Cambridge AS Level Chemistry 9701
Redox and oxidation numbers
Oxidation is the loss of electrons, or an increase in oxidation number; reduction is the gain of electrons, or a decrease in oxidation number. In every redox reaction one species is oxidised and another reduced, and the electrons lost by one equal the electrons gained by the other — which is how redox equations are balanced.
On Cambridge AS Chemistry 9701 Paper 1, most redox questions are either an oxidation-number calculation or “which is the oxidising agent”.
Updated 28 September 2026
How much it is worth
Redox and oxidation numbers: 44 questions across 23 real Cambridge 9701 papers — 4.8% of the total, on 22 of the 23. How that ranks against every other 9701 topic →
Rules for oxidation numbers
- An uncombined element is 0 (Na, O₂, Cl₂).
- The oxidation numbers in a compound add up to 0; in an ion they add up to its charge.
- Fluorine is always −1.
- Oxygen is −2, except in peroxides (−1) and with fluorine.
- Hydrogen is +1, except in metal hydrides such as NaH (−1).
- Group 1 metals are +1 and Group 2 metals +2 in their compounds.
- Chlorine is −1, except when combined with oxygen or fluorine.
Apply them in that order of priority. Names carry the result in Roman numerals: iron(III) chloride has Fe at +3; manganate(VII) has Mn at +7.
Working them out
| Species | Equation | Result |
|---|---|---|
| Mn in KMnO₄ | +1 + x + 4(−2) = 0 | +7 |
| Cr in Cr₂O₇²⁻ | 2x + 7(−2) = −2 | +6 |
| N in NH₄⁺ | x + 4(+1) = +1 | −3 |
| N in NO₃⁻ | x + 3(−2) = −1 | +5 |
| S in S₂O₃²⁻ | 2x + 3(−2) = −2 | +2 |
| Cl in ClO₃⁻ | x + 3(−2) = −1 | +5 |
Nitrogen’s range, from −3 to +5, is why it turns up in so many redox questions — including most of the nitrogen and sulfur topic.
Oxidising and reducing agents
An oxidising agent oxidises something else, so it is itself reduced — it gains electrons and its oxidation number falls. A reducing agent is itself oxidised. This reversal is the single most common trap on the topic.
Zinc goes from 0 to +2: oxidised, so it is the reducing agent. Copper goes from +2 to 0: reduced, so Cu²⁺ is the oxidising agent.
Half-equations
To balance a half-equation in acidic solution:
- Balance the element changing oxidation number.
- Balance oxygen by adding H₂O.
- Balance hydrogen by adding H⁺.
- Balance charge by adding electrons to the more positive side.
Check it with oxidation numbers: Mn goes from +7 to +2, a gain of 5 electrons — matching the 5e⁻.
Disproportionation
When the same element is both oxidised and reduced in one reaction, it is disproportionation. Chlorine in cold dilute sodium hydroxide is the standard example:
Chlorine goes from 0 to −1 in NaCl and from 0 to +1 in NaClO.
Worked example
Worked example
Acidified potassium manganate(VII) oxidises iron(II) ions to iron(III) ions. Write the overall ionic equation.
The two half-equations:
Multiply the iron half-equation by 5 so the electrons cancel, then add:
Check the charge: left, −1 + 8 + 10 = +17; right, +2 + 15 = +17. Manganate(VII) is the oxidising agent; Fe²⁺ is the reducing agent.
Common mistakes
1.Saying the oxidising agent is oxidised.
It is reduced. Oxidising agents take electrons; that is how they oxidise something else.
2.Writing oxidation numbers like charges: 2+ instead of +2.
Oxidation numbers have the sign first (+2); ionic charges have it last (2+).
3.Leaving electrons in the overall equation.
Scale the half-equations so the electrons match, then cancel them — and cancel any H⁺ or H₂O that appears on both sides.
4.Treating oxygen as −2 in hydrogen peroxide.
In H₂O₂ oxygen is −1. The exceptions in the rules are there because questions use them.
Common questions
What is OIL RIG?
A way to remember the electron definitions: Oxidation Is Loss, Reduction Is Gain — of electrons.
Can an oxidation number be a fraction?
As an average, yes. In S₄O₆²⁻ the four sulfur atoms average +2.5, because they are not all in the same environment.
Where does redox fit in the 9701 syllabus?
It opens the electrochemistry topic at AS, and it underpins Group 2, Group 17 and nitrogen and sulfur chemistry. Group 2 guide →
Redox and oxidation numbers on real papers
The papers that leaned on it hardest, as a share of the paper. Each one has its own page on Quanta with the full topic breakdown:
Practise redox and oxidation numbers against real mark schemes
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