Oxidation & Reduction (Edexcel International AS Chemistry)

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Electron Transfer & Oxidation Numbers

  • Oxidation and reduction in a reaction can be demonstrated in terms of electron transfer
    • For example:

2NH3 + 3Br2 → N2 + 6HBr

  • The oxidation number of N in NH3 has changed from -3 to 0
    • As the oxidation number has increased, nitrogen has been oxidised
  • The oxidation number of Br has changed from 0 to -1 
    • As the oxidation number has decreased, bromine has been reduced 
  • Overall, nitrogen has reduced bromine by donating electrons 

Reducing & Oxidising Agents

Oxidising agent

  • An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons
  • An oxidising agent itself gets reduced – gains electrons
  • Therefore, the ox. no. of the oxidising agent decreases

Example of an oxidising agent in a chemical reaction

Reducing agent

  • A reducing agent is a substance that reduces another atom or ion by causing it to gain electrons
  • A reducing agent itself gets oxidised – loses / donates electrons
  • Therefore, the ox. no. of the reducing agent increases

Example of a reducing agent in a chemical reaction

  • For a reaction to be recognised as a redox reaction, there must be both an oxidising and reducing agent
  • Some substances can act both as oxidising and reducing agents
  • Their nature is dependent upon what they are reacting with and the reaction conditions

Worked example

Four reactions are shown.

In which reaction is the species in bold acting as
an oxidising agent?

  1. Cr2O72- + 8H+ + 3SO32- → 2Cr3+ + 4H2O + 3SO42-
  2. Mg + Fe2+ → Mg2+ + Fe
  3. Cl2 + 2Br- → 2Cl- + Br2
  4. Fe2O3 + 3CO → 2Fe + 3CO2

Answer:

    • The correct option is 2
    • Oxidising agents are substances that oxidise other species, gain electrons and are themselves reduced.
    • Write down the oxidation numbers of each species in the reaction

Oxidising & Reducing Agents Worked Example equation, downloadable AS & A Level Chemistry revision notes

    • In equation 2, Fe2+ oxidises Mg(0) to Mg2+(+2) and is itself reduced from Fe2+(+2) to Fe(0)

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Philippa

Author: Philippa

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.