6.4 Redox
- Syllabus
- 0620–2026–2027
- Topic
- 6.4
- Level
- —
Learning objectives
6.4.1Roman numeral to indicate the• Use a Roman numeral to indicate the oxidation number of an element in a compound6.4.2Redox reactions as involving• Define redox reactions as involving simultaneous oxidation and reduction6.4.3Oxidation as gain of oxygen and• Define oxidation as gain of oxygen and reduction as loss of oxygen6.4.4Redox reactions as reactions involving• Identify redox reactions as reactions involving gain and loss of oxygen6.4.5Oxidation and reduction in redox• Identify oxidation and reduction in redox reactions6.4.6Oxidation in terms of: (a) loss of• Define oxidation in terms of: (a) loss of electrons (b) an increase in oxidation number6.4.7Reduction in terms of: (a) gain of• Define reduction in terms of: (a) gain of electrons (b) a decrease in oxidation number6.4.8Redox reactions as reactions involving• Identify redox reactions as reactions involving gain and loss of electrons6.4.9Redox using oxidation numbers: (a)• Identify redox using oxidation numbers: (a) uncombined elements are 0 (b) monatomic ion number equals ion charge (c) compound total is 0 (d) ion total equals ion charge6.4.10Redox reactions by the colour changes• Identify redox reactions by the colour changes involved when using acidified aqueous potassium manganate(VII) or aqueous potassium iodide6.4.11An oxidising agent as a substance that• Define an oxidising agent as a substance that oxidises another substance and is itself reduced6.4.12Reducing agent as a substance that• Define a reducing agent as a substance that reduces another substance and is itself oxidised6.4.13Oxidising agents and reducing agents• Identify oxidising agents and reducing agents in redox reactions
Use Roman numerals for oxidation numbers
A Roman numeral in a compound name gives the oxidation number of the named element. For example, iron(III) oxide contains iron with oxidation number +3.
| Name | Roman numeral meaning |
|---|---|
| copper(II) oxide | Cu has oxidation number +2 |
| iron(III) chloride | Fe has oxidation number +3 |
| potassium manganate(VII) | Mn has oxidation number +7 |
Determine the oxidation number from the formula and ion charges, then write it in Roman numerals immediately after the element name when required.
The numeral is not the number of atoms or ions in the formula and is written without a plus sign inside the name.
Define a redox reaction
A redox reaction is one in which oxidation and reduction happen simultaneously.
One species is oxidised while another is reduced. The two processes are linked because oxygen or electrons transferred from one species are gained by another.
In Mg + CuO → MgO + Cu, magnesium is oxidised and copper(II) oxide is reduced, so the overall reaction is redox.
A reaction is not redox if only mixing, precipitation, or acid–base neutralisation occurs without simultaneous oxidation and reduction.
Define oxidation and reduction by oxygen transfer
| Process | Oxygen change |
|---|---|
| oxidation | gain of oxygen |
| reduction | loss of oxygen |
In CuO + H₂ → Cu + H₂O, hydrogen gains oxygen and is oxidised; copper(II) oxide loses oxygen and is reduced.
Compare each substance before and after the reaction. Track oxygen attached to the substance, not merely whether oxygen gas appears in the equation.
Reduction does not mean a substance becomes physically smaller. In this definition it specifically means loss of oxygen.
Identify redox by oxygen gain and loss
- Compare oxygen in each reactant and product. 2. Identify a species that gains oxygen. 3. Identify another species that loses oxygen. 4. If both occur in the same reaction, classify it as redox.
For 3Fe + 4H₂O → Fe₃O₄ + 4H₂, iron gains oxygen and is oxidised, while water loses oxygen to form hydrogen and is reduced. Both changes make the reaction redox.
In AgNO₃ + NaCl → AgCl + NaNO₃, no species gains oxygen while another loses it, so oxygen transfer does not identify a redox change.
Finding oxygen in an equation is not enough. There must be simultaneous gain and loss of oxygen between species.
Identify which species is oxidised or reduced
| Observation | Classification |
|---|---|
| species gains oxygen | oxidised |
| species loses oxygen | reduced |
| species loses electrons or oxidation number rises | oxidised |
| species gains electrons or oxidation number falls | reduced |
In Zn + CuO → ZnO + Cu, zinc gains oxygen and is oxidised; copper(II) oxide loses oxygen and is reduced.
A complete identification names the species, states oxidation or reduction, and gives the relevant oxygen, electron, or oxidation-number evidence.
Name the whole species shown in the reaction when oxygen is transferred; do not identify oxygen itself as the substance reduced.
Define oxidation by electrons and oxidation number
Oxidation is loss of electrons and an increase in oxidation number.
Zn → Zn²⁺ + 2e⁻ shows oxidation: zinc loses two electrons and its oxidation number increases from 0 to +2.
| Evidence | Oxidation sign |
|---|---|
| electrons in products of a half-equation | electrons were lost |
| oxidation number becomes more positive | oxidation number increased |
Oxidation is electron loss, not gain. The species that loses electrons becomes more positive or less negative.
Define reduction by electrons and oxidation number
Reduction is gain of electrons and a decrease in oxidation number.
Cu²⁺ + 2e⁻ → Cu shows reduction: copper ions gain two electrons and the oxidation number decreases from +2 to 0.
| Evidence | Reduction sign |
|---|---|
| electrons in reactants of a half-equation | electrons were gained |
| oxidation number becomes less positive or more negative | oxidation number decreased |
Reduction is electron gain, not loss. A decreasing oxidation number moves numerically downward, such as +3 to +2 or 0 to −1.
Identify redox by electron transfer
A reaction is redox when electrons are transferred: one species loses electrons and another species gains the same number of electrons.
For Zn + Cu²⁺ → Zn²⁺ + Cu: Zn → Zn²⁺ + 2e⁻ is oxidation, while Cu²⁺ + 2e⁻ → Cu is reduction. The electrons cancel when the half-equations are combined.
Identify the electron donor and electron acceptor. If no oxidation number changes and no electron transfer occurs, the reaction is not redox.
Electrons travel through an external circuit in a cell, not through the electrolyte as free electrons; ions carry charge in the solution.
Use oxidation numbers to identify redox
| Rule | Result |
|---|---|
| uncombined element | oxidation number 0 |
| monatomic ion | oxidation number equals ion charge |
| neutral compound | oxidation numbers sum to 0 |
| polyatomic ion | oxidation numbers sum to the ion charge |
In SO₄²⁻, oxygen is −2: S + 4(−2) = −2, so sulfur is +6. Show the total equation before solving the unknown oxidation number.
Compare oxidation numbers before and after: an increase identifies oxidation and a decrease identifies reduction. A redox reaction contains both changes.
Subscripts multiply oxidation numbers; they are not oxidation numbers themselves. The oxidation number of O₂, Cl₂, metals, and all other uncombined elements is 0.
Recognise redox from manganate(VII) and iodide colours
| Reagent | Starting colour | Redox observation | What it detects |
|---|---|---|---|
| acidified aqueous potassium manganate(VII) | purple | turns colourless | a reducing agent reduces manganate(VII) |
| aqueous potassium iodide | colourless | turns brown as iodine forms | an oxidising agent oxidises iodide ions |
Use the reagent's own change to identify the unknown: manganate(VII) is reduced, so the tested substance is reducing; iodide is oxidised to iodine, so the tested substance is oxidising.
Iodide loses electrons: 2I⁻ → I₂ + 2e⁻. The brown iodine colour is evidence of this oxidation.
Potassium manganate(VII) must be acidified for the stated purple-to-colourless test. Do not confuse brown iodine with a brown precipitate.
Define an oxidising agent
An oxidising agent oxidises another substance and is itself reduced.
It accepts electrons from the other species, so its own oxidation number decreases.
In Zn + Cu²⁺ → Zn²⁺ + Cu, Cu²⁺ is the oxidising agent: it causes zinc to lose electrons and itself gains electrons to form copper.
The agent is named for what it does to the other substance. The oxidising agent is reduced, not oxidised.
Define a reducing agent
A reducing agent reduces another substance and is itself oxidised.
It donates electrons to the other species, so its own oxidation number increases.
In Zn + Cu²⁺ → Zn²⁺ + Cu, zinc is the reducing agent: it supplies electrons that reduce Cu²⁺ and is itself oxidised to Zn²⁺.
The reducing agent is oxidised. Do not select the species whose oxidation number decreases; that species is the oxidising agent.
Identify oxidising and reducing agents
- Assign oxidation numbers or write half-equations. 2. Find the species reduced: it is the oxidising agent. 3. Find the species oxidised: it is the reducing agent. 4. Check that electrons lost equal electrons gained.
| Species change | Role |
|---|---|
| gains electrons / oxidation number decreases / loses oxygen | oxidising agent |
| loses electrons / oxidation number increases / gains oxygen | reducing agent |
For Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂, chlorine gains electrons and is the oxidising agent; bromide ions lose electrons and are the reducing agent.
Do not label an agent from its name alone. Track what happens to that species in the specific reaction.