Oxidation & Reduction (Edexcel IGCSE Chemistry (Modular))

Revision Note

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Oxidation & reduction

Oxidation & reduction in terms of oxygen

  • The reactions of metals with oxygen, such as in iron rusting can be classified as oxidation

  • Oxidation is any reaction in which a substance gains oxygen

  • The opposite of oxidation is reduction

  • Reduction is a reaction in which a substance loses oxygen

  • For example, the displacement reaction between zinc and copper(II)oxide can be classified as a redox reaction

Zn    +     CuO    →    ZnO    +    Cu

zinc + copper(II) oxide → zinc oxide + copper

  • Oxidation cannot occur without reduction happening simultaneously, hence these are called redox reactions

  • The copper(II)oxide supplies the oxygen, so it is the oxidising agent

  • The zinc is the reducing agent because it removes the oxygen

Oxidation & Reduction in terms of electrons

  • Displacement reactions can be analysed in terms redox reactions by studying the transfer of electrons

  • For the example of magnesium and copper sulfate, a balanced equation can be written in terms of the ions involved:

Mg (s) + Cu2+ (aq) + SO42- (aq) → Mg2+ (aq) + SO42- (aq) + Cu (s)

  • The sulfate ions, SO42-, appear on both sides of the equation unchanged hence they are spectator ions and do not participate in the chemistry of the reaction so can be omitted:

Mg (s) + Cu2+ (aq)  → Mg2+ (aq)  + Cu (s)

  • This equation is an example of a balanced ionic equation which can be further split into two half equations illustrating oxidation and reduction individually:

Mg → Mg2+ + 2e

Cu2+ + 2e→ Cu

  • Mg is oxidised as it lose electrons

  • Cu2+ is reduced as it gain electrons

  • Oxidation is the loss of electrons

  • Reduction is the gain of electrons

Oxidising agents in terms of electrons

  • Oxidising agents will oxidise other species in a reaction 

    • They are themselves reduced therefore will gain electrons

  • In the following example Fe has been oxidised and Cu2+ has been reduced

  • Therefore Cu2+ is the oxidising agent

The redox reaction between Fe and Cu2+

oxidation-and-reduction-electrons

The Fe atom is oxidised (loses electrons) and the Cu2+ ion is reduced (gains electrons). Cu2+ is the oxidising agent

OILRIG, downloadable AS & A Level Biology revision notes

'OIL RIG' is a useful mnemonic to help remember the definitions of oxidation and reduction

Worked Example

Which change in the following equation is oxidation?

V3+ + Fe3+  → V4+ + Fe2+

Answer:

  • Step 1 - Identify the changes for each species

    • V3+ to V4+ 

    • V3+ has lost 1 electron 

    • Fe3+ to Fe2+

    • Fe3+ has gained 1 electron

  • Step 2 - Identify each change as either oxidation and reduction

    • V3+ to V4+ is oxidation

    • Fe3+ to Fe2+ is reduction

    • Therefore, V3+ has been oxidised

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