Oxidation & Reduction (DP IB Chemistry): Revision Note

Oxidation & Reduction

Oxidising agents

• An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons

• An oxidising agent itself gets reduced

  • Oxidising agents gain electrons

• Therefore, the oxidation number of the oxidising agent decreases

H₂O₂ acting as the oxidising agent

• Example reaction:

Fe²⁺ + H₂O₂ → Fe³⁺ + H₂O

• Fe²⁺ → Fe³⁺

  • Fe²⁺ loses an electron to become Fe³⁺

  • The oxidation number changes from +2 to +3

• H₂O₂ → H₂O

  • H₂O₂ gains electrons to form H₂O

  • The oxidation number of O changes from –1 to –2

• Therefore, H₂O₂ is acting as an oxidising agent

Reducing agents

• 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 oxidation number of the reducing agent increases

H₂O₂ acting as a reducing agent

Example reaction:

Fe³⁺ + H₂O₂ → Fe²⁺ + O₂

• Fe³⁺ → Fe²⁺

  • Fe³⁺ gains an electron to become Fe²⁺

  • The oxidation number changes from +3 to +2

• H₂O₂ → O₂

  • H₂O₂ loses electrons to form O₂

  • The oxidation number of oxygen changes from –1 to 0

• Therefore, H₂O₂ is acting as a reducing agent

Redox reactions

• 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 depends on:

  • What they are reacting with

  • The reaction conditions

Comparing oxidising and reducing agents:

• Oxidising agents:

  • Oxidise other species

  • Accept electrons

  • Are themselves reduced

• Reducing agents:

  • Reduce other species

  • Donate electrons

  • Are themselves oxidised

• 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 been oxidised or reduced

• Oxidation numbers are also used in naming compounds, especially for elements that form more than one ion

  • For example, iron(II) sulfate contains Fe²⁺, while iron(III) sulfate contains Fe³⁺

  • The Roman numeral shows the oxidation state of the metal in the compound

Oxidation number of redox line