Redox and Oxidation Numbers

Redox is less a topic than a language the rest of 0620 is written in: electrolysis, metal extraction, the halogen displacement reactions all carry redox marks. Paper 4 likes one move in particular: “identify the substance that is oxidised and justify your answer using oxidation numbers”, and justification is where scripts fall apart, because candidates assert rather than quote the numbers.

Three definitions, two tiers

	Oxidation	Reduction
Oxygen (Core)	Gain of oxygen	Loss of oxygen
Electrons (S)	Loss of electrons	Gain of electrons
Oxidation number (S)	Increase	Decrease

OILRIG (Oxidation Is Loss, Reduction Is Gain of electrons) handles the middle row. A redox reaction is one in which oxidation and reduction happen together; one cannot occur without the other, because the electrons lost by one species must be gained by another.

Core example: copper(II) oxide heated with hydrogen, CuO + H2 → Cu + H2O. The copper oxide loses oxygen (reduced); the hydrogen gains oxygen (oxidised).

Oxidation numbers (Supplement)

An oxidation number is the notional charge an atom would have, written with sign first: +2, −1, 0. The working rules:

• Uncombined elements: 0 (Fe, O2, Cl2 all zero).
• Simple ions: the ion’s charge (Na+ is +1, O2− is −2).
• In compounds, oxygen is usually −2 and hydrogen +1.
• The oxidation numbers in a neutral compound sum to zero; in an ion, to the ion’s charge.

Worked count: in SO2, oxygen contributes 2 × (−2) = −4, so sulfur is +4. In SO4^2−, four oxygens give −8 and the ion’s charge is −2, so sulfur is +6. Roman numerals in names broadcast the same information (iron(III) oxide contains Fe at +3, manganate(VII) contains Mn at +7), and naming compounds this way is itself a syllabus point.

To diagnose redox, track one element across the equation. In Zn + CuSO4 → ZnSO4 + Cu: zinc rises from 0 to +2 (oxidised), copper falls from +2 to 0 (reduced), and sulfur and oxygen are unchanged spectators, which is why the sulfate-free ionic equation Zn + Cu2+ → Zn2+ + Cu tells the whole story.

Oxidising and reducing agents (Supplement)

An oxidising agent oxidises another substance and is itself reduced. A reducing agent reduces another substance and is itself oxidised. The cross-over is the testable point: name the agent, then say what happened to it, and the two verbs must be opposites. In CuO + H2 → Cu + H2O, the CuO is the oxidising agent (it is reduced) and H2 is the reducing agent (it is oxidised).

Two test reagents carry their own marks:

• Acidified potassium manganate(VII): purple → colourless in the presence of a reducing agent (Mn falls from +7 to +2).
• Potassium iodide: colourless → brown in the presence of an oxidising agent (iodide ions are oxidised to iodine).

Colour-change identification questions sit naturally alongside the qualitative analysis ion and gas tests.

Where redox marks hide elsewhere

At electrodes: reduction at the cathode, oxidation at the anode, with the electron bookkeeping done by half-equations. The working examples live in Electrolysis of Aqueous Solutions. In Group VII, a more reactive halogen displaces a less reactive halide (chlorine + potassium bromide → bromine), with chlorine as the oxidising agent. In the blast furnace, carbon monoxide is the reducing agent that strips oxygen from iron(III) oxide. Spotting the redox question inside another topic’s clothing is the skill; the parent map is the Chemical Reactions pillar.

Worked exam question

Chlorine is bubbled through aqueous potassium iodide: Cl2 + 2KI → 2KCl + I2. (a) State the colour change observed. [1] (b) Using oxidation numbers, show that the iodide ions are oxidised. [2] (c) Identify the oxidising agent in this reaction and justify your choice. [2]

Model answer: (a) Colourless to brown (1). (b) Iodine in KI has oxidation number −1; in I2 it is 0 (1); the oxidation number increases (−1 → 0), so the iodide ions are oxidised (1). (c) Chlorine (1); it gains electrons / its oxidation number decreases from 0 to −1, so it is reduced, meaning it oxidised the iodide (1).

Mark-by-mark: (a) is a recall observation. (b) demands both numbers quoted and the direction stated: “iodine is oxidised because it loses electrons” answers a different question from the one asked. (c) splits identification from justification, and the justification must describe what happens to chlorine itself.

The mistakes that cost marks

1. Swapping the agents. The oxidising agent is reduced. Write the verb pair down before answering.
2. Asserting without numbers. “Using oxidation numbers” means quote them: −1 → 0, increase, oxidised.
3. Forgetting elements are zero. Cl2, Fe, O2 uncombined all sit at 0, the usual starting point for the change.
4. Treating oxidation and reduction as separable. Every redox equation contains both; if you have found one, the other is in there too.

How examiners want it phrased

Student wording	Mark-scheme wording
”The iodide gets oxidised by the chlorine"	"Iodide increases in oxidation number from −1 to 0, so it is oxidised; chlorine is the oxidising agent"
"Chlorine takes the electrons"	"Each chlorine atom gains one electron: Cl2 + 2e− → 2Cl−; chlorine is reduced"
"The purple stuff goes clear"	"Acidified potassium manganate(VII) turns from purple to colourless, showing a reducing agent is present"
"Copper oxide loses its oxygen"	"CuO loses oxygen, so it is reduced; it acts as the oxidising agent”

Quote the numbers, swap the agents correctly, and redox becomes the most mechanical of the Extended topics. If the agent cross-over still trips you in timed conditions, that is exactly the kind of recurring error a free 1-hour trial lesson with a Chemistry specialist is designed to catch and retrain.

Test yourself

Quote the oxidation numbers in every justification, then check each answer.

Q1 (3 marks). In the blast furnace, iron(III) oxide reacts with carbon monoxide: Fe2O3 + 3CO → 2Fe + 3CO2. Identify the reducing agent and justify your answer. Then state, using oxidation numbers, what happens to the iron.

Show answer

• carbon monoxide is the reducing agent [1] • it removes oxygen from the iron(III) oxide and is itself oxidised (gains oxygen, forming CO2) [1] • iron decreases in oxidation number from +3 to 0, so it is reduced [1]

Q2 (2 marks). (Extended) Deduce the oxidation number of nitrogen in (a) NH3 and (b) NO2.

Show answer

• (a) hydrogen is +1, so 3 × (+1) + N = 0, giving N = −3 [1] • (b) oxygen is −2, so N + 2 × (−2) = 0, giving N = +4 [1]

Q3 (3 marks). (Extended) Chlorine oxidises aqueous iron(II) ions: Cl2 + 2Fe2+ → 2Cl− + 2Fe3+. (a) Show, using oxidation numbers, that the iron(II) ions are oxidised. (b) Identify the oxidising agent and justify your choice.

Show answer

• (a) iron increases in oxidation number from +2 to +3 [1] • an increase in oxidation number is oxidation [1] • (b) chlorine is the oxidising agent: each Cl atom gains an electron / decreases from 0 to −1, so chlorine is itself reduced [1]