IGCSE Chemistry — Electrochemistry: Notes, Practice Questions & Worked Solutions

IGCSE Chemistry 0620 · Topic 4: Electrochemistry

Electrochemistry

Electrolysis uses electricity to split compounds apart. Which product forms at which electrode depends on a few clear rules — whether the electrolyte is molten or dissolved, and how reactive the ions involved are.

1. Key Definitions

Electrolysis — decomposition of an ionic compound by electricity

Electrolyte — the molten or dissolved ionic compound that conducts electricity

Cathode — the negative electrode; attracts positive ions (cations)

Anode — the positive electrode; attracts negative ions (anions)

2. Predicting the Products

For a molten ionic compound, electrolysis simply separates it into its elements: the metal forms at the cathode, the non-metal at the anode.

For an aqueous solution, water is also present, so there’s competition for discharge:

Electrode	Rule
Cathode	Hydrogen is produced, unless the metal is less reactive than hydrogen (e.g. copper), in which case the metal is deposited instead
Anode	OH⁻ (from water) is discharged, producing oxygen — unless a halide ion (Cl⁻, Br⁻, I⁻) is present in reasonable concentration, in which case the halogen is produced instead

Worked example — electrolysis of concentrated aqueous sodium chloride Predict the products at each electrode.
Cathode: sodium is more reactive than hydrogen, so hydrogen gas is produced. Anode: chloride ions are present in concentration, so chlorine gas is produced (instead of oxygen).

Worked example — electrolysis of aqueous copper(II) sulfate with copper electrodes Using copper electrodes (rather than inert ones), the copper anode dissolves, releasing Cu²⁺ ions into solution, while copper metal is deposited at the cathode. This is the basis of electroplating and of purifying copper.

3. Hydrogen–Oxygen Fuel Cells

A fuel cell uses the reaction between hydrogen and oxygen to generate electricity directly, with water as the only product — no combustion, no pollutant gases.

Overall: 2H₂ + O₂ → 2H₂O

Common mistakes to watch for

Mixing up which electrode is which — remember CAThode attracts CATions (positive ions).
Forgetting that dilute halide solutions can still produce oxygen at the anode if the halide concentration is too low — the “halide preference” rule assumes a reasonably concentrated solution.
Describing electroplating as simply “coating” without mentioning that the object to be plated must be made the cathode.

Syllabus Reference

This note covers IGCSE Chemistry 0620 Topic 4, Electrochemistry: electrolysis, electrodes, electrolytes, predicting products of electrolysis, electroplating, half-equations, and hydrogen–oxygen fuel cells.

Practice questions

IGCSE Chemistry 0620 · Topic 4: Electrochemistry — Practice Question Bank (no answers)

Electrochemistry — Practice

Covers: electrolysis rules, electroplating, and fuel cells. 10 questions.

Q1 Core

Define electrolysis.

Q2 Core

State which electrode (anode or cathode) attracts positive ions, and explain why.

Q3 Core

Predict the products at each electrode during the electrolysis of molten lead(II) bromide.

Q4 Extended

Predict the products at each electrode during the electrolysis of concentrated aqueous sodium chloride.

Q5 Extended

Predict the products at each electrode during the electrolysis of dilute sulfuric acid (using inert electrodes).

Q6 Extended

Explain what happens at each electrode during the electrolysis of aqueous copper(II) sulfate using copper electrodes.

Q7 Core

Describe the purpose of electroplating, and state which electrode the object to be plated must be connected to.

Q8 Core

Give one advantage of a hydrogen–oxygen fuel cell over burning a fossil fuel.

Q9 Core

Write the overall equation for the reaction occurring in a hydrogen–oxygen fuel cell.

Q10 Extended

Explain why aluminium is extracted using electrolysis rather than reduction with carbon.

Worked solutions

Show worked solutions

IGCSE Chemistry 0620 · Topic 4: Electrochemistry — Solutions

Electrochemistry — Practice: Solutions

Define electrolysis.

✓Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.

Which electrode attracts positive ions?

✓The cathode (negative electrode) attracts positive ions, since opposite charges attract.

Molten lead(II) bromide electrolysis products.

✓Cathode: lead metal. Anode: bromine gas.

Concentrated aqueous NaCl electrolysis products.

✓Cathode: hydrogen gas (sodium is more reactive than hydrogen). Anode: chlorine gas (chloride ions are present in concentration).

Dilute sulfuric acid electrolysis products.

✓Cathode: hydrogen gas. Anode: oxygen gas (no halide present, so OH⁻ is discharged from water).

Aqueous CuSO₄ with copper electrodes.

✓At the anode, copper metal dissolves into solution as Cu²⁺ ions. At the cathode, Cu²⁺ ions from solution are deposited as copper metal. The overall effect is copper transferring from the anode to the cathode, with the solution’s concentration staying constant — this is used to purify copper and in electroplating.

Purpose of electroplating; electrode connection.

✓Electroplating coats an object with a thin layer of metal, to protect it from corrosion or improve its appearance. The object to be plated is made the cathode (so the metal ions in solution are attracted to it and deposited on its surface).

Advantage of a hydrogen-oxygen fuel cell.

✓Any one of: the only product is water, so it produces no polluting gases (unlike burning a fossil fuel, which produces CO₂ and other pollutants); it is generally more energy-efficient than combustion.

Overall fuel cell equation.

✓2H₂ + O₂ → 2H₂O

Q10

Why is aluminium extracted by electrolysis?

✓Aluminium is too reactive to be extracted by reduction with carbon — carbon cannot remove oxygen from aluminium oxide, since aluminium is more reactive than carbon. Electrolysis (of molten aluminium oxide dissolved in molten cryolite) is used instead, since it can extract even highly reactive metals.

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