Topic 4 of 10

Chemical Changes

From the reactivity series and metal extraction to acids, bases, and electrolysis — understand the reactions that transform substances.

AQA Hub › Topic 4

Metal Oxides

When metals react with oxygen, they form metal oxides. The vigour of this reaction depends on the metal's position in the reactivity series.

2Mg + O₂ → 2MgO

Magnesium burns in air with a bright white flame to produce the white powder magnesium oxide.

Metal oxides are basic oxides. They react with acids in neutralisation reactions to produce a salt and water.

The Reactivity Series

The reactivity series ranks metals in order of how vigorously they react. The most reactive metals react most readily with water and acids.

Order (most → least reactive)

Potassium → Sodium → Lithium → Calcium → Magnesium → Aluminium → Carbon → Zinc → Iron → Hydrogen → Copper → Silver → Gold

Carbon and hydrogen are included in the reactivity series even though they are non-metals. Carbon is used as a benchmark for extraction, and hydrogen is used to compare acid reactions.

Displacement Reactions

A more reactive metal can displace a less reactive metal from a compound. For example, iron displaces copper from copper sulfate solution:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
In a displacement reaction, the more reactive metal is oxidised (loses electrons) and the ion of the less reactive metal is reduced (gains electrons).

Extraction of Metals

Most metals are found in the Earth's crust as ores — rocks containing enough metal to make extraction worthwhile.

Position in Reactivity Series Determines Method

  • Metals below carbon (zinc, iron, copper): extracted by reduction with carbon.
  • Metals above carbon (aluminium, sodium, potassium): must be extracted by electrolysis.

Reduction with Carbon

2Fe₂O₃ + 3C → 4Fe + 3CO₂

The carbon removes oxygen from the metal oxide (reducing it), producing the pure metal.

Biological Methods

Phytomining uses plants to absorb metal compounds from soil, then the plant ash is smelted. Bioleaching uses bacteria to produce a leachate solution containing metal compounds.

Oxidation & Reduction (OIL RIG)

These are complementary processes that always happen together (redox reactions).

  • Oxidation Is Loss of electrons (OIL).
  • Reduction Is Gain of electrons (RIG).
A substance that is oxidised is the reducing agent (it causes another substance to be reduced). A substance that is reduced is the oxidising agent.
Use the mnemonic OIL RIG to remember: Oxidation Is Loss, Reduction Is Gain. In ionic equations, show the electron transfer explicitly.

Reactions of Acids

Acid + Metal → Salt + Hydrogen

Mg + 2HCl → MgCl₂ + H₂

Only metals above hydrogen in the reactivity series react with dilute acids.

Acid + Metal Oxide → Salt + Water

CuO + H₂SO₄ → CuSO₄ + H₂O

Acid + Metal Hydroxide → Salt + Water

NaOH + HCl → NaCl + H₂O

Acid + Metal Carbonate → Salt + Water + CO₂

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
The salt produced depends on the acid: hydrochloric acid → chloride salts, sulfuric acid → sulfate salts, nitric acid → nitrate salts.

Neutralisation

Neutralisation is the reaction between an acid and a base (or alkali) to produce a salt and water.

acid + base → salt + water

In terms of H⁺ and OH⁻ ions:

H⁺(aq) + OH⁻(aq) → H₂O(l)

The hydrogen ions from the acid combine with the hydroxide ions from the base to form water — the solution becomes less acidic (or less alkaline).

Making Soluble Salts

To make a pure, dry salt from an insoluble base:

  1. Warm acid in a beaker.
  2. Add the insoluble base (excess) and stir until no more dissolves.
  3. Filter to remove excess base.
  4. Heat the filtrate gently to evaporate some water.
  5. Leave to crystallise, then dry the salt crystals.
Always add excess base so all the acid reacts — then filter to remove the unreacted base. This ensures the salt solution is neutral.

pH Scale & Indicators

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is.

  • pH 0–6: Acidic (lower = stronger acid). Acids produce H⁺ ions in solution.
  • pH 7: Neutral.
  • pH 8–14: Alkaline (higher = stronger alkali). Alkalis produce OH⁻ ions.

Universal indicator is a mixture of dyes that changes colour across the pH range. A pH probe gives a more precise numerical reading.

Strong & Weak Acids (HT)

Strong Acids

Strong acids fully ionise in water — every molecule releases H⁺ ions. Examples: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃).

HCl → H⁺ + Cl⁻ (complete ionisation)

Weak Acids

Weak acids only partially ionise in water — most molecules remain intact. An equilibrium exists. Examples: ethanoic acid (CH₃COOH), citric acid.

CH₃COOH ⇌ CH₃COO⁻ + H⁺ (partial ionisation)
Strong vs concentrated are different! Strong = fully ionised. Concentrated = large amount of solute per volume. A dilute strong acid and a concentrated weak acid are equally valid concepts.

Electrolysis

Electrolysis uses electricity to decompose an ionic compound when it is molten or dissolved in water, allowing ions to move.

Key Terminology

  • Electrolyte: The ionic compound (molten or in solution).
  • Cathode: The negative electrode — attracts positive ions (cations). Reduction occurs here.
  • Anode: The positive electrode — attracts negative ions (anions). Oxidation occurs here.

Electrolysis of Aluminium Oxide

Aluminium is too reactive to be extracted by carbon. Aluminium oxide (Al₂O₃) is dissolved in molten cryolite (to lower the melting point) and electrolysed.

  • Cathode: Al³⁺ + 3e⁻ → Al (reduction).
  • Anode: 2O²⁻ → O₂ + 4e⁻ (oxidation). The carbon anodes must be replaced regularly as the oxygen reacts with them.

Electrolysis of Brine (NaCl solution)

Three useful products are made:

  • Chlorine gas (Cl₂) at the anode — used in bleach and disinfection.
  • Hydrogen gas (H₂) at the cathode — used in margarine production.
  • Sodium hydroxide (NaOH) left in solution — used in soap and paper.
At the cathode: if the metal is more reactive than hydrogen, hydrogen gas is produced instead. At the anode: if halide ions (Cl⁻, Br⁻, I⁻) are present, the halogen gas is produced; otherwise oxygen is produced.