catalysis is a very important branch of chemistry in commercial terms as they 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, GCSE Chemistry

Nickel is used to catalyse the production of ethane from ethylene

Activation Energy

Specification Point 7.7:
  • Explain how the addition of a catalyst increases the rate of a reaction in terms of 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 are a higher number of successful collision.
  • 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 a Catalyst

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.


Specification Point 7.8:
  • Recall that enzymes are biological catalysts and that enzymes are used in the production of alcoholic drinks.
  • 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

Edexcel 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.