Redox chemistry is the transfer of electrons. Oxidation and reduction always happen together: whatever one species loses, another gains. The same idea drives metal extraction from ores, electrolysis, simple cells and fuel cells, rusting, combustion and bleaching.
In the O-Level exam, redox questions almost always ask you to identify which species is oxidised and which is reduced, then justify your answer using oxidation states or electron transfer. That makes the definitions and oxidation state rules essential before moving on to agents, tests, electrolysis and cells.
What these notes cover
- •Why redox matters
- •The four definitions of oxidation and reduction
- •Oxidation state rules
- •Oxidising and reducing agents
- •Tests with acidified KMnO₄ and aqueous KI
- •Electrolysis and the electrode rules
- •Electrolysis of molten lead(II) bromide
- •Simple cells and electron flow
- •The hydrogen fuel cell
- •Everyday redox: combustion, rusting and bleaching
- •Common mistakes and exam technique
- •The five takeaways for Topic 7
Key definitions
- Oxidation
- Loss of electrons, or an increase in oxidation state. When oxygen or hydrogen is transferred, it is also gain of oxygen or loss of hydrogen.
- Reduction
- Gain of electrons, or a decrease in oxidation state. When oxygen or hydrogen is transferred, it is also loss of oxygen or gain of hydrogen.
- Oxidation state
- A bookkeeping number assigned to each atom, showing how many electrons it has effectively lost (+) or gained (−).
- Oxidising agent
- A substance that causes another substance to be oxidised. It is the electron acceptor, so it is itself reduced.
- Reducing agent
- A substance that causes another substance to be reduced. It is the electron donor, so it is itself oxidised.
Key formulas
Sum rule for oxidation states
sum of all oxidation states = overall charge of the species
The sum is 0 for a neutral compound and equals the ion's charge for an ion. Free elements are 0; O in compounds is −2 (−1 in peroxides such as H₂O₂); H in compounds is +1 (−1 in metal hydrides such as NaH).
Cathode rule (electrolysis)
Mⁿ⁺ + ne⁻ → M
Cations (+) migrate to the cathode (−) and are reduced.
Anode rule (electrolysis)
Xⁿ⁻ → X + ne⁻
Anions (−) migrate to the anode (+) and are oxidised.
Acidified manganate(VII) test
MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ → Mn²⁺(aq) + 4H₂O(l)
Purple MnO₄⁻ is reduced to almost colourless Mn²⁺ (Mn +7 → +2), showing a reducing agent is present. The H⁺ ions are needed, so the reagent must be acidified.
Potassium iodide test
2I⁻(aq) → I₂(aq) + 2e⁻
Colourless iodide is oxidised to brown iodine (I −1 → 0), showing an oxidising agent is present. K⁺ ions are spectators.
Hydrogen fuel cell — overall reaction
2H₂(g) + O₂(g) → 2H₂O(l)
Generates electricity directly. Hydrogen is oxidised and oxygen is reduced; the only product is water, so no CO₂ is released at the point of use.
Worked example
Chlorine gas is bubbled into aqueous potassium iodide. Predict the observation, write the ionic equation, and identify the oxidising and reducing agents.
- 1Observation: the solution changes from colourless to brown, because iodine, I₂(aq), is formed.
- 2Write the full ionic picture: Cl₂(g) + 2K⁺(aq) + 2I⁻(aq) → 2K⁺(aq) + 2Cl⁻(aq) + I₂(aq).
- 3Potassium ions appear unchanged on both sides, so cross them out as spectators, leaving Cl₂(g) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq).
- 4Chlorine: the oxidation state falls from 0 in Cl₂ to −1 in Cl⁻, so chlorine gains electrons and is reduced, making Cl₂ the oxidising agent.
- 5Iodine: the oxidation state rises from −1 in I⁻ to 0 in I₂, so iodide loses electrons and is oxidised, making I⁻ the reducing agent.
Watch out: Name the actual species — Cl₂ and I⁻, not 'KI' — and give the oxidation state change with numbers, linked to electron transfer.
Common mistakes
Reversing what happens to the agent
Wrong: The oxidising agent is oxidised.
Right: The oxidising agent oxidises its partner, so it accepts electrons and is itself reduced. The reducing agent donates electrons and is itself oxidised.
Setting the sum to zero for a polyatomic ion
Wrong: For MnO₄⁻: x + 4(−2) = 0, so x = +8.
Right: The sum equals the ion's charge: x + 4(−2) = −1, so x = +7. Zero applies only to neutral compounds.
Wrong polarity in a simple cell
Wrong: The more reactive metal is the positive electrode because it reacts more.
Right: The more reactive metal is the negative electrode, where oxidation occurs. In electrolysis, oxidation is at the positive electrode (anode) — the labels are opposite.
Questions students ask
What is the difference between an oxidising agent and a reducing agent?
An oxidising agent is an electron acceptor: it oxidises another substance and is itself reduced. A reducing agent is an electron donor: it reduces another substance and is itself oxidised. The name describes what the agent does to the other species, not what happens to itself.
How do you test for an oxidising agent and a reducing agent in O-Level Chemistry?
Acidified potassium manganate(VII) changes from purple to colourless when a reducing agent is present. Aqueous potassium iodide changes from colourless to brown, as iodine forms, when an oxidising agent is present. Always quote both the starting and final colours.
Why do ionic compounds conduct electricity only when molten or aqueous?
In the solid, the ions are held in fixed positions in the giant ionic lattice and cannot move from place to place. When melted or dissolved, the lattice breaks apart and the ions become mobile, carrying charge towards the electrodes. Inside the electrolyte the current is carried by ions; electrons flow only in the external wires.
Which metal is the negative electrode in a simple cell?
The more reactive metal is the negative electrode: it is oxidised, loses electrons and dissolves. Electrons flow through the external wire from the more reactive metal to the less reactive metal. The larger the gap in the reactivity series, the higher the voltage.
In the full PDF
- •Labelled diagrams of the electrolysis of molten lead(II) bromide and the zinc–copper simple cell
- •Further worked examples: oxidation states in MnO₄⁻, SO₄²⁻ and NH₃, molten sodium chloride electrolysis, and ranking cell voltages
- •The remaining common mistakes, including swapping the two chemical tests and treating oxidation as if it can happen alone
- •Everyday redox — combustion, rusting and bleaching — with equations and oxidation state changes
- •Exam technique for full-credit answers, including a model three-mark test answer
- •A five-point summary of Topic 7