4 high-yield, syllabus-aligned questions on Oxidation & reduction, covering 14 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
Define4 marks
Define oxidation and reduction in terms of electron transfer and in terms of oxidation number.
TopMarks model answer
In terms of electron transfer, oxidation is the loss of electrons by a species and reduction is the gain of electrons by a species. In terms of oxidation number, oxidation is an increase in the oxidation number of an element and reduction is a decrease in its oxidation number.
Suggested marking guide
1Oxidation is loss of electrons
2Reduction is gain of electrons
3Oxidation is an increase in oxidation number
4Reduction is a decrease in oxidation number
Why it scores
The question names TWO frameworks, so four marks means two definitions in each. Candidates who give only the electron definitions have answered half the question perfectly and scored half the marks — which is worse than it sounds, because the oxidation-number half takes one extra line to write.
A common incomplete answer
“Oxidation is the loss of electrons and reduction is the gain of electrons.”
What it costs: Two marks of four. Both are correct, but the question explicitly asked for the oxidation-number definitions as well.
Key ideas to include: loss of electrons, gain of electrons, oxidation number, increase, decrease.
Determine4 marks
Determine the oxidation number of manganese in KMnO₄ and of chromium in K₂Cr₂O₇.
TopMarks model answer
In KMnO₄ potassium is +1 and each oxygen is -2, and the compound is neutral, so the oxidation numbers sum to zero: +1 + x + 4(-2) = 0. Therefore x = +7, and the oxidation number of manganese is +7. In K₂Cr₂O₇: 2(+1) + 2y + 7(-2) = 0, so 2y = +12 and y = +6. The oxidation number of chromium is +6.
Suggested marking guide
1Uses the rule that the oxidation numbers sum to zero in a neutral compound
2Manganese is +7
3Sets up the equation correctly for chromium
4Chromium is +6
The working, line by line
1Rule: the oxidation numbers in a neutral compound add up to zero — For an ion they add up to the charge on the ion. State the rule you are using — it is a method mark.
2KMnO₄: +1 + x + 4(-2) = 0 → x - 7 = 0 → x = +7 — There are FOUR oxygens, so it is 4 x (-2), not -2.
3K₂Cr₂O₇: 2(+1) + 2y + 7(-2) = 0 → 2y = +12 → y = +6 — There are TWO chromiums, so divide by 2 at the end. Forgetting this gives +12, a very common wrong answer.
Why it scores
Two of the four marks are for the working. Notice the two traps built into the second compound: seven oxygens and two chromiums. A candidate who forgets to halve gets +12 — an oxidation number no element can have — and the impossible answer is itself the clue that a step was missed.
A common incomplete answer
“Manganese is +7 and chromium is +12.”
What it costs: Three marks of four. The manganese answer is right. There are two chromium atoms sharing the +12, so each is +6; and with no working shown, neither method mark can be awarded.
Key ideas to include: oxidation number, neutral compound, sum to zero, polyatomic ion.
Explain3 marks
Explain the term oxidising agent, and give one example, stating what happens to it during a redox reaction.
TopMarks model answer
An oxidising agent is a substance that oxidises another species by accepting electrons from it, and in doing so it is itself reduced, so its own oxidation number decreases. Acidified potassium tetraoxomanganate(VII), KMnO₄, is an example: it oxidises iron(II) to iron(III) and is itself reduced, the manganese falling from +7 to +2.
Suggested marking guide
1Accepts electrons from another species
2Is itself reduced in the process
3A valid example named
Why it scores
The second mark is the one that catches people out: an oxidising agent is ITSELF REDUCED. It sounds contradictory until you see that the agent must take the electrons the other species loses. State it explicitly — the examiner is checking that you have not simply memorised the label.
A common incomplete answer
“An oxidising agent is a substance that gains electrons from another substance.”
What it costs: Two marks of three. The electron transfer is credited, but nothing says the agent is itself reduced, and no example is given.
Key ideas to include: oxidising agent, accepts electrons, itself reduced, oxidation number decreases.
State3 marks
Chlorine gas is bubbled into potassium iodide solution. State two observations and write the equation for the reaction, indicating the changes in oxidation number.
TopMarks model answer
The colourless potassium iodide solution turns brown. A dark brown or black solid is deposited as more iodine is displaced. The equation is Cl₂ + 2KI → 2KCl + I₂; the chlorine is reduced from 0 to -1 while the iodide is oxidised from -1 to 0.
Suggested marking guide
1The solution turns brown
2Iodine is displaced or deposited
3Correct equation with the oxidation-number changes
Why it scores
“Observation” means what you would SEE. “Iodine is formed” is a deduction, not an observation — the observable fact is that the solution turns brown. Examiners separate the two deliberately, and answers that give only the chemistry lose the observation marks.
A common incomplete answer
“The solution turns brown.”
What it costs: Two marks of three. A correct observation, but the second observation is missing and the equation with its oxidation-number changes — which is where the redox reasoning lives — is not attempted.
Key ideas to include: observation, displacement, more reactive halogen, oxidation number change.
Practise Oxidation & reduction on real questions
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