A-Level Chemistry AQA Notes

3.1.7 Oxidation, reduction, and redox equations

Oxidation, reduction, and redox equations
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Oxidation Numbers
  • Oxidation number is a number representing the number of electrons lost or gained by an atom in a compound.
  • Oxidation is a loss of electrons during a reaction or an increase in oxidation number.
  • Reduction is a gain of electrons during a reaction or a decrease in oxidation number
  • The rules for assigning oxidation numbers:
    • An uncombined element has an oxidation number of 0
    • A simple ion (of a single element) has an oxidation number equal to the charge on the ion
    • The sum of oxidation numbers of the elements in a compound is equal to the overall charge of the compound
    • The charge on a complex ion, e.g. NH4+, is equal to the sum of the oxidation numbers
    • The most electronegative element in a compound always has a negative oxidation number
    • Oxygen is always -2 except in peroxides where its -1
    • Hydrogen is +1 except in metal hydrides where its -1
  • Oxidation numbers are represented by Roman numerals when naming compounds​
Redox Reactions
  • Redox reactions involve both oxidation and reduction
  • Oxidising agents cause oxidation of other species, and so are themselves reduced
  • Reducing agents cause reduction of other species, and so are themselves oxidised
  • Disproportionation reactions involve both oxidation and reduction of the same element
e.g. The decomposition of hydrogen peroxide
2H2O2 2H2O + O2
Oxidation Number of O: -1 -2 0
O is both oxidised and reduced during this reaction

Redox Equations
  • A redox reaction can be constructed from two half-equations; one representing an oxidation process, and the other a reduction process
  • To construct a full equation from half equations:
    • Balance the electrons
    • Combine the equations
    • Cancel the electrons
    • Check the charge balance and stoichiometry​
e.g. Mg -> Mg2+ + 2e-
Cu2+ + 2e--> Cu
1. Mg -> Mg2+ + 2e-
Cu2+ + 2e--> Cu
2. Mg + Cu2+ + 2e-->Mg2++2e-+Cu
3. Mg + Cu2+ -> Mg2++Cu

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Oxidation, reduction, and redox equations
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