Identify redox, oxidation, reduction and disproportionation
Classify redox processes by electron transfer and oxidation-number changes, including disproportionation.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Check atom balance, total charge and oxidation-state changes separately. Written practice is self-checked
Before you begin
- Recognise common ions and write formulas for simple compounds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to oxidation number during reduction?
What you will learn
- Explain oxidation and reduction in terms of electron transfer and oxidation number
- Define redox as oxidation and reduction occurring together
- Recognise disproportionation
Oxidation and reduction happen together
Oxidation is electron loss and an increase in oxidation number. Reduction is electron gain and a decrease in oxidation number. In a redox reaction, one species is oxidised while another is reduced.
Track the same element or species from reactants to products. The oxidation-number method is a reliable check for electron transfer, especially when the equation does not show electrons explicitly.
Disproportionation sends one element in two directions
Disproportionation is a redox reaction in which the same element in one oxidation state is simultaneously oxidised and reduced into products with higher and lower oxidation numbers.
For chlorine reacting with cold aqueous alkali, Cl₂ at oxidation number 0 forms Cl⁻ at −1 and ClO⁻ at +1: Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O. Chlorine is both oxidised and reduced in this reaction.
Pause and explain
What must happen in disproportionation?
Worked example
Show that Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O is disproportionation.
Show the worked solution
- Cl in Cl₂ has oxidation number 0.
- Cl in Cl⁻ is −1: reduction.
- Cl in ClO⁻ is +1: oxidation. The same reactant element undergoes both changes.
Answer It is disproportionation because chlorine at 0 is both reduced to −1 and oxidised to +1.
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Classify Fe²⁺ → Fe³⁺ + e⁻ as oxidation or reduction using both definitions.
Give me a hint
Track electron direction and oxidation number.
Compare my reasoning
- An electron appears as a product, so it is lost.
- Oxidation number increases from +2 to +3.
- Both definitions identify oxidation.
It is oxidation: Fe²⁺ loses an electron and its oxidation number rises from +2 to +3.
Look for these in your work
- Electron loss identified
- Increase in oxidation number identified
- Classification consistent
In 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻, identify which species is oxidised and which is reduced.
Give me a hint
Compare each element's oxidation number on both sides.
Compare my reasoning
- Fe increases from +2 to +3.
- Cl decreases from 0 in Cl₂ to −1 in Cl⁻.
- Iron is oxidised; chlorine is reduced.
Fe²⁺ is oxidised to Fe³⁺; Cl₂ is reduced to Cl⁻.
Look for these in your work
- Iron change correct
- Chlorine change correct
- Both processes named
In Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O, state the chlorine oxidation-number changes and explain the reaction label.
Give me a hint
Assign chlorine values in the element, chloride and hypochlorite.
Compare my reasoning
- Cl₂ has Cl at 0.
- Cl⁻ has Cl at −1; ClO⁻ has Cl at +1.
- The same element is reduced and oxidised, so the reaction is disproportionation.
Chlorine changes from 0 to −1 and +1; the same element undergoes reduction and oxidation, so this is disproportionation.
Look for these in your work
- All oxidation numbers correct
- Both directions identified
- Term justified
Common mistakes
- Defining oxidation as oxygen gain only
- Calling electron gain oxidation
- Looking at different elements instead of the same element for disproportionation
- Ignoring atom or charge balance in the example equation
What can you explain now?
What happens to oxidation number during reduction?
Compare with the explanation
It decreases
Reduction corresponds to electron gain and a lower oxidation number.
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Make it stick
What paired electron and oxidation-number tests distinguish oxidation from reduction?
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