Chemistry 0620 · syllabus 4
Cambridge IGCSE Chemistry 0620 — Electrochemistry topical past papers
Electrolysis of molten and aqueous ionic compounds, the products at each electrode, electroplating, ionic half-equations, and hydrogen–oxygen fuel cells.
- Questions
- 471
- Years
- 2009–2026
- With mark scheme
- 99%
- Questions 2022–2026
- 175
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Electrochemistry covers
Electrolysis breaks down an ionic compound, molten or in aqueous solution, by passing an electric current through it. The anode is the positive electrode, the cathode the negative electrode, and the electrolyte the molten or dissolved substance that is broken down. Positive ions move to the cathode and negative ions to the anode, so metals or hydrogen form at the cathode and non-metals other than hydrogen at the anode. A molten binary compound gives its two elements: molten lead(II) bromide gives lead at the cathode and red-brown bromine vapour at the anode.
A solution also contains hydrogen ions and hydroxide ions from the water, which changes the products. With inert electrodes, concentrated aqueous sodium chloride gives hydrogen at the cathode, because sodium is more reactive than hydrogen, and chlorine at the anode, leaving sodium hydroxide in solution; dilute sulfuric acid gives hydrogen and oxygen. Electroplating puts the process to use: the object is the cathode, the plating metal the anode, and the electrolyte contains ions of that metal, giving the object a better appearance and more resistance to corrosion.
The Extended route asks more. You describe how charge is transferred — electrons in the external circuit, ions in the electrolyte, electrons gained and lost at the electrodes — and write ionic half-equations showing reduction at the cathode and oxidation at the anode. You predict the products for halides in dilute and concentrated solution, and compare aqueous copper(II) sulfate with graphite electrodes, which gives copper and oxygen, with copper electrodes, where the anode dissolves and copper plates the cathode. The topic ends with the hydrogen–oxygen fuel cell, which makes electricity with water as its only chemical product, and its advantages and disadvantages against petrol engines in vehicles.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 4 Electrochemistry
- 4.1 Electrolysis
- 4.2 Hydrogen–oxygen fuel cells
What you should be able to do
- Identify the anode, cathode and electrolyte, and predict the products of electrolysing a molten binary compound
- Give the products and observations for molten lead(II) bromide, concentrated aqueous sodium chloride and dilute sulfuric acid with inert electrodes
- Describe how an object is electroplated and why
- Write ionic half-equations at each electrode and say which shows oxidation and which reduction (Extended)
- Predict the products for dilute and concentrated halide solutions, and for aqueous copper(II) sulfate with graphite or copper electrodes (Extended)
- State what a hydrogen–oxygen fuel cell produces, and compare it with a petrol engine on the Extended route
Key ideas
- Cathode (negative): positive ions gain electrons, which is reduction, e.g. Cu²⁺ + 2e⁻ → Cu
- Anode (positive): negative ions lose electrons, which is oxidation, e.g. 2Cl⁻ → Cl₂ + 2e⁻ or 4OH⁻ → O₂ + 2H₂O + 4e⁻
- From a solution, hydrogen forms at the cathode unless the metal is less reactive than hydrogen, as copper and silver are
- A concentrated halide solution gives the halogen at the anode; a dilute one gives oxygen
- Hydrogen–oxygen fuel cell: 2H₂ + O₂ → 2H₂O, transferring chemical energy to electrical energy
How Electrochemistry has been examined
From 2022 to 2026, Electrochemistry was on Paper 3 · Theory (Core) in 11 of 14 exam series, worth about 12% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 56 | 4% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 69 | 5% | 14 of 14 |
| Paper 3 · Theory (Core) | 26 | 12% | 11 of 14 |
| Paper 4 · Theory (Extended) | 20 | 11% | 11 of 14 |
| Paper 6 · Alternative to Practical | 4 | 2% | 4 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1.
Filed in Momentum under: Electricity and Chemical Changes.
A worked example
Aqueous copper(II) sulfate is electrolysed with graphite electrodes until 0.640 g of copper has been deposited on the cathode. Write the half-equation at each electrode and calculate the volume of oxygen, measured at r.t.p., given off at the anode.
- Cathode (reduction): Cu²⁺(aq) + 2e⁻ → Cu(s)
- Anode (oxidation): 4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻
- Moles of copper = 0.640 ÷ 64 = 0.0100 mol, so electrons transferred = 2 × 0.0100 = 0.0200 mol
- The same electrons pass through the anode, and each O₂ molecule needs 4: moles of O₂ = 0.0200 ÷ 4 = 0.00500 mol
- Volume of O₂ = 0.00500 × 24 = 0.120 dm³ = 120 cm³
Answer: Cathode: Cu²⁺ + 2e⁻ → Cu; anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻; 120 cm³ of oxygen at r.t.p.
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Saying electrons flow through the electrolyte — electrons move only in the wires and electrodes; in the electrolyte the current is carried by moving ions
- Putting the electrons on the wrong side of an anode half-equation — oxidation is loss of electrons, so they appear as products: 2Br⁻ → Br₂ + 2e⁻
- Predicting sodium at the cathode from aqueous sodium chloride — sodium is more reactive than hydrogen, so hydrogen is given off and sodium ions stay in solution
- Making the object to be plated the anode — the anode loses metal; the object must be the cathode, where metal ions are reduced and deposited
- Saying the blue colour fades when copper electrodes are used in aqueous copper(II) sulfate — the anode dissolves as fast as copper is deposited, so the colour stays the same; it fades with graphite electrodes
Exam technique
- Begin every electrolysis answer by listing the ions present — from the compound and, in a solution, H⁺ and OH⁻ from water — and the electrode each moves to
- Give observations as well as names: bubbles of a pale yellow-green gas at the anode for chlorine, a red-brown solid forming on the cathode for copper
- Balance the electrons in half-equations, then check that the total charge is the same on both sides
- In fuel-cell comparisons, give specific points, such as no carbon dioxide from the vehicle or the difficulty of storing hydrogen safely, rather than 'better for the environment'
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.