Electrolysis Applications
Summary: Industrial applications of electrolysis: extraction of aluminium from bauxite (Hall-Heroult process with cryolite), copper refining, electroplating with chromium/silver/nickel, anodising aluminium, and hydrogen production via water electrolysis. Tags: igcse chemistry electrolysis Created: 2026-07-14 Last Updated: 2026-07-16
Learning Objectives
By the end of this topic, you should be able to:
- Describe the extraction of aluminium using the Hall-Heroult process, including the role of cryolite
- Explain how copper is refined by electrolysis
- Describe electroplating with silver, chromium, and nickel, including the setup and half-equations
- Explain the process and purpose of anodising aluminium
- Describe how hydrogen is produced by the electrolysis of water
Content
1. Extraction of Aluminium — The Hall-Heroult Process
Aluminium is too reactive to be extracted by reduction with carbon (it is above carbon in the reactivity series). Instead, it is extracted by electrolysis of molten aluminium oxide (Al2O3).
The Problem: Aluminium oxide has a very high melting point (~2072°C). Melting it directly would be extremely expensive in energy terms.
The Solution — Cryolite: Aluminium oxide is dissolved in molten cryolite (Na3AlF6). This lowers the melting point to about 950°C, saving enormous amounts of energy and making the process economically viable.
The Setup
| Feature | Detail |
|---|---|
| Electrolyte | Al2O3 dissolved in molten cryolite |
| Temperature | ~950°C |
| Cathode (-) | Carbon (graphite) lining of the steel cell |
| Anode (+) | Large carbon (graphite) blocks |
| Container | Steel cell lined with carbon |
Reactions
At the Cathode (-): Al3+ + 3e- → Al(l)
Molten aluminium is produced. It is denser than the electrolyte and collects at the bottom of the cell, where it is periodically tapped off.
At the Anode (+): 2O2- → O2(g) + 4e-
The oxygen produced reacts with the carbon anodes, forming CO2: C(s) + O2(g) → CO2(g)
Key consequence: The carbon anodes are gradually consumed (they burn away) and need to be replaced regularly. This adds to the cost.
Overall Equation
2Al2O3(l) → 4Al(l) + 3O2(g)
Why This Process is Expensive
- High temperature required (~950°C) → high energy cost
- Carbon anodes need regular replacement
- Electricity is expensive
- Aluminium is often extracted near hydroelectric power stations for cheap electricity
2. Copper Refining (Electrolytic Purification)
Impure copper from smelting is about 98-99% pure. For use in electrical wiring, purity must be >99.99%. Electrolysis achieves this.
| Feature | Detail |
|---|---|
| Anode (+) | Impure copper (large block) |
| Cathode (-) | Pure copper (thin sheet/starter plate) |
| Electrolyte | Copper(II) sulfate solution (CuSO4(aq)) + dilute H2SO4 |
Reactions
At the Anode (+) — Oxidation: Cu(s) → Cu2+(aq) + 2e-
The impure copper anode dissolves. More reactive impurities (Zn, Fe) also dissolve but are NOT deposited at the cathode because they are harder to reduce than Cu2+. Less reactive impurities (Ag, Au, Pt) do NOT dissolve and fall to the bottom as anode sludge — these are valuable and are recovered.
At the Cathode (-) — Reduction: Cu2+(aq) + 2e- → Cu(s)
Pure copper (99.99%) plates onto the cathode. The cathode grows in size.
Result: Pure copper transfers from the anode to the cathode. The anode sludge contains precious metals (silver, gold, platinum) which can be sold to offset costs.
3. Electroplating
Electroplating is the process of coating one metal with a thin layer of another metal using electrolysis. It is done for:
- Protection from corrosion (e.g., chromium on steel)
- Improved appearance (e.g., silver on cutlery)
- Reduced cost (thin layer of expensive metal over cheap base metal)
The General Electroplating Setup
| Feature | Detail |
|---|---|
| Anode (+) | Bar of the plating metal (e.g., silver, chromium, nickel) |
| Cathode (-) | The object to be plated |
| Electrolyte | Solution containing ions of the plating metal |
Specific Electroplating Examples
| Plating Metal | Electrolyte | Anode | Purpose |
|---|---|---|---|
| Silver (Ag) | Silver nitrate or silver cyanide solution | Silver bar | Jewellery, cutlery, decorative items |
| Chromium (Cr) | Chromium(III) sulfate / chromic acid solution | Lead anode (not Cr — CrO3 used) | Car bumpers, bathroom fittings, corrosion protection |
| Nickel (Ni) | Nickel(II) sulfate solution | Nickel bar | Undercoat for chromium, corrosion resistance |
Half-Equations for Silver Plating
At the Anode (+): Ag(s) → Ag+(aq) + e- (silver bar dissolves)
At the Cathode (-): Ag+(aq) + e- → Ag(s) (silver plates onto object)
The concentration of Ag+ in the electrolyte remains constant because the rate of dissolution at the anode equals the rate of deposition at the cathode.
Conditions for Good Electroplating
- The object to be plated must be thoroughly cleaned (grease/dirt prevents adhesion)
- The electrolyte must contain ions of the plating metal
- Current and time must be controlled for even, adherent coating
4. Anodising Aluminium
Anodising is an electrolytic process that thickens the natural aluminium oxide layer on aluminium, making it more resistant to corrosion and enabling it to absorb dyes.
| Feature | Detail |
|---|---|
| Anode (+) | The aluminium object to be anodised |
| Cathode (-) | Another piece of aluminium or lead |
| Electrolyte | Dilute sulfuric acid (H2SO4(aq)) |
What Happens
- At the anode, oxygen is produced: 4OH- → O2 + 2H2O + 4e-
- This oxygen reacts with the aluminium surface, building up a thicker Al2O3 layer
- The oxide layer is porous — dyes can be absorbed into the pores
- After dyeing, the pores are sealed by boiling in water or steam
Uses of anodised aluminium: Window frames, kitchen utensils, phone cases, architectural panels (especially when dyed different colours).
5. Hydrogen Production — Electrolysis of Water
Hydrogen can be produced on an industrial scale by the electrolysis of water. This is important as a potential source of green hydrogen (when powered by renewable electricity).
Setup
| Feature | Detail |
|---|---|
| Electrolyte | Water with a small amount of acid (H2SO4) or alkali (NaOH) to improve conductivity |
| Cathode (-) | 2H+ + 2e- → H2(g) |
| Anode (+) | 4OH- → O2(g) + 2H2O(l) + 4e- |
| Volume ratio | 2 volumes H2 : 1 volume O2 |
Pure water is a poor conductor. Adding an acid or alkali provides ions that make the solution conductive without changing the net products (H2 and O2).
Advantages: Produces very pure hydrogen; can use renewable electricity (solar/wind).
Disadvantages: Expensive (high electricity cost); currently most hydrogen is made from natural gas (steam reforming), which produces CO2.
Worked Examples (Exam-Style)
Example 1: Aluminium Extraction
Question: Explain why aluminium is extracted by electrolysis rather than by reduction with carbon. [2 marks]
Solution:
- Aluminium is more reactive than carbon / aluminium is above carbon in the reactivity series ✓
- Carbon cannot reduce aluminium oxide ✓
Example 2: Role of Cryolite
Question: Explain the role of cryolite in the extraction of aluminium. [2 marks]
Solution:
- Cryolite dissolves the aluminium oxide ✓
- It lowers the melting point (from ~2072°C to ~950°C), reducing energy costs ✓
Example 3: Copper Refining
Question: During the electrolytic refining of copper, what happens to the silver impurity present in the anode? [1 mark]
Solution: Silver is less reactive than copper, so it does NOT dissolve. It falls to the bottom of the cell as part of the anode sludge. ✓
Practice Questions
-
Name the ore from which aluminium is extracted and state its chemical name. [2 marks]
-
Explain why the carbon anodes need to be replaced regularly during the extraction of aluminium. [2 marks]
-
Describe, with a labelled diagram, how a steel spoon can be electroplated with silver. Include the half-equation at the cathode. [5 marks]
-
State two reasons why objects are electroplated. [2 marks]
-
Explain the difference between electroplating and anodising. [2 marks]
Key Facts to Memorise
- Aluminium is extracted from bauxite (Al2O3) by electrolysis, dissolved in molten cryolite at ~950°C
- Cryolite lowers the melting point of Al2O3, saving energy
- Cathode: Al3+ + 3e- → Al(l). Anode: 2O2- → O2 + 4e-
- Carbon anodes burn away (C + O2 → CO2) and must be replaced
- Copper refining: impure Cu anode → pure Cu cathode; impurities fall as anode sludge
- Electroplating: plating metal as anode, object to be plated as cathode, electrolyte contains ions of the plating metal
- Anodising: aluminium object as anode in H2SO4(aq) → thickens oxide layer, absorbs dyes
Common Misconceptions
| Students often think… | But the correct understanding is… |
|---|---|
| ”Aluminium is extracted by reduction with carbon like iron” | Aluminium is too reactive — above carbon in the reactivity series — and can only be extracted by electrolysis |
| ”Cryolite is a catalyst in aluminium extraction” | Cryolite is a solvent — it dissolves Al2O3 to lower the melting point. It is not chemically changed in the process but it is not acting as a catalyst |
| ”In electroplating, the plating metal is the cathode” | The plating metal is the ANODE (it dissolves). The object to be plated is the CATHODE (metal deposits on it) |
| “In copper refining, the pure copper is the anode” | The IMPURE copper is the anode; the PURE copper is the cathode |
Key Concepts from Past Papers
Definitions You MUST Know (Exact Mark Scheme Wording)
- Bauxite: the ore of aluminium, mainly aluminium oxide (Al2O3)
- Cryolite: Na3AlF6, used to dissolve Al2O3 and lower its melting point
- Electroplating: coating an object with a thin layer of metal using electrolysis
- Anodising: thickening the natural oxide layer on aluminium by electrolysis
Recurring Mark Scheme Answers
- negative electrode: potassium positive electrode: chlorine observations: (yellow-) green vapour
- anode (left) AND cathode (right) correctly labelled leads connected correctly to electrodes test-tubes over both electrodes and dipping into electrolyte
- positive electrode: chlorine negative electrode: magnesium
- positive electrode: iodine negative electrode: sodium
- left hand electrode labelled anode electrolyte labelled
- negative electrode: lead positive electrode: bromine
- anode (on right) and cathode (on left) power supply added AND connecting wire from each end of power supply to separate electrodes
- positive electrode: bromine / Br2 negative electrode: magnesium / Mg
Common Question Types
- Extended writing (describe/explain/suggest): “Describe how aluminium is extracted from its ore”
- Diagram/labelling task: “Label the diagram of the Hall-Heroult cell”
- Short recall (state/give/name): “State the role of cryolite in aluminium extraction”
- Comparison: “Compare copper refining with electroplating”
Exam Tips
- This topic appears in 26 papers in the database
- Total Q+A entries: 41
- Average marks per question: 1.7
- The most commonly tested content is the Hall-Heroult process — know cryolite’s role and why carbon anodes need replacing
- For electroplating diagrams, anode = plating metal, cathode = object, electrolyte = solution of plating metal ions
- Be able to distinguish anodising (object is the ANODE, thickens oxide layer) from electroplating (object is the CATHODE, coated with different metal)
Keywords from Past Papers
electrode, labelled, negative, positive, anode, cathode, correctly, electrolyte, chlorine, bromine, magnesium, observations, left, electrodes, power
Related Notes
- Electrolysis — Molten electrolysis basics
- Electrolysis of Aqueous Solutions — Aqueous electrolysis and ion competition rules
- Reactivity Series — Why aluminium cannot be extracted with carbon
- IGCSE-Chem-Index — Full IGCSE Chemistry index
Sources
- OpenStax Chemistry 2e — Chapter 17: Electrochemistry, Rice University (free, CC BY 4.0)
- BBC Bitesize GCSE Chemistry — Electrolysis Applications, BBC (free educational resource)
- Cambridge IGCSE Chemistry 0620 — Syllabus topic 4 (Electrochemistry), Cambridge Assessment International Education
- CK-12 Chemistry for High School — Electrolysis Applications, CK-12 Foundation (free, CC BY-NC 3.0)
Past Paper Sources
- 0620/32 Feb/March 2017: Q22(a)(ii) (1m)
- 0620/32 Feb/March 2018: Q55(a)(i) (1m)
- 0620/32 Feb/March 2019: Q66(a)(ii) (2m), Q66(d)(i) (2m)
- 0620/32 Feb/March 2020: Q77(a)(i) (3m), Q77(a)(i) (2m)
- 0620/32 Feb/March 2021: Q77(b)(ii) (1m)
- 0620/32 Feb/March 2022: Q66(c)(iii) (2m)
- 0620/32 Feb/March 2023: Q55(b)(i) (2m), Q55(b)(ii) (1m)
- 0620/32 May/June 2018: Q55(a)(i) (1m), Q55(a)(ii) (1m), Q55(a)(iii) (1m)
- 0620/32 May/June 2020: Q55(a)(i) (2m), Q55(a)(ii) (2m)
- 0620/33 May/June 2016: Q22(c)(ii) (1m)
- 0620/33 May/June 2017: Q22(d)(i) (1m)
- 0620/33 May/June 2022: Q33(b)(iv) (2m), Q77(b)(i) (2m), Q77(b)(ii) (2m)