Ch 1 · Electrolysis · IGCSE Chemistry 0620
Topic 1 of 2 in Electrochemistry — IGCSE Chemistry 0620 — 48 lessons.
Introduction
Aluminium, chlorine, sodium hydroxide and pure copper are all made by passing electricity through ionic compounds. Questions reward students who name the electrodes correctly, explain where the electrons go and predict the products for the four standard electrolytes; Extended candidates also write the half-equations.
Key ideas — Definitions and the parts of a cell
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor where everyone is paired up (ions). Normally, they just move around randomly. Electrolysis is like a sudden, powerful magnetic force pulling all the 'positive' dancers to one side and all the 'negative' dancers to the other. Once they reach the walls (electrodes), they undergo a chemical change to become something entirely new.
📖 Definitions
🔍 Worked example — Easy
Identify the products formed during the electrolysis of molten lead(II) bromide (PbBr_2) using inert electrodes.
🔍 Worked example — Standard
Calculate the mass of copper produced at the cathode during the electrolysis of aqueous CuSO_4 if 0.5 moles of electrons are transferred, given A_r(Cu) = 64.
🔍 Worked example — Exam-style
Predict the products at the electrodes for the electrolysis of concentrated aqueous sodium chloride (NaCl) with inert electrodes.
⚠️ Common errors
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🎯 Exam question
Key ideas — How charge is transferred
Three different movements carry the charge round the complete circuit.
🧑🏫 Teacher's explanation
Think of electrolysis like a water delivery system in a city. The wires are the pipes where the 'flow' is the movement of electrons. The electrolyte is like a large reservoir of water containing different types of particles (ions). When you turn on the pump (the power supply), the particles in the reservoir start moving toward specific gates.
📖 Definitions
🔍 Worked example — Easy
In the electrolysis of molten lead(II) bromide, Pb^2+ ions move to the cathode. Write the half-equation for the reduction of Pb^2+ at the cathode.
🔍 Worked example — Standard
During the electrolysis of aqueous sodium chloride, Cl^- ions move to the anode. Write the half-equation for the oxidation of Cl^- ions.
🔍 Worked example — Exam-style
A current of 0.5 A flows through an electrolytic cell for 100 s. Calculate the total charge Q transferred in Coulombs (Q = I × t).
⚠️ Common errors
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🎯 Exam question
Key ideas — Predicting products with inert electrodes
A molten binary compound contains only two kinds of ion, so the products are simply the metal and the non-metal. Molten lead(II) bromide gives lead at the cathode and brown bromine vapour at the anode.
🧑🏫 Teacher's explanation
Think of electrolysis as a high-stakes competition at the surface of two metal plates (electrodes). The ions in the liquid are like commuters rushing toward two different exits. The cathode is the 'negative' exit, attracting positive ions, while the anode is the 'positive' exit, attracting negative ions.
📖 Definitions
🔍 Worked example — Easy
Predict the products of the electrolysis of molten PbBr_2. Write the half-equations for the cathode and anode.
🔍 Worked example — Standard
Predict the products at each electrode for the electrolysis of concentrated aqueous NaCl.
🔍 Worked example — Exam-style
A student electrolyzes dilute sulfuric acid (H_2 extSO_4) using inert platinum electrodes. Write the ionic half-equation for the reaction at the anode.
⚠️ Common errors
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🎯 Exam question
(a) State the products formed at the cathode and the anode. [2] (b) Write the ionic half-equation for the reaction occurring at the anode. [1] [3 marks]
Key ideas — Copper electrodes: purification and electroplating
🧑🏫 Teacher's explanation
Imagine you are building a LEGO model. In standard electrolysis with inert electrodes (like graphite), the electrodes are just 'tools' that don't change. But in copper refining, the electrodes are 'active participants.' Think of the anode as a source of raw material and the cathode as the construction site. The anode literally melts away, releasing copper ions into the solution, which then travel to the cathode to be rebuilt into a solid layer.
📖 Definitions
🔍 Worked example — Easy
In the purification of copper, the half-equation at the anode is Cu(s) arrow Cu^2+(aq) + 2e^-. If 0.5 moles of copper atoms dissolve from the anode, how many moles of electrons are released?
🔍 Worked example — Standard
A student wants to silver-plate a steel spoon. They use a silver anode and a silver nitrate (AgNO_3) electrolyte. Write the half-equation for the reaction occurring at the cathode.
🔍 Worked example — Exam-style
During the refining of copper, 12.0 g of copper is deposited at the cathode. Calculate the mass of copper that must have dissolved from the anode to maintain a constant concentration of Cu^2+ ions in the electrolyte. (Assume A_r of Cu = 64)
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Worked example
Question. Concentrated aqueous sodium chloride is electrolysed using inert electrodes. (a) Name the product at each electrode and state a test for the anode product. [3] (b) (Supplement) Write half-equations for the reactions at the cathode and the anode. [2] (c) Explain why the solution becomes alkaline. [2]
🔍 Worked example — Easy
Predict the products when molten lead(II) bromide (PbBr_2) is electrolysed using inert electrodes.
🔍 Worked example — Standard
Aqueous copper(II) sulfate (CuSO_4) is electrolysed using inert electrodes. Predict the products at the cathode and anode.
🔍 Worked example — Exam-style
Write the ionic half-equation for the oxidation of chloride ions at the anode during the electrolysis of concentrated NaCl.
⚠️ Common errors
Common mistakes
Practice
Answers The decomposition of an ionic compound, when molten or in aqueous solution [B1], by the passage of an electric current [B1]. Lead at the cathode [B1]; bromine at the anode [B1]; brown (orange-brown) vapour or bubbles seen [B1]. Hydrogen and oxygen [B1]; 2H^+ + 2e^- arrow H_2 and 4OH^- arrow O_2 + 2H_2O + 4e^-, so four electrons give two hydrogen molecules but only one oxygen molecule [M1]; equal numbers of electrons pass each electrode, so the mole (and volume) ratio is 2 : 1 [A1]. 4OH^-(aq) arrow O_2(g) + 2H_2O(l) + 4e^- [B1]; the blue colour fades as copper ions are removed [B1]. Spoon as the cathode [B1]; silver as the anode [B1]; electrolyte containing silver ions, e.g. silver nitrate solution [B1].