Oxidation & Reduction in Terms of ElectronsOxford AQA IGCSE Chemistry: Revision notes
Section 1
How are oxidation and reduction defined in terms of electrons?
In terms of electron transfer:
- Oxidation is the loss of electrons
- Reduction is the gain of electrons
A useful way to remember this is the mnemonic OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
When asked to define oxidation or reduction 'in terms of electrons', you must use the words 'loss' and 'gain' of electrons — definitions using oxygen will not gain the mark if electrons are specifically asked for.
Section 2
What is a redox reaction?
When oxidation and reduction happen at the same time in the same reaction, it is called a redox reaction (reduction and oxidation). In any redox reaction, one substance loses electrons (is oxidised) while another substance gains those same electrons (is reduced) — the two processes always occur together.
Section 3
How does this apply to metal displacement reactions?
In a metal displacement reaction, a more reactive metal displaces a less reactive metal from a solution of its compound. In electron terms, the more reactive metal atom loses electrons to form a positive ion (it is oxidised), and the ion of the less reactive metal gains those electrons to form a metal atom (it is reduced).
When zinc is added to copper sulfate solution: Zn → Zn²⁺ + 2e⁻ (oxidation, zinc loses electrons) and Cu²⁺ + 2e⁻ → Cu (reduction, copper ions gain electrons).
Section 4
How does this apply to electrolysis?
During electrolysis, at the cathode (negative electrode), positive ions gain electrons — this is reduction. At the anode (positive electrode), negative ions lose electrons — this is oxidation. This gives a consistent way of describing electrode reactions across all electrolysis processes.
Do not confuse which electrode is which — the cathode (reduction) is the negative electrode; the anode (oxidation) is the positive electrode.
Must Know
- Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons) — OIL RIG
- A redox reaction is one where oxidation and reduction occur simultaneously
- In a redox reaction, the electrons lost by one substance are gained by another
- In metal displacement, the more reactive metal is oxidised (loses electrons); the less reactive metal ion is reduced (gains electrons)
- In electrolysis, reduction (gain of electrons) occurs at the cathode; oxidation (loss of electrons) occurs at the anode
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