Uses of Electrolysis

This lesson covers the practical applications of electrolysis for Edexcel GCSE Chemistry (1CH0). You need to understand electroplating, the purification of copper, industrial uses, and the economic and environmental considerations associated with electrolysis.

---

Electroplating

Electroplating is the process of coating an object with a thin layer of metal using electrolysis. It is used to improve the appearance of objects, protect them from corrosion, or increase their hardness and durability.

How Electroplating Works

The electrolysis cell for electroplating is set up as follows:

Component	What It Is
Cathode (−)	The object to be plated (e.g. a steel spoon, a piece of jewellery)
Anode (+)	A piece of the plating metal (e.g. silver, nickel, chromium, gold)
Electrolyte	A solution containing ions of the plating metal (e.g. silver nitrate solution for silver plating)

The Process

1. The object to be plated is connected as the cathode (negative electrode).
2. The plating metal is connected as the anode (positive electrode).
3. The electrolyte contains dissolved ions of the plating metal.
4. When the d.c. power supply is switched on:
   • Metal ions from the solution are deposited onto the cathode (the object) — reduction.
   • The anode dissolves, releasing metal ions into the solution to replace those deposited — oxidation.

Example: Silver Plating a Spoon

Component	Detail
Cathode	Steel spoon
Anode	Pure silver bar
Electrolyte	Silver nitrate solution (AgNO₃)

At the cathode (spoon): Ag⁺(aq) + e⁻ → Ag(s) — silver is deposited onto the spoon

At the anode (silver bar): Ag(s) → Ag⁺(aq) + e⁻ — silver dissolves to replace ions used up

The spoon becomes coated in a thin, even layer of silver.

Common Electroplating Applications

Metal Used	Object Being Plated	Reason
Silver	Cutlery, jewellery	Attractive appearance
Gold	Jewellery, electrical contacts	Appearance; excellent electrical conductivity; does not tarnish
Chromium	Car bumpers, taps, bicycle parts	Shiny appearance; corrosion resistance; hard-wearing
Nickel	Tools, machine parts	Corrosion resistance; hard surface
Tin	Steel cans (tin cans)	Prevents rusting; non-toxic for food contact
Zinc	Steel structures (galvanising)	Sacrificial protection against corrosion

Exam Tip: In an exam question on electroplating, you must state which component is the cathode (the object), which is the anode (the plating metal), and what the electrolyte is (a solution of the plating metal salt). A common mistake is to get the anode and cathode the wrong way round.

---

Purification of Copper by Electrolysis

Copper extracted from its ore is not pure enough for use in electrical wiring (impurities increase electrical resistance). Electrolysis is used to purify it.

Setup

Component	Detail
Cathode (−)	A thin strip of pure copper
Anode (+)	A large lump of impure copper
Electrolyte	Copper sulfate solution (CuSO₄)

The Process

1. The impure copper anode dissolves. Copper atoms lose electrons and enter the solution as Cu²⁺ ions.

   Anode: Cu(s) → Cu²⁺(aq) + 2e⁻ (oxidation)

2. Cu²⁺ ions from the solution are deposited onto the pure copper cathode.

   Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s) (reduction)

3. Impurities in the anode that are less reactive than copper (e.g. gold, silver, platinum) do not dissolve. They fall to the bottom of the cell and collect as anode sludge (or anode mud). This sludge can be processed to recover valuable metals.

4. Impurities that are more reactive than copper (e.g. zinc, iron) dissolve into the solution but are not deposited at the cathode because copper ions are preferentially reduced.

Result

• The cathode grows larger as pure copper is deposited on it.
• The anode shrinks as impure copper dissolves.
• The concentration of Cu²⁺ in the solution remains approximately constant.
• The final product is copper of 99.99% purity — suitable for electrical wiring.

Exam Tip: Questions on copper purification often ask you to explain what happens to the impurities. Less reactive impurities (precious metals) fall as sludge. More reactive impurities dissolve into solution but are not deposited.

---

Industrial Uses of Electrolysis

1. Extraction of Reactive Metals

Metals that are more reactive than carbon cannot be extracted by reduction with carbon. They must be extracted by electrolysis of their molten compounds.

Metal	Source Compound	Key Detail
Aluminium	Al₂O₃ dissolved in cryolite	Very high electricity costs
Sodium	Molten NaCl (Downs process)	Produces sodium metal and chlorine gas

2. Production of Chlorine and Sodium Hydroxide (Chlor-Alkali Industry)

The electrolysis of brine (concentrated sodium chloride solution) produces three important industrial chemicals:

Product	Electrode/Location	Uses
Chlorine (Cl₂)	Anode	Water purification, making PVC plastic, bleach, disinfectants
Hydrogen (H₂)	Cathode	Making margarine (hydrogenation), ammonia (Haber process), fuel
Sodium hydroxide (NaOH)	Left in solution	Making soap, paper, bleach, ceramics

This is one of the most economically important electrolysis processes in the world.

3. Anodising Aluminium

Electrolysis can be used to thicken the oxide layer on aluminium, making it more resistant to corrosion and allowing it to be dyed different colours. The aluminium object is made the anode in an electrolysis cell containing dilute sulfuric acid. Oxygen produced at the anode reacts with the aluminium surface to form a thicker Al₂O₃ layer.

---

Economic and Environmental Considerations

Economic Factors

Factor	Detail
Electricity costs	Electrolysis requires large amounts of electrical energy, especially for extracting reactive metals. This is the largest running cost.
Electrode replacement	Graphite anodes in aluminium extraction burn away and must be replaced regularly.
Raw material costs	The cost of the ore, cryolite, and other chemicals.
Value of by-products	Anode sludge in copper purification contains gold and silver, which can be sold to offset costs.

Environmental Factors

Factor	Detail
Energy source	If the electricity comes from burning fossil fuels, the process has a large carbon footprint. Using renewable energy (hydroelectric, solar, wind) reduces this.
Mining impacts	Extracting the ores causes habitat destruction, noise, dust, and visual pollution.
Toxic products	Chlorine gas is toxic and must be handled carefully. Waste electrolyte solutions must be treated before disposal.
Recycling	Recycling metals (especially aluminium) uses far less energy than extracting them from ore by electrolysis. Recycling aluminium uses only about 5% of the energy needed for extraction.

Exam Tip: If a question asks you to evaluate the use of electrolysis, you need to give both advantages (high purity product, can extract reactive metals, produces useful by-products) and disadvantages (high energy cost, carbon footprint, environmental damage from mining). Always link your points to specific examples.

---

Exam Technique for Electrolysis Questions

Electrolysis questions are among the most common on Edexcel Chemistry papers. Here is a checklist for answering them:

For Any Electrolysis Question:

1. State the electrolyte and whether it is molten or aqueous.
2. Identify the ions present (including H⁺ and OH⁻ if aqueous).
3. State the product at each electrode and explain why (using the rules for prediction).
4. Write half equations with state symbols.
5. Label oxidation and reduction (OIL RIG).
6. Describe observations at each electrode.

For 6-Mark Extended Writing:

• Use a logical sequence (setup → ions present → ion movement → reaction at cathode → reaction at anode → observations).
• Include technical vocabulary: electrolyte, electrode, anode, cathode, cation, anion, oxidation, reduction, half equation.
• Write balanced half equations with state symbols.
• If comparing two setups (e.g. inert vs reactive electrodes), use a structured comparison.