Most tested C5.4

Identifying Redox Reactions

This topic covers how to identify electron transfer in chemical reactions. For the ESAT, you must analyse a given equation and classify it as oxidation, reduction, a combined redox reaction, or a non-redox process by tracking electron movement.

Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.

Key points

  • The mnemonic OIL RIG is essential: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.
  • A full redox reaction must have both an oxidation and a reduction process occurring simultaneously; electrons lost by one species are gained by another.
  • Reactions where electrons (e-) are shown as a product represent 'oxidation only'. If electrons are a reactant, it is 'reduction only'.
  • Many common reaction types, such as acid-base neutralisations and ionic precipitation, are NOT redox reactions because no species changes its charge or gains/loses electrons.
  • Look for changes in charge to spot redox. A neutral metal atom forming a positive ion (e.g., Mg → Mg2+) is oxidation. A non-metal molecule forming negative ions (e.g., Cl2 → 2Cl-) is reduction.

Formulae

Species → Product + n e-

To represent an oxidation process where a species loses 'n' electrons. Note the electrons are on the product side.

Species + n e- → Product

To represent a reduction process where a species gains 'n' electrons. Note the electrons are on the reactant side.

Definitions

Oxidation
The process where a chemical species loses electrons. In simpler cases, it can be seen as the gain of oxygen.
Reduction
The process where a chemical species gains electrons. In simpler cases, it can be seen as the loss of oxygen.
Redox Reaction
A reaction in which both oxidation and reduction occur together. The number of electrons lost must equal the number of electrons gained.
Half-Equation
An equation showing either the oxidation or reduction part of a redox reaction, explicitly including the electrons transferred.

Worked example

Which of the following equations describes a reaction that is NOT a redox reaction?

  1. 1

    Analyse each equation to see if any element undergoes a change in its electronic state (charge).

  2. 2

    A) 2Mg(s) + O2(g) → 2MgO(s):

    Solid Mg (neutral) becomes Mg2+ ions in the ionic compound MgO.

    This is oxidation.

    O2 (neutral) becomes O2- ions.

    This is reduction.

    Therefore, this is a redox reaction.

  3. 3

    B) Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq):

    Zn (neutral) becomes a Zn2+ ion, losing electrons (oxidation).

    Cu2+ ion becomes neutral Cu, gaining electrons (reduction).

    This is a redox reaction.

  4. 4

    C) Fe3+(aq) + e- → Fe2+(aq):

    The Fe3+ ion gains an electron to become an Fe2+ ion.

    This is a gain of electrons, which is reduction only.

    It is still a redox process.

  5. 5

    D) BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq):

    This is an ionic precipitation reaction.

    The ions start as Ba2+, Cl-, Na+, and SO42- and end as BaSO4 (containing Ba2+ and SO42-) and dissolved Na+ and Cl- ions.

    No ion changes its charge.

    Therefore, this is not a redox reaction.

Answer: D) BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)

Common mistakes

  • ×Confusing the definitions of oxidation and reduction is a frequent error. Lock in the OIL RIG mnemonic to prevent this.
  • ×Incorrectly identifying precipitation or neutralisation reactions as redox. Always check for a change in charge or electron transfer; if none occurs, it's not a redox reaction.
  • ×Forgetting to check all species in an equation. A reaction is only redox if one species is oxidised AND another is reduced.
  • ×Overlooking changes in elemental forms. A reaction converting a neutral element like Cl2 into ions like Cl- (or vice-versa) is always a redox process.

No-calculator tips

  • ✓Focus on the charges. An increase in positive charge (e.g., from 0 to +2, or +1 to +3) means oxidation. A decrease in charge means reduction.
  • ✓If you see a pure, neutral element on one side of an equation and it's part of an ionic compound on the other, a redox reaction has almost certainly occurred.
  • ✓Mentally track the electrons. If a species loses electrons, ask yourself which species gained them. If you can't find a destination for the electrons, it might not be a full redox reaction.
  • ✓The presence of `e-` in an equation is a direct sign. Electrons as a product means oxidation; electrons as a reactant means reduction.

Read this topic in the official UAT-UK ESAT guide →

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