Redox is one of the most predictable topics in O-Level Chemistry, and yet it is where a lot of students lose marks that were basically free. The reason is almost always the same: students try to feel whether something is oxidised instead of proving it with oxidation states. Examiners reward the proof, not the feeling.
Here is the exact method we teach at Dojo, the same one we used when we sat these papers.
Step 1: Know the oxidation state rules cold
You cannot do redox reasoning without these. Memorise them until they are automatic.
- Uncombined elements have an oxidation state of 0. That includes Cl₂, O₂, Zn, C and I₂.
- A simple ion has an oxidation state equal to its charge. Na⁺ is +1, Cl⁻ is −1, Fe³⁺ is +3.
- Oxygen is usually −2, except in peroxides such as H₂O₂ where it is −1.
- Hydrogen is usually +1, except in metal hydrides such as NaH where it is −1.
- Group I is +1, Group II is +2, aluminium is +3, fluorine is always −1.
- The oxidation states in a neutral compound sum to 0. In a polyatomic ion they sum to the charge.
Rule 6 is the workhorse. It is how you get every number the question does not hand you.
Quick practice with rule 6
- MnO₄⁻: Mn + 4(−2) = −1, so Mn = +7
- Cr₂O₇²⁻: 2Cr + 7(−2) = −2, so Cr = +6
- SO₄²⁻: S + 4(−2) = −2, so S = +6
- NO₃⁻: N + 3(−2) = −1, so N = +5
Write the algebra out in your working. If your final number is wrong but the method line is there, you can still pick up the method mark.
Step 2: Compare before and after, then use the right words
Oxidation is an increase in oxidation state. Reduction is a decrease. That is it.
The language examiners want is tight:
- Oxidised: oxidation state increases, species loses electrons
- Reduced: oxidation state decreases, species gains electrons
- Oxidising agent: the species that is itself reduced while it oxidises something else
- Reducing agent: the species that is itself oxidised while it reduces something else
That last pair is where most marks are dropped. Students correctly say chlorine is reduced, then also call it the reducing agent. Say it in your head every time: the oxidising agent is the one that gets reduced.
Step 3: Worked example, displacement
2KI + Cl₂ → 2KCl + I₂
- Iodine: −1 in KI → 0 in I₂. Increase, so iodide is oxidised.
- Chlorine: 0 in Cl₂ → −1 in KCl. Decrease, so chlorine is reduced.
- Therefore Cl₂ is the oxidising agent and KI is the reducing agent.
A full-mark answer reads: "The oxidation state of iodine increases from −1 to 0, so iodide is oxidised. The oxidation state of chlorine decreases from 0 to −1, so chlorine is reduced and acts as the oxidising agent."
Notice the structure: element, from, to, direction, conclusion. Use that sentence frame every single time.
Step 4: Worked example, blast furnace
Fe₂O₃ + 3CO → 2Fe + 3CO₂
- Iron: +3 → 0, decrease, reduced.
- Carbon: +2 in CO → +4 in CO₂, increase, oxidised.
- So CO is the reducing agent, Fe₂O₃ is the oxidising agent.
This one appears constantly in metal extraction questions. Do not just write "carbon monoxide removes oxygen", because the mark scheme often wants oxidation state numbers.
Step 5: Link redox to observations
Paper 1 and Paper 2 love pairing redox with a colour change. Learn these:
- Acidified potassium manganate(VII), KMnO₄: purple to colourless, it is being reduced, so it tests for a reducing agent.
- Acidified potassium dichromate(VI), K₂Cr₂O₇: orange to green, also reduced, also tests for a reducing agent.
- Aqueous potassium iodide with an oxidising agent: colourless to brown, iodide is oxidised to iodine.
- Iron(II) to iron(III): pale green to yellow or brown, and the precipitate with NaOH goes green to reddish-brown.
If a question says "state the observation and explain in terms of oxidation states", you need both halves. Observation, then numbers.
Common traps that cost marks
- Confusing charge with oxidation state. SO₄²⁻ has a charge of 2−, but sulfur has an oxidation state of +6. Different things.
- Forgetting signs. Always write +2, not 2. A missing sign can lose the mark.
- Peroxides and hydrides. H₂O₂ and NaH break the usual rules.
- Assuming everything is redox. Neutralisation and simple precipitation are not redox, because no oxidation state changes. If asked to justify, say so explicitly: "No element changes oxidation state, so this is not a redox reaction."
- Disproportionation. In Cl₂ + 2NaOH, chlorine goes from 0 to −1 and to +1. The same element is oxidised and reduced. Rare, but it appears.
Build the habit
Take any ten equations from your textbook and write oxidation states above every element before you answer anything. Two weeks of that, and redox questions become mechanical marks instead of guesswork.
Want a tutor who recently sat this exact paper to walk you through your weakest redox questions? Message us on WhatsApp to book a free trial class: https://wa.link/dsgbkf