IB Chemistry SL

Revision Notes

9.1.3 Oxidizing & Reducing agents

Oxidizing & 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

Oxidising & Reducing Agents Table







Identifying Oxidizing & Reducing Agents

  • Applying the definitions of oxidising and reducing agents allows you to identify them in chemical equations
  • By deducing the oxidation numbers of the species you can determine whether it has ben oxidised or reduced






A visual reminder of oxidation numbers and redox. This is like an elevator in a building going up to higher floors is oxidation and going down to the basement is reduction

Worked Example

Four reactions are shown. In which reaction is the species in blue acting as an oxidising agent?

A.  Cr2O72- + 8H+ + 3SO32- → 2Cr3+ + 4H2O+ 3SO42-

B.  Mg + Fe2+ → Mg2+ + Fe

C.  Cl2 + 2Br → 2Cl–  + Br2

D.  Fe2O3 + 3CO → 2Fe + 3CO2


The correct option is B.

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

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