Oxidation and reduction in terms of electrons (HT only) — AQA GCSE Combined Science
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Oxidation and reduction in terms of electrons (HT only) explained
In terms of electrons, oxidation is loss of electrons and reduction is gain of electrons.
Read the full explanation
This applies to reactions involving ions, such as displacement and electrolysis. For example, in Zn + Cu²⁺ → Zn²⁺ + Cu, zinc atoms lose electrons to form Zn²⁺, so zinc is oxidised, while Cu²⁺ ions gain electrons to form Cu, so copper(II) ions are reduced. To identify which species is oxidised or reduced, compare the charge and formula of each species before and after reaction. An increase in positive charge or loss of negative charge indicates electron loss (oxidation); a decrease in positive charge or gain of negative charge indicates electron gain (reduction). This electron-transfer definition is required for higher tier and supports understanding of redox in electrolysis and metal displacement.
write ionic equations for displacement reactions
This Higher Tier only statement requires converting a full displacement equation into an ionic equation showing only the particles that change. In displacement, a more reactive element takes the place of a less reactive one. For example, zinc displaces copper from copper sulfate: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). Sulfate ions are spectator ions as they remain unchanged in solution. Cancelling them gives the ionic equation: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). To succeed, identify the ions, cancel spectator ions appearing unchanged on both sides, and write the remaining species with correct state symbols and charges. This also applies to halogens: Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq) becomes Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq).
identify in a given reaction, symbol equation or half equation which species are oxidised and which are reduced.
In terms of electrons (Higher Tier only), oxidation is loss of electrons and reduction is gain of electrons. To identify which species is oxidised or reduced, track electron loss and gain using ionic charges or half equations. In a full symbol equation, split it into half equations: the species whose charge increases is oxidised; the species whose charge decreases is reduced. For example, in Zn + Cu²⁺ → Zn²⁺ + Cu, zinc goes from 0 to 2⁺ (loses electrons, oxidised) and copper goes from 2⁺ to 0 (gains electrons, reduced). In a half equation, electrons on the right mean oxidation; electrons on the left mean reduction. Always balance atoms and charge, and remember that oxidation and reduction occur together.
Your focus
- Define oxidation and reduction in terms of electron loss and gain.
- Identify which species is oxidised or reduced in a given ionic equation.
- Write and interpret half-equations showing electron transfer.
Show all 9 objectives
- Convert a full displacement equation into an ionic equation by removing spectator ions.
- Balance ionic equations for atoms and charge.
- Explain why spectator ions are omitted from ionic equations.
- Define oxidation and reduction in terms of electron loss and gain.
- Determine which species is oxidised and which is reduced from a given symbol equation or half equation.
- Apply changes in ionic charge to justify the identification of oxidised and reduced species.
Oxidation and reduction in terms of electrons (HT only) exam tips
Marking Points
- State that oxidation is loss of electrons and reduction is gain of electrons.
- Identify electron loss or gain by comparing the charge of a species before and after reaction.
- Apply the definition to ionic equations, such as displacement reactions and electrolysis.
- Use oxidation states or ion charges to confirm which species is oxidised or reduced.
- Write the full balanced equation first, including correct formulae and state symbols, before converting to an ionic equation.
- Identify spectator ions as those that appear unchanged in form, charge and state on both sides of the equation.
- Cancel spectator ions to leave only the reacting ions and atoms in the final ionic equation.
- Balance the final ionic equation for both atoms and total charge, including coefficients where needed.
- Apply the method to both metal displacement and halogen displacement reactions.
- State that oxidation is loss of electrons and reduction is gain of electrons, using the mnemonic OIL RIG if helpful.
- In a symbol equation, check the charges of elements on both sides and identify the species whose charge increases (oxidised) and decreases (reduced).
- In a half equation, locate the electrons: if they appear as products on the right, the reactant is oxidised; if they appear as reactants on the left, the reactant is reduced.
- Check that the number of electrons lost equals the number gained in the overall reaction, and that both oxidation and reduction are identified.
- For a named species, state clearly whether it is oxidised or reduced, referring to electron transfer rather than oxygen or hydrogen.
Examiner Tips
- 💡Write the relevant half-equations to show electron loss or gain clearly.
- 💡Check the charge on each side of the equation to confirm whether electrons are lost or gained.
- 💡Write the full equation first and underline the ions that change; this makes spectator ions obvious.
- 💡Check that the charges on both sides of the ionic equation are equal, not just the number of atoms.
- 💡Underline the species and write its charge above each symbol before and after the reaction to make the change clear.
- 💡For half equations, draw a quick arrow from the electron term to show whether electrons are lost or gained.
Common Mistakes
- Reversing the definitions: remember 'OIL RIG' — Oxidation Is Loss, Reduction Is Gain of electrons.
- Confusing gain of electrons with gain of oxygen: in electron terms, reduction is gain of electrons, not oxygen.
- Forgetting to balance charge when writing half-equations: ensure the number of electrons lost equals the number gained overall.
- Including spectator ions such as sulfate or nitrate in the final ionic equation: correct this by checking each ion appears on both sides unchanged and removing it.
- Forgetting state symbols or writing them incorrectly: correct this by labelling aqueous ions as (aq) and solid metals or halogens as (s) or as appropriate.
- Failing to balance charge after cancelling ions: correct this by adding coefficients and checking that total charge is equal on both sides.
- Confusing oxidation and reduction: remember OIL RIG (Oxidation Is Loss, Reduction Is Gain) and check electron position in half equations.
- Assuming oxygen must be present for oxidation: in electron-transfer terms, oxidation can occur without oxygen, such as Na → Na⁺ + e⁻.
- Ignoring charge changes in a full equation: determine the charges of ions on both sides before deciding which species is oxidised or reduced.