Chemical reactions, acids and bases, rates and energy changes
This domain covers equations, neutralisation, collision theory and energy changes. Expect to balance equations, use the pH scale, name salts, and explain how rate changes without confusing it with yield.
Reactions and equations
Reactants → products, and mass is conserved. Balance equations by changing coefficients only, never subscripts. Reaction types include combination, decomposition, combustion, displacement and neutralisation; state symbols (s), (l), (g), (aq) add information. Rate can be measured by gas volume, mass loss or colour change.
Acids, bases and pH
Acids produce H⁺ ions in water (HCl, H₂SO₄). Bases neutralise acids; alkalis are soluble bases (NaOH). The pH scale runs 0–14: below 7 acidic, 7 neutral, above 7 basic. Universal indicator or litmus gives a rough value; a pH meter is more precise. Acid + base → salt + water, e.g. HCl + NaOH → NaCl + H₂O. Acid + metal → salt + hydrogen; acid + carbonate → salt + water + carbon dioxide. Strong acids ionise fully; 'concentrated' means more acid per volume - not the same thing.
Rates and energy
Rate increases with higher concentration, higher temperature, greater surface area and a catalyst. Collision theory explains this: more frequent and more energetic collisions. A catalyst lowers activation energy and is not used up. Exothermic reactions release energy (combustion, neutralisation); endothermic reactions absorb it (thermal decomposition, photosynthesis). Energy-level diagrams show activation energy.
Worked example
2H₂ + O₂ → 2H₂O: hydrogen has 4 atoms on each side and oxygen has 2, so it is balanced. If 4 g of hydrogen reacts fully with 32 g of oxygen, 36 g of water forms - conservation of mass.
Traps
- Changing subscripts instead of coefficients to balance.
- Saying a catalyst is 'used up'.
- Confusing rate with yield.
- Describing pH 7 as acidic.