Oxidation and reduction in terms of electrons (HT only) — AQA GCSE Chemistry
Verified courseTeaching note checked against the official specification.
Oxidation and reduction in terms of electrons (HT only) explained
Oxidation and reduction in terms of electrons (HT only) covers 2 specification points, including Oxidation is the loss of electrons and… and Write ionic equations for displacement….
Read the full explanation
Open a point below for its explanation and revision guidance.
Oxidation is the loss of electrons and reduction is the gain of electrons.
At the higher tier, the definitions of oxidation and reduction expand beyond oxygen transfer to involve the movement of electrons. This is often remembered using the acronym OILRIG: Oxidation Is Loss, Reduction Is Gain of electrons. In displacement reactions or electrolysis, metals and non-metals change their oxidation state by transferring electrons. For example, when magnesium reacts with copper(II) sulfate, the neutral magnesium atoms lose two electrons to form Mg²⁺ ions, meaning magnesium is oxidised. Simultaneously, the Cu²⁺ ions gain two electrons to form neutral copper atoms, meaning the copper ions are reduced. Understanding these half-equations is crucial for explaining the underlying ionic chemistry.
Student should be able to: write ionic equations for displacement reactions identify in a given reaction, symbol equation or half equation which species are oxidised and which are reduced.
A displacement reaction occurs when a more reactive metal takes the place of a less reactive metal in a compound. To write the ionic equation, show only the ions that change: the solid metal atom loses electrons to form aqueous ions, while the aqueous ion of the less reactive metal gains electrons and forms a solid atom. For example, zinc added to copper(II) sulfate solution gives Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). The zinc is oxidised because it loses electrons; the copper ion is reduced because it gains electrons. Spectator ions, such as sulfate, are omitted. In a half equation, electrons appear on the right for oxidation and on the left for reduction. Identifying which species is oxidised or reduced means tracking electron loss or gain, not simply naming the more reactive metal.
Your focus
- Define oxidation and reduction in terms of electron transfer.
- Identify which species are oxidised and reduced in an ionic equation.
- Construct balanced half-equations for oxidation and reduction processes.
Show all 6 objectives
- Write a balanced ionic equation for a displacement reaction by removing spectator ions.
- Identify the species oxidised and the species reduced in a given reaction, symbol equation or half equation.
- Explain oxidation and reduction in terms of loss and gain of electrons.
Oxidation and reduction in terms of electrons (HT only) exam tips
Marking Points
- Define oxidation as the loss of electrons.
- Define reduction as the gain of electrons.
- Identify the species losing electrons in a given reaction as being oxidised.
- Identify the species gaining electrons in a given reaction as being reduced.
- Write balanced half-equations to demonstrate the transfer of electrons.
- Write the full symbol equation first, then cancel spectator ions to leave the ionic equation.
- Show the solid metal atom losing electrons to form a positive ion in the ionic equation.
- Show the metal ion in solution gaining electrons to form a solid metal atom.
- State that oxidation is loss of electrons and reduction is gain of electrons.
- Identify the species oxidised as the one that loses electrons and the species reduced as the one that gains electrons.
- Check that charge and mass balance on both sides of the ionic equation.
Examiner Tips
- 💡Always write the charges on ions clearly, such as Fe²⁺ or Cl⁻, to help track electron movement.
- 💡Split full ionic equations into two half-equations to easily identify which species is oxidised and which is reduced.
- 💡Remember that spectator ions do not participate in electron transfer and can be ignored when identifying oxidation and reduction.
- 💡Underline the ions that change before writing the final ionic equation.
- 💡For each half equation, count the charge on each side to confirm electrons are placed correctly.
- 💡Use the phrase 'loss of electrons is oxidation, gain of electrons is reduction' to justify your identification.
Common Mistakes
- Reversing the definitions of oxidation and reduction. Correction: Use the OILRIG mnemonic to remember Oxidation Is Loss, Reduction Is Gain of electrons.
- Stating that the whole compound is reduced. Correction: Specify the exact ion that gains electrons, such as the copper(II) ion rather than copper sulfate.
- Putting electrons on the wrong side of a half-equation. Correction: Electrons should be on the right side for oxidation and on the left side for reduction.
- Including spectator ions such as sulfate in the ionic equation; the correction is to cancel ions that appear unchanged on both sides.
- Saying the more reactive metal is reduced because it 'wins'; the correction is to track electrons: the more reactive metal loses electrons and is oxidised.
- Writing electrons on the wrong side of a half equation; the correction is electrons on the right for oxidation and on the left for reduction.