Edexcel A Level Chemistry:复习笔记2.1.2 Types of Reduction & Oxidation

Electron Transfer & Change in Oxidation Number


  • 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

Oxidising & Reducing 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


    • 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




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