UNIT 2 CHEMISTRY · PEARSON AAQ · C1.2.3
Oxidation and Reduction: Electron Transfer
FREE CHEMISTRY REVISION · TOPIC 21
Follow the electrons to identify both changes and both agents.
Understand oxidation, reduction and agent roles through accurate electron-transfer diagrams and ionic reaction examples.
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · C1.2.3
What you will learn
Define oxidation and reduction through electron transfer; identify the species oxidised and reduced; name oxidising and reducing agents; connect electron loss/gain to oxidation-number changes; distinguish reacting ions from spectators in ionic reactions. C1.2.4 develops the full equation-balancing method next.
1. Oxidation is loss; reduction is gain
Oxidation is the loss of electrons. Reduction is the gain of electrons. Remember OIL RIG: Oxidation Is Loss; Reduction Is Gain. Electrons are negatively charged. Losing them makes a species more positive in charge; gaining them makes it less positive or more negative.
For example, Mg → Mg²⁺ + 2e⁻ is oxidation. A magnesium atom loses two electrons while its nucleus remains magnesium: the proton number does not change. Cu²⁺ + 2e⁻ → Cu is reduction. A positive ion gains electrons to become a neutral atom; reduction does not require the final species to be negative.
A positive ion can also be oxidised: Fe²⁺ → Fe³⁺ + e⁻. A negative ion can be oxidised too: 2I⁻ → I₂ + 2e⁻. Always compare the starting and finishing species rather than guessing from the sign of the initial charge.
2. The two changes are coupled in a redox reaction
In a chemical redox reaction an electron donor loses the electrons that an acceptor gains. The total number lost equals the total number gained. Electrons are conserved; they are not created by one species and destroyed by another. The two half-processes describe one overall transfer.
In Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), zinc loses two electrons and copper(II) ions gain two. The zinc atoms are oxidised; Cu²⁺ ions are reduced. The overall equation contains no uncancelled electrons. In this direct displacement the transfer occurs at the metal–solution interface rather than through a reservoir of free electrons in water.
3. Oxidation numbers provide a useful cross-check
An increase in oxidation number indicates oxidation; a decrease indicates reduction. Zinc changes 0 → +2 and Cu changes +2 → 0 in the displacement above. This agrees with two-electron loss and gain. For Fe²⁺ → Fe³⁺, +2 → +3 is an increase of one, so one electron is lost per Fe ion.
Oxidation numbers are formal electron-accounting assignments. In a simple ionic transfer they match the changes in ionic charge. In covalent reactions, changes can describe shifts in electron allocation rather than the formation of free highly charged ions. For example, oxidation of a non-metal by oxygen does not automatically create separate monatomic cations of that non-metal.
The familiar description “oxidation is addition of oxygen” is useful for many combustion reactions, but the electron-transfer definition is broader. A metal can be oxidised by Cu²⁺ or H⁺ without oxygen being present. Similarly, reduction does not always involve removing oxygen.
4. Oxidising and reducing agents
An oxidising agent accepts electrons from another species, causing that species to be oxidised. The oxidising agent is itself reduced. A reducing agent donates electrons to another species, causing that species to be reduced. The reducing agent is itself oxidised.
In the Zn/Cu²⁺ reaction, Zn is the reducing agent because it supplies electrons; Cu²⁺ is the oxidising agent because it accepts them. Do not name the copper metal product as the agent in the forward reaction. State the reacting ion and charge precisely: Cu²⁺, not just “copper”, when the distinction matters.
An agent’s identity depends on the reaction partner and conditions. The same element can appear in different species with different roles. A species containing Fe²⁺ can be oxidised to Fe³⁺ by a suitable oxidising agent, while Fe³⁺ can act as an electron acceptor in another reaction. Neither a positive sign nor the word “metal” by itself defines the agent.
| Role | Electron change | Oxidation-number change | What happens to the agent |
|---|---|---|---|
| Reducing agent | Donates / loses electrons | Increases | Oxidised |
| Oxidising agent | Accepts / gains electrons | Decreases | Reduced |
5. Ionic equations and spectator ions
An ionic equation shows the species that actually change. For zinc in aqueous copper(II) sulfate, sulfate ions remain in solution before and after the reaction and are spectators. Removing unchanged sulfate gives Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Omitting spectators does not omit the charge that belongs to the reacting copper and zinc ions.
Use state symbols where required: (s) solid, (l) liquid, (g) gas, (aq) aqueous. Cu²⁺(aq) and Cu(s) have different chemical identities, charges and physical states. A statement that “copper is reduced” is less precise than “Cu²⁺ ions gain two electrons to form Cu metal”.
Not every ionic reaction is redox. In H⁺(aq) + OH⁻(aq) → H₂O(l), hydrogen remains +1 and oxygen remains −2. No oxidation-number change or net electron transfer is involved: this is acid–base neutralisation. A new bond, gas, precipitate or colour change alone is not proof of redox.
6. A reliable identification routine
1. Write the reacting species with charges and states.
2. Compare the same element before and after; use oxidation numbers where helpful.
3. Identify electron loss and gain, including the number per atom or ion.
4. Name the species oxidised and reduced.
5. Name the reducing agent (donor) and oxidising agent (acceptor).
6. Check that the total electron loss and gain can match; ignore unchanged spectator ions when naming agents.
Worked examples
Example 1 · Magnesium displaces copper
Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s). Mg changes 0 → +2 and loses two electrons: oxidised and the reducing agent. Cu²⁺ changes +2 → 0 and gains two: reduced and the oxidising agent. The total charge is +2 on both sides.
Example 2 · Zinc reduces silver ions
Zn(s) + 2Ag⁺(aq) → Zn²⁺(aq) + 2Ag(s). Zn loses 2e⁻ per atom; each Ag⁺ gains 1e⁻. Two Ag⁺ ions accept the two electrons. Zn is oxidised and reduces the silver ions. Ag⁺ is reduced and acts as the oxidising agent.
Example 3 · A positive ion is oxidised
2Fe²⁺(aq) + Cl₂(aq) → 2Fe³⁺(aq) + 2Cl⁻(aq). Each Fe²⁺ loses one electron (+2 → +3), so two Fe ions lose two in total. Cl₂ gains two electrons (each Cl: 0 → −1). Fe²⁺ is the reducing agent; Cl₂ is the oxidising agent. Charge: +4 on each side.
Example 4 · Acid is reduced without oxygen
Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g). Mg loses 2e⁻ and is oxidised. Two H⁺ ions gain one electron each and form H₂: H changes +1 → 0. H⁺ is the oxidising species in this reaction; the acid’s unchanged anion is a spectator.
Example 5 · A negative ion is oxidised
Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq). I⁻ loses electrons (−1 → 0), so iodide is oxidised and is the reducing agent. Cl₂ gains electrons (0 → −1), so chlorine is reduced and is the oxidising agent. Starting negative does not imply reduction.
Example 6 · Test whether a reaction is redox
H⁺ + OH⁻ → H₂O: H stays +1 and O stays −2. No element changes oxidation number, so do not assign an oxidising or reducing agent. The charges balance (+1 − 1 = 0), but charge balance alone does not make a reaction redox.
Common misconceptions and exam tips
“Oxidation always needs oxygen.” Electron loss is the general definition.
“A reducing agent is reduced.” It donates electrons and is oxidised.
“Only neutral atoms can be oxidised.” Fe²⁺ and I⁻ can lose electrons.
“Reduction must make a negative ion.” Cu²⁺ can be reduced to neutral Cu.
“The spectator anion is the oxidising agent.” Identify the species that accepts electrons.
“The nucleus changes when electrons are lost.” Ordinary redox does not change proton number.
Use a complete explanation: species → electrons lost/gained → oxidation/reduction → agent. Include the ion charge and distinguish “per atom” from the total transferred in the balanced reaction. Naming the mnemonic alone does not explain which species changes.
Your revision summary
Oxidation is electron loss and an increase in oxidation number; reduction is electron gain and a decrease. The reducing agent donates electrons and is oxidised. The oxidising agent accepts electrons and is reduced. Match total loss and gain and omit unchanged spectators from the net ionic description.
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