Chemical Changes
What's covered
Key facts
Acids and Alkalis
Acid + metal carbonate → salt + water + carbon dioxide.
Electrolysis
During electrolysis, negative ions (anions) move towards the anode and positive ions (cations) move towards the cathode.
Electrolysis of Aluminium Oxide
Aluminium is deposited at the cathode because Al³⁺ ions are attracted to the negative electrode.
Electrolysis of Brine
In brine electrolysis, chlorine gas is produced at the anode.
Oxidation and Reduction
In a redox reaction, the oxidising agent is itself reduced, and the reducing agent is itself oxidised.
Preparing Salts
Acid + base → salt + water is the core neutralisation reaction used to make most soluble salts — e.g. HCl + NaOH → NaCl + H₂O.
Reactivity Series
Copper cannot displace iron from iron sulfate because copper is less reactive than iron.
Acids and Alkalis
Acid + reactive metal → salt + hydrogen gas.
Electrolysis
At the anode (positive electrode), ions lose electrons — this is called oxidation.
Electrolysis of Aluminium Oxide
Aluminium must be extracted by electrolysis (not reduction with carbon) because it is above carbon in the reactivity series, so carbon cannot displace it.
Sample questions
A taste of the 81 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
What is produced when an acid neutralises an alkali?
- •Carbon dioxide and water
- •Hydrogen gas and salt
- •Oxygen and salt
- ✓Salt and water
During electrolysis, what happens at the cathode?
- •Negative ions lose electrons and are oxidised
- ✓Positive ions (cations) gain electrons and are reduced
- •Positive ions lose electrons and are oxidised
- •The compound decomposes by thermal decomposition
What are the cathode and anode half-equations for electrolysis of aluminium oxide?
- •Cathode: 2O²⁻ → O₂ + 4e⁻. Anode: Al³⁺ + 3e⁻ → Al.
- •Cathode: Al → Al³⁺ + 3e⁻. Anode: O₂ + 4e⁻ → 2O²⁻.
- ✓Cathode: Al³⁺ + 3e⁻ → Al. Anode: 2O²⁻ → O₂ + 4e⁻.
- •Cathode: Al³⁺ + e⁻ → Al⁺. Anode: O²⁻ → O + 2e⁻.
What are the three products of the electrolysis of brine (concentrated sodium chloride solution)?
- ✓Chlorine gas (at anode), hydrogen gas (at cathode), and sodium hydroxide solution
- •Oxygen gas (at anode), hydrogen gas (at cathode), and sodium chloride solution (unchanged)
- •Sodium gas (at cathode), chlorine gas (at anode), and water vapour (produced)
- •Sodium metal (at cathode), chlorine gas (at anode), and water (left over)
What is oxidation in terms of electrons?
- •Gain of electrons
- •Gain of neutrons
- ✓Loss of electrons
- •Loss of protons
Describe how to prepare pure, dry copper sulfate crystals from copper oxide and dilute sulfuric acid.
- ✓Add excess CuO to warm dilute H₂SO₄; stir, filter, evaporate
- •Add H₂SO₄ to copper chloride solution and evaporate until dry crystals form
- •Electrolyse copper sulfate solution with copper electrodes and collect crystals
- •Mix copper powder with dilute H₂SO₄ in a boiling tube; crystals form on cooling
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