Electrolysis
Electrolysis is the process of using an electric current to break down, or decompose, an ionic compound that is molten or dissolved in solution, because only in these states are the ions free to move and carry charge. Its main industrial uses are the extraction of aluminium from molten aluminium oxide and the production of chlorine, hydrogen and sodium hydroxide from concentrated sodium chloride solution.
Method
- Learn the electrode names and what happens at each: the cathode is the negative electrode, where positively charged ions (cations) move to and gain electrons (reduction); the anode is the positive electrode, where negatively charged ions (anions) move to and lose electrons (oxidation). The mnemonic OIL RIG (oxidation is loss, reduction is gain) helps here.
- For a molten ionic compound, write the half equation at each electrode directly from its ions, making sure the number of electrons gained at the cathode equals the number lost at the anode within each half equation, for example Pb2+ + 2e- -> Pb at the cathode.
- For an aqueous solution, apply the extra rule at the cathode: hydrogen gas is produced instead of the metal, unless the metal is less reactive than hydrogen (only copper, silver and gold are commonly tested), in which case the metal itself is deposited.
- For an aqueous solution, apply the extra rule at the anode: a halogen gas (chlorine, bromine or iodine) is produced if a halide ion is present in the solution; if no halide ion is present, oxygen gas is produced instead, from the hydroxide ions/water.
- For the required practical investigating the electrolysis of aqueous solutions with inert electrodes, know how to identify each product: a glowing splint relights in oxygen, a lit splint gives a squeaky pop in hydrogen, and damp litmus paper is bleached (turns white) in chlorine.
- For aluminium extraction, remember that aluminium is too reactive to be extracted by reduction with carbon, so it is extracted by electrolysis instead; the aluminium oxide is dissolved in molten cryolite, which lowers its melting point and reduces the energy needed.
- For the electrolysis of concentrated sodium chloride solution (brine) with inert electrodes, learn the three useful products: hydrogen at the cathode, chlorine at the anode, and sodium hydroxide solution left behind.
Worked example
Molten lead bromide, PbBr2, is electrolysed using inert graphite electrodes. Write the half equation for the reaction at each electrode, and state the products formed.
- Identify the ions present in molten lead bromide: lead ions, Pb2+, and bromide ions, Br-.
- The positive lead ions move to the cathode (negative electrode) and gain electrons: Pb2+ + 2e- -> Pb.
- The negative bromide ions move to the anode (positive electrode) and lose electrons: 2Br- -> Br2 + 2e-.
- Check that the half equations balance: the cathode equation gains 2 electrons and the anode equation loses 2 electrons, so the electrons cancel overall.
- Final answer: lead metal is produced at the cathode, and orange bromine gas is produced at the anode.
Practice questions
Type your answer and press Check to be marked straight away, or reveal the answer and mark yourself.
Q1State the name given to the negative electrode in electrolysis.Show answer
Answer: The cathode.
Q2State the two materials commonly used as inert electrodes in electrolysis, and explain why they must be inert.Show answer
Answer: Graphite (carbon) and platinum; they must be inert (unreactive) so that the electrode itself does not react during electrolysis, which would interfere with the products formed.
Q3Molten zinc chloride, ZnCl2, is electrolysed using inert electrodes. Write the half equation for the reaction at (a) the cathode, and (b) the anode.Show answer
Answer: (a) Zn2+ + 2e- -> Zn (b) 2Cl- -> Cl2 + 2e-
Q4Aqueous copper sulfate solution is electrolysed using inert graphite electrodes. State the products formed at (a) the cathode and (b) the anode, giving a reason for each.Show answer
Answer: (a) Copper, because copper is less reactive than hydrogen so copper ions are discharged (gain electrons) in preference to hydrogen ions from the water. (b) Oxygen, because there is no halide ion present in copper sulfate solution, so hydroxide ions/water are discharged instead.
Q5Concentrated sodium chloride solution (brine) is electrolysed using inert electrodes. State the products formed at the cathode and at the anode.Show answer
Answer: Hydrogen at the cathode, because sodium is more reactive than hydrogen so hydrogen is discharged instead, and chlorine at the anode, because chloride is a halide ion so it is discharged.
Q6Explain why solid sodium chloride does not conduct electricity, but molten sodium chloride does.Show answer
Answer: In solid sodium chloride the ions are held in a fixed lattice and cannot move, so there are no charge carriers free to move; when molten, the ions are free to move throughout the liquid and can carry charge, so it conducts electricity.
Q7In the industrial extraction of aluminium, explain why the aluminium oxide is dissolved in molten cryolite rather than melted on its own.Show answer
Answer: Aluminium oxide has a very high melting point, so melting it alone would need a huge amount of energy; dissolving it in molten cryolite lowers the melting point of the mixture, reducing the energy needed and the cost of the process.
Q8In the industrial extraction of aluminium, explain why the carbon anodes need to be replaced regularly.Show answer
Answer: Oxygen gas produced at the anode reacts with the hot carbon of the anode to form carbon dioxide, so the anode gradually burns away and must be replaced.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Molten potassium bromide, KBr, is electrolysed using inert electrodes. (a) Name the products formed at the cathode and at the anode. (b) Write the half equation for the reaction at the anode.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 3 available
A student electrolyses a concentrated aqueous solution of sodium chloride using inert graphite electrodes. Predict the products formed at each electrode, and explain your predictions using the reactivity of the ions present.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 4 available
Aluminium is extracted from aluminium oxide by electrolysis rather than by reduction with carbon. Explain why electrolysis must be used to extract aluminium, why the aluminium oxide is dissolved in molten cryolite before electrolysis, and why this process uses large amounts of electrical energy.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 6 available
See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the electrolysis worksheet pack - 16 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 15 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
Next topics
Not quite what you needed?
Tell us what is missing on electrolysis, or which topic to write up next. Every request is read, and we reply to every one.
Build a full practice pack.
This topic is one of hundreds in the library - pick the ones a student needs and generate a printable PDF in minutes.