What you'll learn
This revision guide covers the two most important industrial applications of electrolysis tested at AQA GCSE level: extracting aluminium from its ore and the chlor-alkali process for producing chlorine and sodium hydroxide. You'll learn why these processes use specific conditions, how to write half equations, and the economic and environmental considerations surrounding large-scale electrolysis operations.
Key terms and definitions
Electrolysis — the decomposition of an ionic compound when molten or in aqueous solution by passing an electric current through it
Cryolite — a compound (Na₃AlF₆) added to aluminium oxide to lower its melting point from 2050°C to approximately 950°C during extraction
Anode — the positive electrode where oxidation occurs (electrons are lost)
Cathode — the negative electrode where reduction occurs (electrons are gained)
Bauxite — the main ore of aluminium, containing aluminium oxide (Al₂O₃)
Brine — a concentrated solution of sodium chloride in water used in the chlor-alkali process
Oxidation — the loss of electrons (or gain of oxygen)
Reduction — the gain of electrons (or loss of oxygen)
Core concepts
Why aluminium requires electrolysis for extraction
Aluminium is more reactive than carbon, so it cannot be extracted from its oxide by reduction with carbon in a blast furnace (unlike iron). The high position of aluminium in the reactivity series means only electrolysis provides sufficient energy to break the strong ionic bonds in aluminium oxide.
Bauxite is first purified to obtain aluminium oxide (Al₂O₃), which is then dissolved in molten cryolite. This mixture is electrolysed in large steel-lined cells.
Key reasons for using cryolite:
- Lowers the melting point from 2050°C to approximately 950°C
- Reduces energy costs significantly
- Makes the process economically viable
- The aluminium oxide dissolves in molten cryolite
The aluminium extraction process
The electrolysis cell consists of:
- A steel case lined with graphite (acts as the cathode)
- Large graphite blocks suspended in the molten mixture (act as anodes)
- Molten cryolite with dissolved aluminium oxide
- Operating temperature of approximately 950°C
At the cathode (negative electrode):
Aluminium ions gain electrons (reduction):
Al³⁺ + 3e⁻ → Al
Molten aluminium forms and sinks to the bottom of the cell (density 2.7 g/cm³), where it is tapped off periodically.
At the anode (positive electrode):
Oxide ions lose electrons (oxidation):
2O²⁻ → O₂ + 4e⁻
Oxygen gas forms at the anode. This reacts with the hot graphite anodes to produce carbon dioxide:
C + O₂ → CO₂
The graphite anodes gradually burn away and must be replaced regularly, adding to operating costs.
Overall equation:
2Al₂O₃ → 4Al + 3O₂
Economic and environmental considerations of aluminium extraction
Economic factors:
- Electrolysis requires enormous amounts of electricity (approximately 15,000 kWh per tonne of aluminium)
- Aluminium smelters are often located near sources of cheap electricity (hydroelectric power in Norway, Iceland)
- Graphite anodes need regular replacement
- High initial capital costs for building extraction plants
- Aluminium recycling uses only 5% of the energy needed to extract new aluminium
Environmental factors:
- High energy consumption contributes to greenhouse gas emissions if electricity generated from fossil fuels
- Carbon dioxide produced at anodes is a greenhouse gas
- Mining bauxite causes habitat destruction and landscape scarring
- Waste products (red mud) from purifying bauxite are toxic and difficult to dispose of
- Recycling aluminium significantly reduces environmental impact
The chlor-alkali process
The chlor-alkali process electrolyses concentrated sodium chloride solution (brine) to produce three commercially important products:
- Chlorine gas at the anode
- Hydrogen gas at the cathode
- Sodium hydroxide solution (the alkali)
The process uses an inert electrode system (typically titanium) to prevent reactions with the products.
At the cathode (negative electrode):
Water molecules gain electrons, not sodium ions (sodium is too reactive and would react immediately with water):
2H₂O + 2e⁻ → H₂ + 2OH⁻
Hydrogen gas bubbles off, leaving hydroxide ions in solution.
At the anode (positive electrode):
Chloride ions lose electrons (oxidation):
2Cl⁻ → Cl₂ + 2e⁻
Chlorine gas bubbles off at the anode.
In solution:
Sodium ions (Na⁺) remain in solution with the hydroxide ions (OH⁻) produced at the cathode, forming sodium hydroxide solution.
Overall equation:
2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH
Products and uses from the chlor-alkali process
Chlorine:
- Water treatment and disinfection (swimming pools, drinking water)
- Manufacturing bleach
- Production of plastics (especially PVC)
- Manufacturing of hydrochloric acid
Hydrogen:
- Production of ammonia in the Haber process
- Margarine manufacture (hydrogenation of vegetable oils)
- Rocket fuel
- Potential clean fuel for vehicles
Sodium hydroxide:
- Soap and detergent manufacture
- Paper manufacture
- Neutralising acidic waste
- Production of ceramics and rayon
The chlor-alkali process is economically attractive because it produces three valuable products simultaneously from cheap, abundant raw materials (salt and water).
Half equations and electron transfer
Writing correct half equations is essential for electrolysis questions at GCSE.
Key rules:
- Identify which species gains or loses electrons
- Balance atoms first
- Balance charge by adding electrons
- Check electrons match between cathode and anode (multiply if needed)
For aluminium extraction:
Cathode: Al³⁺ + 3e⁻ → Al
Anode: 2O²⁻ → O₂ + 4e⁻
To balance electrons, multiply the cathode equation by 4 and anode equation by 3:
4Al³⁺ + 12e⁻ → 4Al
6O²⁻ → 3O₂ + 12e⁻
For chlor-alkali:
Cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻
Anode: 2Cl⁻ → Cl₂ + 2e⁻
Electrons already balance (2e⁻ in each).
Worked examples
Example 1: Aluminium extraction calculation
Question: A company extracts 5400 kg of aluminium in one day. The electrolysis requires 15,000 kWh of electrical energy per tonne of aluminium produced.
(a) Calculate the total electrical energy required in kWh. [2 marks]
(b) Explain why recycling aluminium is preferable to extracting new aluminium. [2 marks]
Answer:
(a) 5400 kg = 5.4 tonnes [1 mark]
Energy = 5.4 × 15,000 = 81,000 kWh [1 mark]
(b) Recycling uses only 5% of the energy needed to extract new aluminium [1 mark], which reduces costs and carbon dioxide emissions / environmental impact [1 mark].
Mark scheme notes: Part (a) requires conversion to tonnes and correct multiplication. Part (b) needs one point about energy/cost and one about environmental benefit.
Example 2: Half equations in chlor-alkali process
Question: The diagram shows electrolysis of concentrated sodium chloride solution.
(a) Write the half equation for the reaction at the negative electrode. [2 marks]
(b) Explain why sodium metal is not produced at the negative electrode. [2 marks]
(c) State one use of the gas produced at the positive electrode. [1 mark]
Answer:
(a) 2H₂O + 2e⁻ → H₂ + 2OH⁻ [2 marks for correct equation with state symbols OR species and electrons; 1 mark if electrons on wrong side or minor error]
(b) Sodium is too reactive [1 mark] and would react immediately with water [1 mark].
Alternative: Hydrogen ions / water molecules are preferentially discharged [1 mark] because they are easier to reduce than sodium ions [1 mark].
(c) Any one from: water treatment / disinfection, making bleach, making plastics / PVC, making hydrochloric acid [1 mark]
Mark scheme notes: Half equations must show correct charge balance and electron positioning. "Explain" requires a reason, not just a statement.
Example 3: Aluminium extraction reasoning
Question: Aluminium is extracted by electrolysis of a molten mixture containing aluminium oxide and cryolite.
(a) Explain why cryolite is added to aluminium oxide. [2 marks]
(b) The graphite anodes need replacing regularly. Explain why. [3 marks]
Answer:
(a) Cryolite lowers the melting point [1 mark] which reduces energy costs / makes process economically viable [1 mark].
(b) Oxygen is produced at the anode [1 mark]. The oxygen reacts with the hot carbon / graphite [1 mark] to form carbon dioxide, so the anodes gradually burn away [1 mark].
Mark scheme notes: Part (a) requires both the effect and benefit. Part (b) needs the complete sequence: oxygen production → reaction with carbon → consumption of electrode.
Common mistakes and how to avoid them
Confusing which electrode is which — Remember: anode is positive (both start with vowels); cathode is negative. Oxidation occurs at the anode, reduction at the cathode (OILRIG: Oxidation Is Loss, Reduction Is Gain).
Writing Na instead of NaOH in chlor-alkali products — Sodium metal is NOT produced because it's too reactive. Water is preferentially reduced to produce hydrogen, leaving sodium ions with hydroxide ions in solution.
Forgetting that anodes burn away in aluminium extraction — The oxygen produced reacts with the graphite at high temperature. This doesn't happen in chlor-alkali because inert electrodes (titanium) are used and temperature is much lower.
Not balancing electrons in half equations — Always check that electrons lost at anode equal electrons gained at cathode. You may need to multiply whole equations.
Stating uses without specificity — "Water treatment" scores marks; "cleaning" doesn't. "Manufacture of plastics/PVC" scores marks; "making things" doesn't.
Mixing up energy and environmental points — Lower melting point reduces energy costs (economic) which then reduces carbon footprint (environmental). Make the connection clear in "explain" questions.
Exam technique for industrial electrolysis
Command word awareness — "State" requires facts without explanation (1 mark each). "Explain" requires reasons and typically scores 2-3 marks. "Write a half equation" needs correct species, electrons on correct side, and balanced atoms and charge.
Extract information from diagrams — Industrial electrolysis questions often include labelled diagrams. Use these to identify which electrode is which, direction of electron flow, and where products form.
Link structure to marks — A 3-mark "explain" question needs three distinct points or a three-step sequence (e.g., oxygen forms → reacts with carbon → electrode consumed). Don't repeat the same point in different words.
Use precise chemical terminology — Write "aluminium ions gain electrons" not "aluminium gets electricity." Write "oxide ions are oxidised" not "oxygen is made." Examiners reward scientific accuracy.
Quick revision summary
Industrial electrolysis has two key GCSE applications. Aluminium extraction electrolyses aluminium oxide dissolved in molten cryolite at 950°C, producing aluminium at the cathode and oxygen at the anode (which burns away graphite electrodes). The chlor-alkali process electrolyses brine, producing chlorine at the anode, hydrogen at the cathode, and sodium hydroxide in solution. Both processes require significant electrical energy but produce economically valuable products. Master half equations, electrode identification, and product uses for exam success.