Electrolysis of Molten and Aqueous Compounds
Electrolysis at IGCSE goes beyond electrolysing simple molten ionic compounds to cover the selective discharge of ions in aqueous solutions, where water itself can be discharged at each electrode instead of the dissolved ions, writing balanced half-equations for the reactions at each electrode, and industrial applications such as the electrolytic purification of copper and electroplating.
Before you start
Make sure you're comfortable with these topics first:
Method
- Recap that electrolysis is the decomposition of an ionic compound (the electrolyte) by an electric current: positive ions (cations) move to and are reduced (gain electrons) at the negative cathode, and negative ions (anions) move to and are oxidised (lose electrons) at the positive anode.
- For a molten binary compound, the metal forms at the cathode and the non-metal forms at the anode; write a balanced half-equation for each electrode, balancing the electrons transferred - for example, for molten lead bromide: cathode Pb2+ + 2e- -> Pb, and anode 2Br- -> Br2 + 2e-.
- For an aqueous solution, water itself also provides H+ and OH- ions from its own slight ionisation, so apply the selective discharge rules: at the cathode, hydrogen gas is produced unless the metal is less reactive than hydrogen (such as copper), in which case the metal is deposited instead; at the anode, oxygen gas is produced unless a halide ion (chloride, bromide or iodide) is present in a high enough concentration, in which case the halogen gas is produced instead.
- Write half-equations for the electrodes commonly formed from water: cathode (reduction) 2H+ + 2e- -> H2, or 4H2O + 4e- -> 2H2 + 4OH-; anode (oxidation) 4OH- -> O2 + 2H2O + 4e-, or 2H2O -> O2 + 4H+ + 4e-.
- Describe the electrolytic purification of copper: an impure copper anode dissolves into solution as Cu2+ ions (Cu -> Cu2+ + 2e-), and pure copper is deposited on the cathode (Cu2+ + 2e- -> Cu) from a copper sulfate electrolyte, leaving the more reactive metal impurities in solution and the less reactive metal impurities as a solid 'anode sludge'.
- Describe electroplating: an object is made the cathode in a solution containing ions of the plating metal (for example, silver nitrate solution to silver-plate cutlery), so the plating metal is reduced and deposited as an even coating on the object's surface, often to improve appearance or resist corrosion.
Worked example
Copper sulfate solution is electrolysed using inert (graphite) electrodes. Predict, with reasons, the products at the cathode and the anode, and write a balanced half-equation for each.
- Identify the ions present: Cu2+ and SO4^2- from the copper sulfate, plus H+ and OH- from the water.
- At the cathode, compare copper to hydrogen in reactivity: copper is less reactive than hydrogen, so copper ions are discharged in preference to hydrogen ions.
- Write the cathode half-equation: Cu2+ + 2e- -> Cu.
- At the anode, since there is no halide ion present, hydroxide ions from water are discharged in preference to sulfate ions, producing oxygen gas.
- Write the anode half-equation: 4OH- -> O2 + 2H2O + 4e-.
- Final answer: copper is deposited at the cathode and oxygen gas is produced at the anode.
Practice questions
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Q1State whether reduction or oxidation occurs at the cathode during electrolysis.Show answer
Answer: Reduction (the ion gains electrons).
Q2Write the half-equation for the formation of chlorine gas at the anode during the electrolysis of molten sodium chloride.Show answer
Answer: 2Cl- -> Cl2 + 2e-
Q3State the product formed at the cathode when dilute sulfuric acid is electrolysed using inert electrodes.Show answer
Answer: Hydrogen gas.
Q4Predict the product formed at the anode when concentrated sodium chloride solution (brine) is electrolysed.Show answer
Answer: Chlorine gas; the chloride ion is discharged in preference to hydroxide ions, because it is present in a high concentration.
Q5In the electrolytic purification of copper, state which electrode, the anode or the cathode, is made of impure copper.Show answer
Answer: The anode.
Q6Describe what happens at the cathode when an object is electroplated with silver from a silver nitrate solution.Show answer
Answer: Silver ions (Ag+) in the solution are reduced (gain an electron) and deposited as a layer of solid silver metal onto the surface of the object, which acts as the cathode: Ag+ + e- -> Ag.
Q7State two reasons why an object might be electroplated.Show answer
Answer: To improve its appearance, giving it an attractive, shiny coating, and/or to protect it from corrosion using a less reactive metal coating.
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
Molten zinc chloride, ZnCl2, is electrolysed using inert electrodes. (a) Identify the product formed at each electrode. (b) Write a balanced half-equation for the reaction at the cathode.
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Dilute sulfuric acid is electrolysed using inert (platinum) electrodes. (a) Name the gas produced at the cathode and the gas produced at the anode. (b) Write a balanced half-equation for the reaction at the anode.
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A student electrolyses concentrated aqueous sodium chloride solution (brine) using inert graphite electrodes, and separately electrolyses molten sodium chloride using inert electrodes. Explain, in terms of the ions present and the rules of selective discharge, why the products at the anode are the same in these two experiments while the products at the cathode are different.
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Free printable worksheet
Want more practice on paper? Download the electrolysis of molten and aqueous compounds worksheet pack - 5 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 18 of IGCSE Science Workbook, the whole course as one free printable PDF.
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