Electrolytic processes
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Revision notes for Edexcel GCSE Chemistry Electrolytic processes. Open the guide for explanations and worked examples. Written against the Edexcel GCSE Chemistry (1CH0) specification, so the content matches what's examinable rather than general Chemistry background.

Electrolytic processes

What you'll learn

  • What an electrolyte is, and why it must be molten or dissolved in water.
  • How ions move to the cathode and anode during electrolysis.
  • How to predict products for molten and aqueous electrolytes.
  • How copper sulfate electrolysis links to copper purification and the core practical.

1. The particles that make electrolysis possible

An ionic compound is made from positive and negative ions held together in a regular structure. For example, sodium chloride contains sodium ions, Na⁺, and chloride ions, Cl⁻.

In a solid ionic compound, the ions are fixed in place, so they cannot move and carry charge. If the ionic compound is molten — melted into a liquid — or dissolved in water, the ions are free to move.

Definition

Electrolyte

An electrolyte is an ionic compound in the molten state or dissolved in water that can conduct electricity because its ions are free to move.

The word aqueous means “dissolved in water”, shown using the state symbol (aq). For example, copper sulfate solution contains Cu²⁺(aq) and SO₄²⁻(aq) ions.

Key Idea

Why solids do not work

Electrolysis needs moving ions. Solid ionic compounds do not act as electrolytes because their ions are locked in position.

2. What electrolysis is

Electrolysis is a process in which electrical energy from a direct current supply decomposes an electrolyte. Decomposes means “breaks down into simpler substances”.

A direct current, often shortened to d.c., is an electric current that flows in one direction only. Electrolysis uses two conductors called electrodes dipped into the electrolyte.

  • The cathode is the negative electrode in electrolysis.
  • The anode is the positive electrode in electrolysis.
  • Inert electrodes, such as graphite or platinum, conduct electricity but do not react with the electrolyte or products.

The diagram shows the key parts of an electrolysis cell: the d.c. supply, the electrodes, the electrolyte, and the direction ions move.

Labelled electrolysis cell showing cathode, anode, direct current supply, and ion movement

3. How ions move

A cation is a positively charged ion. Cations are attracted to the negative cathode.

An anion is a negatively charged ion. Anions are attracted to the positive anode.

Key Idea

Ion movement

During electrolysis, cations migrate to the cathode and anions migrate to the anode. “Migrate” just means move through the liquid or solution.

At the electrodes, ions are changed into atoms or molecules by gaining or losing electrons.

This next electron-transfer language is Higher Tier only, but it is very useful:

  • Reduction is gain of electrons.
  • Oxidation is loss of electrons.
  • Reduction happens at the cathode.
  • Oxidation happens at the anode.
Tip

OIL RIG

Remember: Oxidation Is Loss, Reduction Is Gain — of electrons.

Common Mistake

Mixing up electrode signs

In electrolysis, the cathode is negative and the anode is positive. Do not assume “cathode” always means positive — it depends on the type of electrical cell.

4. Predicting products from molten ionic compounds

A binary ionic compound is an ionic compound made from ions of only two elements, such as sodium chloride, NaCl, or lead bromide, PbBr₂.

Molten binary ionic compounds are the simplest cases because there is no water present. The electrolyte contains only the ions from the compound.

For a molten binary ionic compound:

  • the metal cation forms the metal at the cathode
  • the non-metal anion forms the non-metal at the anode

For example, molten lead bromide contains Pb²⁺ ions and Br⁻ ions. This experiment is usually done as a teacher demonstration because molten lead bromide is very hot and bromine is harmful.

Example

Predicting products from molten lead bromide

  1. Identify the ions in molten lead bromide: Pb²⁺(l) and Br⁻(l).

  2. The positive Pb²⁺ ions move to the negative cathode and gain electrons, so lead forms: Pb²⁺(l) + 2e⁻ → Pb(l).

  3. The negative Br⁻ ions move to the positive anode and lose electrons. Bromine atoms pair up as Br₂ molecules: 2Br⁻(l) → Br₂(g) + 2e⁻.

  4. The products are lead at the cathode and bromine at the anode.

5. Predicting products from aqueous electrolytes

Aqueous solutions are trickier because water is present. Water can provide H⁺(aq) and OH⁻(aq) ions, so the products may come from the water rather than from the dissolved ionic compound.

At the cathode

At the negative cathode, positive ions compete to gain electrons.

  • If the metal is less reactive than hydrogen, the metal forms.
  • If the metal is more reactive than hydrogen, hydrogen gas forms instead.

So copper ions form copper metal, but sodium ions do not form sodium metal in aqueous solution.

At the anode with inert electrodes

At the positive anode, negative ions compete to lose electrons.

  • If a halide ion is present, such as Cl⁻, Br⁻ or I⁻, a halogen forms.
  • If there is no halide ion, oxygen gas forms from water or hydroxide ions.

Key examples you need to know

For the Edexcel 1CH0 specification, you should know these:

  • Copper chloride solution, CuCl₂(aq): copper forms at the cathode and chlorine forms at the anode.
  • Sodium chloride solution, NaCl(aq): hydrogen forms at the cathode and chlorine forms at the anode.
  • Sodium sulfate solution, Na₂SO₄(aq): hydrogen forms at the cathode and oxygen forms at the anode.
  • Water acidified with sulfuric acid: hydrogen forms at the cathode and oxygen forms at the anode.
  • Molten lead bromide, PbBr₂(l): lead forms at the cathode and bromine forms at the anode.
Example

Predicting products from sodium sulfate solution

  1. At the cathode, the possible positive ions are Na⁺(aq) from sodium sulfate and H⁺(aq) from water. Sodium is more reactive than hydrogen, so hydrogen forms instead of sodium.

  2. The cathode product is hydrogen gas: 2H⁺(aq) + 2e⁻ → H₂(g).

  3. At the anode, the possible negative ions include SO₄²⁻(aq) and OH⁻(aq). There is no halide ion, so oxygen forms.

  4. The anode product is oxygen gas. Sodium ions and sulfate ions mainly remain in solution.

Tip

Gas tests

Hydrogen gives a squeaky pop with a lit splint, oxygen relights a glowing splint, and chlorine bleaches damp litmus paper.

6. Higher Tier: writing half equations

A half equation shows what happens to electrons at one electrode. This is Higher Tier only.

At the cathode, electrons are on the left because ions gain electrons.

At the anode, electrons are on the right because ions lose electrons.

Example

Writing half equations for copper chloride solution

  1. Copper chloride solution contains Cu²⁺(aq) and Cl⁻(aq) ions. Copper is less reactive than hydrogen, so Cu²⁺ ions are discharged at the cathode.

  2. Balance the copper cathode reaction by adding two electrons to cancel the 2+ charge: Cu²⁺(aq) + 2e⁻ → Cu(s).

  3. Chloride ions are halide ions, so chlorine forms at the anode. Two chloride ions are needed to make one Cl₂ molecule: 2Cl⁻(aq) → Cl₂(g) + 2e⁻.

  4. Check the electron pattern: reduction at the cathode uses electrons, while oxidation at the anode releases electrons.

Common Mistake

Forgetting diatomic molecules

Chlorine, bromine, iodine, hydrogen and oxygen form molecules in pairs: Cl₂, Br₂, I₂, H₂ and O₂. This matters when balancing half equations.

7. Copper sulfate solution and copper purification

Copper sulfate solution, CuSO₄(aq), is especially important because it is part of the core practical.

Using inert electrodes

With inert electrodes, such as graphite:

  • Cu²⁺(aq) ions gain electrons at the cathode and form copper metal.
  • Oxygen gas forms at the anode.
  • The blue colour of the solution may become paler because Cu²⁺ ions are removed.

Useful Higher Tier half equations are:

  • Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s)
  • Anode: 4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻

Using copper electrodes

Copper electrodes are not inert. The copper anode reacts and dissolves into the solution.

At the anode: Cu(s) → Cu²⁺(aq) + 2e⁻
At the cathode: Cu²⁺(aq) + 2e⁻ → Cu(s)

Overall, copper is transferred from the anode to the cathode. The anode loses mass, the cathode gains mass, and the blue colour of the copper sulfate solution stays roughly the same because Cu²⁺ ions are replaced as they are used up.

This is how electrolysis can be used to purify copper: impure copper is used as the anode and pure copper is used as the cathode. Pure copper plates onto the cathode, while insoluble impurities fall below the anode as anode sludge.

Copper purification by electrolysis showing impure copper anode, pure copper cathode, copper sulfate electrolyte, copper ion movement, and anode sludge

Example

Explaining mass changes with copper electrodes

  1. At the copper anode, copper atoms lose electrons and become Cu²⁺(aq), so copper leaves the anode: Cu(s) → Cu²⁺(aq) + 2e⁻.

  2. At the cathode, Cu²⁺(aq) ions gain electrons and become copper atoms: Cu²⁺(aq) + 2e⁻ → Cu(s).

  3. Because copper leaves the anode but is deposited on the cathode, the anode gets lighter and the cathode gets heavier.

8. Core practical: copper sulfate electrolysis

You need to be able to describe investigating copper sulfate solution using both inert electrodes and copper electrodes.

A typical method is:

  1. Add copper sulfate solution to a beaker.
  2. Place two electrodes into the solution, making sure they do not touch.
  3. Connect the electrodes to a low-voltage direct current power supply.
  4. For inert electrodes, observe the copper deposit at the cathode and bubbles at the anode.
  5. For copper electrodes, clean, dry and weigh the electrodes before the experiment.
  6. Run the current for a fixed time, then remove, rinse, dry and reweigh the copper electrodes.
  7. Compare the mass change of the anode and cathode.

Important control variables include the concentration and volume of copper sulfate solution, the voltage or current, the time, the distance between electrodes, and the surface area of the electrodes.

Common Mistake

Practical safety

Wear eye protection and avoid touching copper sulfate solution. Do not let the electrodes touch, because that can cause a short circuit.

Exam technique

In the exam

  1. For product predictions, always decide whether the electrolyte is molten or aqueous first.

  2. For aqueous solutions, apply the cathode rule using the reactivity of the metal compared with hydrogen, then apply the anode halide rule.

  3. For half equations, put electrons on the left for cathode reactions and on the right for anode reactions.

  4. For copper purification, mention both electrodes: the impure copper anode dissolves and pure copper is deposited on the cathode.

Self review

Check yourself

  • Why does solid sodium chloride not conduct electricity, but molten sodium chloride does?
  • What products form when sodium chloride solution is electrolysed using inert electrodes?
  • In copper purification, why does the cathode gain mass while the anode loses mass?

Recap questions

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