Catalysts (Oxford AQA International A Level Chemistry)

Revision Note

Philippa Platt

Written by: Philippa Platt

Reviewed by: Stewart Hird

Catalysts & Rate of Reaction

  • Catalysis is the process in which the rate of a chemical reaction is increased, by adding a catalyst

  • A catalyst increases the rate of a reaction by providing the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalysed reaction

  • Catalysts can be divided into two types:

    • Homogeneous

    • Heterogeneous

  • Homogeneous means that the catalyst is in the same phase as the reactants, for example

    • Concentrated sulfuric acid in esterification

  • Heterogeneous means that the catalyst is in a different phase to the reactants, for example

    • Aluminium oxide in cracking hydrocarbon chains from crude oil

    • Iron in the Haber process (production of ammonia)

    • Catalytic convertors in cars exhausts

reaction-kinetics-catalyst-pathway
The diagram shows that the catalyst allows the reaction to take place through a different mechanism, which has a lower activation energy than the original reaction

Maxwell-Boltzmann distribution curve

  • Catalysts provide the reactants another pathway which has a lower activation energy

  • By lowering Ea, a greater proportion of molecules in the reaction mixture have the activation energy, and therefore have sufficient energy for an effective collision

  • As a result of this, the rate of the catalysed reaction is increased compared to the uncatalyzed reaction

Reaction Kinetics Catalyst Boltzmann Distribution,
The diagram shows that the total shaded area (both dark and light shading) under the curve shows the number of particles with energy greater than the Ea when a catalyst is present. This area is much larger than the dark shaded area which shows the number of particles with energy greater than the Ea without a catalyst

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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.