5 high-yield, syllabus-aligned questions on Electrolysis, covering 17 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.
Define2 marks
Define electrolysis.
TopMarks model answer
Electrolysis is the decomposition of an electrolyte by the passage of an electric current through it when the electrolyte is molten or in aqueous solution.
Suggested marking guide
1Decomposition of an electrolyte
2By an electric current, in the molten or aqueous state
Why it scores
The second mark carries a condition that candidates drop constantly: the electrolyte must be MOLTEN or in AQUEOUS SOLUTION. Solid sodium chloride does not conduct, because its ions are not free to move. Leaving the state out makes the definition wrong, not merely incomplete.
A common incomplete answer
“Electrolysis is the breaking down of a substance using electricity.”
What it costs: One mark of two. The decomposition is credited, but nothing says the electrolyte must be molten or in solution — the condition that makes electrolysis possible at all.
Key ideas to include: decomposition, electrolyte, electric current, molten, aqueous solution.
State4 marks
State the products formed at each electrode during the electrolysis of concentrated sodium chloride solution using inert electrodes, and write the equation for each electrode reaction.
TopMarks model answer
At the cathode, hydrogen gas is liberated: 2H⁺ + 2e⁻ → H₂. At the anode, chlorine gas is liberated: 2Cl⁻ → Cl₂ + 2e⁻. Sodium hydroxide remains in solution as the third product.
Suggested marking guide
1Hydrogen at the cathode
2Correct cathode half-equation
3Chlorine at the anode
4Sodium hydroxide left in solution
Why it scores
The fourth product is the one that decides the grade. Candidates name hydrogen and chlorine and stop — but the whole industrial value of this electrolysis is the sodium hydroxide left behind, and the examiner asks about it precisely because it is forgotten. Note also that chlorine, not oxygen, is discharged here because the brine is CONCENTRATED.
A common incomplete answer
“Hydrogen is produced at the cathode and chlorine at the anode.”
What it costs: Two marks of four. Both products are correct, but neither half-equation is written and the sodium hydroxide is not mentioned.
State three factors that determine which ion is discharged at an electrode during electrolysis.
TopMarks model answer
The position of the ion in the electrochemical series: the ion that is more easily discharged, that is lower in the series, is discharged preferentially. The concentration of the ion in the electrolyte: a very concentrated ion may be discharged in preference to one that is easier to discharge. The nature of the electrode: an active electrode such as copper may itself dissolve instead of an anion being discharged.
Suggested marking guide
1Position in the electrochemical series
2Concentration of the ion
3Nature of the electrode
Why it scores
These three factors are the reason the same electrolyte can give different products in different conditions — dilute brine gives oxygen, concentrated brine gives chlorine. If you can name all three, every “explain why X was discharged” question in the topic becomes answerable.
A common incomplete answer
“The electrochemical series determines which ion is discharged.”
What it costs: Two marks of three. Correct, but concentration and the nature of the electrode are the two factors that explain the interesting cases, and neither appears.
Key ideas to include: electrochemical series, preferential discharge, concentration, inert electrode, active electrode.
State4 marks
State two industrial applications of electrolysis and name the electrolyte used in each.
TopMarks model answer
Electroplating: an object is coated with a metal such as silver, using a solution of a salt of the plating metal — for example silver trioxonitrate(V) solution — as the electrolyte. Extraction of aluminium: aluminium is obtained from purified bauxite, that is alumina dissolved in molten cryolite, which serves as the electrolyte.
Suggested marking guide
1Electroplating named
2A correct electrolyte for it
3Extraction of a reactive metal named
4A correct electrolyte for it
Why it scores
Read the question again: it asks for the application AND the electrolyte. That is two marks per application, so naming both applications and neither electrolyte scores exactly half. Whenever a question has an “and”, count the marks — the “and” almost always doubles them.
A common incomplete answer
“Electroplating and the extraction of aluminium.”
What it costs: Two marks of four. Both applications are right, but the question also asked for the electrolyte in each and neither was named.
Key ideas to include: electroplating, anode, bauxite, alumina, cryolite.
Calculate4 marks
State Faraday's first law of electrolysis and calculate the mass of copper deposited when a current of 5 A flows for 30 minutes through copper(II) tetraoxosulphate(VI) solution. [Cu = 64, F = 96500 C mol⁻¹]
TopMarks model answer
Faraday's first law states that the mass of a substance liberated at an electrode is directly proportional to the quantity of electricity passed through the electrolyte. Quantity of electricity Q = I x t = 5 x 1800 = 9000 C. From Cu²⁺ + 2e⁻ → Cu, 2 x 96500 = 193000 C are needed to deposit 64 g of copper. Mass deposited = 9000 x 64 ÷ 193000 = 2.98 g.
Suggested marking guide
1Correct statement of the law
2Q = I x t worked out in coulombs
3Two faradays per mole of copper recognised
4Correct answer with the unit
The working, line by line
1Convert the time: 30 minutes = 1800 s — Q = I x t requires SECONDS. Using minutes multiplies the answer by sixty and loses every mark that follows.
2Q = 5 x 1800 = 9000 C — Show the substitution, not just the result.
3Cu²⁺ + 2e⁻ → Cu, so 2 x 96500 = 193000 C deposit 64 g — Copper is DIVALENT. Two moles of electrons are needed per mole of copper — this is the step that is skipped most often.
4Mass = 9000 x 64 ÷ 193000 = 2.98 g — State the unit.
Why it scores
The charge on the ion is the hinge of the whole calculation. A candidate who uses 96500 instead of 193000 does every other step perfectly and arrives at exactly double the right answer. Always write the half-equation first, and let it tell you how many faradays you need.
A common incomplete answer
“Q = I x t = 5 x 1800 = 9000 C, so the mass of copper = 9000 ÷ 96500 x 64 = 5.97 g.”
What it costs: Two marks of four. The charge is right, but copper is divalent and needs two faradays per mole. Using one faraday doubles the answer, and the law itself was never stated.
Key ideas to include: quantity of electricity, coulomb, faraday, valency, half-equation.
Practise Electrolysis on real questions
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