Chemical Reactions and Equations
What this domain tests
Chemical Reactions and Equations tests balanced equations, mole calculations, acids, bases, salts and common reaction types.
Core rules
- Physical changes are reversible and make no new substance; chemical changes make new substances and are usually irreversible.
- Balanced symbol equations must have equal numbers of each atom on both sides. State symbols (s), (l), (g) and (aq) are required in precise answers.
- Mole: n = m/Mr. One mole contains 6.02 × 10²³ particles. At room temperature and pressure, one mole of gas occupies 24 dm³.
- Concentration = moles ÷ volume in dm³, giving mol dm⁻³.
- Acid + base → salt + water; acid + metal → salt + hydrogen; acid + carbonate → salt + water + carbon dioxide.
- pH below 7 is acidic, 7 is neutral and above 7 is alkaline. Neutralisation is H⁺ + OH⁻ → H₂O.
Worked example
4.8 g of magnesium (Ar = 24) is added to excess hydrochloric acid. n(Mg) = 4.8 ÷ 24 = 0.20 mol. The equation Mg + 2HCl → MgCl₂ + H₂ shows that 1 mol Mg gives 1 mol H₂, so n(H₂) = 0.20 mol. Volume at r.t.p. = 0.20 × 24 = 4.8 dm³. If asked for the mass of MgCl₂ (Mr = 95), it is 0.20 × 95 = 19 g.
Traps
Never balance an equation by changing subscripts. Multiply by the coefficient before converting moles to mass or volume. Do not use 22.4 dm³ for Malaysian r.t.p.; use 24 dm³. Check which reactant is limiting before calculating product amount.