Chemical reactions — balanced equations, types of reaction and conservation of mass

Conservation of mass

In a chemical reaction, atoms are neither created nor destroyed — they are rearranged. This means the total mass of reactants always equals the total mass of products (the Law of Conservation of Mass).

Why a reaction in a sealed container shows no mass change: all atoms are accounted for. Why an open container may appear to lose mass: a gas is produced and escapes (e.g. CO₂ in a reaction with an acid). Why an open container may appear to gain mass: a gas from the atmosphere is incorporated (e.g. magnesium burning in air gains oxygen).

Balancing equations

A symbol equation shows the formulae of reactants and products. It must be balanced: the same number of each type of atom on both sides. Only change the coefficients (numbers in front of formulae) — never change the formulae themselves.

Method:

1. Write the unbalanced equation.
2. Count atoms of each element on each side.
3. Adjust coefficients systematically (start with elements that appear in fewest compounds).
4. Recount until balanced.

Example — combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O (unbalanced) Carbon: 3 left, 1 right → need 3CO₂ Hydrogen: 8 left, 2 right → need 4H₂O Oxygen: 2 left, 3×2 + 4×1 = 10 right → need 5O₂ Balanced: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water). OCR expects state symbols in ionic equations.

Types of chemical reaction

Combustion

Fuel + oxygen → carbon dioxide + water (complete combustion). If oxygen is limited, incomplete combustion produces carbon monoxide (CO) and/or carbon (soot) instead of CO₂.

Oxidation and reduction

• Oxidation: gain of oxygen, or loss of hydrogen, or loss of electrons (OIL).
• Reduction: loss of oxygen, or gain of hydrogen, or gain of electrons (RIG).
• Redox: both oxidation and reduction occur simultaneously.
• Oxidising agent: the substance that gets reduced (accepts electrons).
• Reducing agent: the substance that gets oxidised (donates electrons).

Neutralisation

Acid + base → salt + water. This is an exothermic reaction.

• Acid + metal oxide (or hydroxide) → salt + water.
• Acid + carbonate → salt + water + carbon dioxide.
• Acid + metal → salt + hydrogen.

Ionic equation for neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l)

Thermal decomposition

A compound breaks down into simpler substances on heating.

• CaCO₃(s) → CaO(s) + CO₂(g) (used in cement manufacture — J258 often asks this)
• Cu₂CO₃(OH)₂ → 2CuO + CO₂ + H₂O (green copper carbonate → black copper oxide)

Precipitation

Two aqueous solutions mix to form an insoluble solid (precipitate). Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) (white precipitate — test for sulfate ions) Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (white precipitate — test for chloride ions)

Calculating masses using equations (Higher)

Moles: 1 mole of any substance contains 6.02 × 10²³ particles (Avogadro's constant). Moles = mass ÷ Mᵣ (relative formula mass).

Using a balanced equation to find masses:

1. Write the balanced equation.
2. Write the moles ratio from the equation.
3. Calculate moles of the known substance.
4. Use the ratio to find moles of the unknown.
5. Convert moles to mass.

Example: How many grams of CO₂ are produced when 40 g of CaCO₃ decomposes? CaCO₃ → CaO + CO₂ (1:1 ratio) Mᵣ of CaCO₃ = 40+12+48 = 100; moles CaCO₃ = 40/100 = 0.4 mol Moles CO₂ = 0.4 mol; mass CO₂ = 0.4 × 44 = 17.6 g

OCR PAG C2 — preparing a salt by neutralisation

Students react an acid with an excess of an insoluble base (e.g. copper oxide + sulfuric acid), filter, then evaporate/crystallise. Mass conservation is checked by weighing before and after. Common J258 extended response question.