The most frequent Redox Equilibrium errors in SPM, from confusing the oxidising and reducing agent to mixing up electrode signs and forgetting the discharge factors, each shown with why it loses marks and the correct version to write instead.
Most marks lost in Redox Equilibrium come from a small set of repeated errors. Each entry below shows what students often write, why it loses marks, and the correct version. Group your revision around these and you will keep the marks that are easiest to drop.
9.1 Oxidation and reduction
1. Naming the agent by what happens to it. Students write “magnesium is oxidised, so it is the oxidising agent.” Why it loses marks: the definitions are the other way round. Correct: the species that is oxidised is the reducing agent; the species that is reduced is the oxidising agent. Magnesium is oxidised, so it is the reducing agent.
2. Reversing OIL RIG. Students say oxidation is the gain of electrons. Why it loses marks: it is the opposite, and every later half-equation then comes out wrong. Correct: Oxidation Is Loss, Reduction Is Gain of electrons. Rehearse it until it is automatic.
3. Claiming a reaction is redox without checking. Students label a reaction redox because “electrons move.” Why it loses marks: not every reaction is redox, precipitation and neutralisation are not. Correct: assign oxidation numbers; only if one rises and one falls is it redox.
9.2 Oxidation number
4. Confusing oxidation number with ionic charge. Students write the oxidation number of sulfur in SO₄²⁻ as −2 (the ion charge). Why it loses marks: the ion charge is not the oxidation number of the central atom. Correct: set the sum of oxidation numbers equal to the ion charge and solve, sulfur is +6.
5. Forgetting the exceptions for oxygen and hydrogen. Students always use O = −2 and H = +1. Why it loses marks: in peroxides oxygen is −1, and in metal hydrides hydrogen is −1. Correct: learn the usual value and its exception, then check which compound you are in.
6. Dropping the sign or the Roman numeral. Students write “oxidation number 2” or name a compound “iron chloride.” Why it loses marks: an oxidation number must carry its sign (+2, −2), and Stock names must show it (iron(II) vs iron(III)). Correct: always write the sign, and use the Roman numeral that matches the oxidation number.
9.3 Displacement
7. Writing the wrong starting species in a half-equation. Students write Br → Br₂ + e⁻ when bromide is oxidised. Why it loses marks: bromide already exists as Br⁻, and the electron count is wrong. Correct: 2Br⁻ → Br₂ + 2e⁻. Start from the ion actually present and balance the electrons.
8. Getting displacement direction backwards. Students say copper displaces zinc from zinc sulfate. Why it loses marks: only a more reactive metal displaces a less reactive one. Correct: zinc (more reactive) displaces copper; copper cannot displace zinc.
9.4 Rusting
9. Naming only one condition for rusting. Students say “iron rusts because of water” (or only oxygen). Why it loses marks: both are required. Correct: rusting needs water and oxygen together; removing either prevents rust.
10. Reversing sacrificial protection. Students attach a less reactive metal to protect iron. Why it loses marks: only a more reactive metal (zinc, magnesium) corrodes in place of the iron. Correct: a more reactive metal is sacrificed; a less reactive one makes the iron rust faster.
9.5 Electrolysis
11. Confusing electrode signs between cell types. Students assume the cathode is always positive, or apply the voltaic rule to electrolysis. Why it loses marks: the signs differ. Correct: in electrolysis the cathode is negative and the anode positive; in a voltaic cell the more reactive metal is the negative terminal. Reduction always happens at the cathode in both.
12. Ignoring the discharge factors in aqueous solutions. Students predict the same product as the molten compound. Why it loses marks: water adds H⁺ and OH⁻, so selective discharge applies. Correct: use position in the electrochemical series, concentration and electrode type to decide which ion is discharged.
13. Forgetting the electrode can react. Students always release oxygen at the anode. Why it loses marks: an active copper anode dissolves instead. Correct: with a copper anode, Cu → Cu²⁺ + 2e⁻ and no gas forms; only inert electrodes give oxygen.
9.6 Cells
14. Making the more reactive metal the positive terminal. Students label copper negative in a magnesium–copper cell. Why it loses marks: it is the reverse. Correct: the more reactive metal is negative; electrons flow from it through the external wire to the less reactive metal.
9.7 Reactivity series and extraction
15. Saying carbon can extract any metal. Students use carbon reduction for aluminium. Why it loses marks: carbon cannot reduce the oxide of a metal more reactive than itself. Correct: metals above carbon (potassium to aluminium) are extracted by electrolysis; those below (zinc, iron, tin, lead) by carbon reduction.
More slips worth fixing
16. Not balancing charge and electrons together. Students write Fe²⁺ → Fe³⁺ + e⁻ but then combine it with a two-electron half without scaling. Why it loses marks: the electrons on each side of the combined equation must be equal. Correct: multiply each half-equation so the electrons cancel exactly before adding them.
17. Confusing “discharged” with “moved.” Students say an ion is discharged simply because it moves to an electrode. Why it loses marks: many ions arrive at an electrode but are not discharged. Correct: an ion is discharged only when it gains or loses electrons there; state the half-equation to show it.
Turning these into marks
Every mistake here has the same cure: define terms by their electron direction, assign oxidation numbers before you judge a reaction, and keep the electrode rules for electrolysis and voltaic cells separate in your mind. A one-to-one teacher can watch your working for exactly these slips as you practise. Revise this list beside the worked examples for SPM Chemistry, then attempt the practice questions and check that none of these errors appear in your answers.
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