Electrochemistry: IGCSE Chemistry 0620

Electrolysis questions are the most predictable structured questions in 0620 (a labelled cell, “name the product at each electrode”, then an explanation or half-equation), worth 4-7 marks on Paper 3 or 4 plus 2-3 MCQs, and the topic is a regular subject for the 6-mark extended response. Examiner reports highlight one repeated failure: students memorise products for one electrolyte and transfer them blindly to another, giving oxygen from concentrated sodium chloride or sodium metal from brine. The rules below remove the guesswork.

Electrolysis: the core ideas (Core)

Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current. Four definitions are tested as single marks:

• Electrolyte: the molten or aqueous ionic compound that conducts and is decomposed.
• Cathode: the negative electrode. Positive ions (cations) move to it and are reduced.
• Anode: the positive electrode. Negative ions (anions) move to it and are oxidised.
• Inert electrodes: carbon (graphite) or platinum: they conduct but do not react.

Solid ionic compounds do not conduct because the ions are held in fixed positions in the lattice; melting or dissolving frees them to move. That sentence ties this topic to bonding in atoms, elements and compounds.

For a molten binary compound the products are forced: the metal at the cathode, the non-metal at the anode. Molten lead(II) bromide, the Core set-piece, gives lead at the cathode and bromine (orange-brown vapour) at the anode.

The syllabus also names two aqueous Core examples to learn as facts: concentrated aqueous sodium chloride gives hydrogen at the cathode and chlorine at the anode (with sodium hydroxide left in solution), and dilute sulfuric acid gives hydrogen at the cathode and oxygen at the anode.

(S) Charge transfer earns its own mark: electrons flow through the external circuit from anode to cathode via the power supply, while inside the electrolyte the current is carried by moving ions. Saying “electrons flow through the solution” is a standard examiner-report error.

Electrolysis of aqueous solutions (S)

Water adds H+ and OH− ions to the electrolyte, so each electrode becomes a competition.

At the cathode: hydrogen is produced unless the metal is less reactive than hydrogen. Copper and silver ions discharge as the metal; sodium, potassium and magnesium ions stay in solution and hydrogen forms instead. The reactivity series is the deciding list.

At the anode with inert electrodes: oxygen is produced from dilute solutions, but if a halide ion (Cl−, Br−, I−) is present in high concentration, the halogen is produced instead. So concentrated NaCl(aq) gives chlorine; dilute NaCl(aq) gives mostly oxygen.

Half-equations carry 1 mark each and follow two templates:

Electrode	Example half-equation	Process
Cathode	Cu2+ + 2e− → Cu	Reduction (electron gain)
Cathode	2H+ + 2e− → H2	Reduction
Anode	2Cl− → Cl2 + 2e−	Oxidation (electron loss)
Anode	4OH− → O2 + 2H2O + 4e−	Oxidation

Reduction at the cathode, oxidation at the anode. Electrolysis is applied redox, and OILRIG does the bookkeeping.

Copper(II) sulfate is the favourite exam electrolyte because the electrodes matter. With inert carbon electrodes: copper at the cathode, oxygen at the anode, and the blue colour fades as Cu2+ ions are removed. With copper electrodes: the anode dissolves (Cu → Cu2+ + 2e−), the cathode gains copper of equal mass, and the colour stays constant, the basis of copper refining. A question that swaps carbon for copper electrodes mid-question is testing exactly this switch.

Electroplating (Core)

Electroplating coats an object with a thin layer of metal by electrolysis. The set-up never changes: the object is the cathode, the anode is made of the plating metal, and the electrolyte is an aqueous salt of the plating metal. Chromium-plating a tap: tap as cathode, chromium anode, chromium salt solution. The two accepted reasons for plating are improved appearance and protection from corrosion.

Exam questions usually award one mark per labelled component, so a 15-second sketch (labelled cathode, anode, electrolyte) banks 3 marks before any explanation starts.

Hydrogen–oxygen fuel cells (Core statement, Supplement evaluation)

Core candidates state one fact: a hydrogen–oxygen fuel cell uses hydrogen and oxygen to produce electricity, with water as the only product. The overall equation is 2H2 + O2 → 2H2O.

(S) Extended candidates weigh fuel cells against gasoline (petrol) engines. Advantages: water is the only product at the point of use, so no carbon dioxide or carbon monoxide; fuel cells are more efficient. Disadvantages: hydrogen is difficult and expensive to store and transport (it is a gas with very low density, stored at high pressure), and producing the hydrogen may itself burn fossil fuels. Balanced evaluation (at least one advantage and one disadvantage) is what the mark scheme rewards, and this “evaluate” structure is the same skill drilled in the 6-mark extended response technique.

Worked exam question

Concentrated aqueous sodium chloride is electrolysed using inert electrodes. (a) Name the product at each electrode. (b) Write the half-equation for the reaction at the anode. (c) Explain why sodium is not produced at the cathode. [5]

Model answer: (a) Cathode: hydrogen (1). Anode: chlorine (1). (b) 2Cl− → Cl2 + 2e− (also accepted as 2Cl− − 2e− → Cl2) (1). (c) Sodium is more reactive than hydrogen (1), so H+ ions from the water are discharged instead of Na+ ions (1).

Mark-by-mark: marks 1-2 are recall, but reversed electrodes is the classic throwaway: cathode is negative, and hydrogen (a positive ion’s product) forms there. Mark 3 needs a balanced half-equation with electrons on the correct side; Cl− → Cl + e− fails on both counts because chlorine is diatomic. Marks 4-5 split the explanation: the reactivity comparison is one idea, the consequence (H+ discharged in preference) is the second. One sentence covering both still earns both.

The mistakes that cost marks

1. Swapping the electrodes. The cathode is negative and attracts positive ions. Write “PANIC: Positive Anode, Negative Is Cathode” on your plan line before answering.
2. Giving sodium as a product from aqueous NaCl. Sodium is above hydrogen in the reactivity series, so hydrogen is discharged. Sodium metal only forms from molten NaCl.
3. Unbalanced half-equations: forgetting chlorine and oxygen are diatomic (Cl2, O2), or putting electrons on the wrong side. Check charge balances on both sides.
4. “Electrons flow through the electrolyte.” Ions carry the charge in the solution; electrons travel only through the electrodes and external wires.
5. Predicting oxygen at the anode for concentrated halide solutions. Concentration tips the competition to the halogen. That one word in the question controls the answer.

How to phrase it for full marks

Student wording	Mark-scheme wording
”The copper sticks to the negative rod"	"Cu2+ ions gain two electrons at the cathode: Cu2+ + 2e− → Cu"
"Sodium is too reactive to come out"	"Sodium is more reactive than hydrogen, so hydrogen is discharged at the cathode"
"Electricity goes through the liquid"	"Ions move through the electrolyte; electrons flow through the external circuit"
"The blue fades because copper is used up"	"Cu2+ ions are removed from solution and deposited as copper at the cathode"
"Fuel cells are cleaner"	"Water is the only product, so no carbon dioxide is released at the point of use”

The pattern for explanations here: name the ion, name the electrode, state gain or loss of electrons. Any answer carrying those three pieces matches the marking points.

The Malaysia note

Most international schools in KL, Penang and Johor teach electrochemistry late in Year 10 or early Year 11, close enough to the May/June series that content recall is fine. Phrasing is the gap. Malaysian students also benefit from a local anchor: Malaysia smelts aluminium (Press Metal in Sarawak runs on Bakun hydropower) using exactly the molten-electrolysis chemistry this topic covers, which makes the “why molten, why cryolite” extraction questions in the metals topic feel less abstract. Electrolysis prediction tables are a staple of our exam-technique sessions; a free 1-hour trial lesson with a Chemistry specialist will show you the full cathode/anode decision routine on a past-paper question before you commit to anything.

Every sub-topic in Electrochemistry

• Electrolysis
• Electrolysis of Aqueous Solutions (Supplement)
• Electroplating
• Hydrogen–Oxygen Fuel Cells (Supplement)