Edexcel IAL Chemistry Unit 4 — Rates, Equilibria and Further Organic Chemistry

I classified every question part in 18 WCH14 papers from 2020 to 2026 — 891 parts, 1,620 marks. Organic chemistry (carbonyls, carboxylic acids, chirality and spectroscopy) is 35% of the paper on its own. Acid–base equilibria and entropy and energetics are 19% each, kinetics 17% and chemical equilibria 11%. Together, organic plus acid–base is more than half the marks.

Section A multiple choice ("Which" and "What" stems) is 35% of question parts. "Calculate" is 15% — rate constants, half-lives, Kc and Kp with units, pH of weak acids and buffers, Born–Haber and entropy values — and "Explain" 9%. Twenty percent of parts are high-difficulty, concentrated in mechanisms drawn with curly arrows and in multi-step pH and equilibrium calculations. The pattern is simple: learn the organic mechanisms cold, then drill the five calculation types until each takes under three minutes.

Unit 4 turns the qualitative kinetics of Unit 2 into rate equations. You must deduce the order with respect to each reactant from concentration–time and rate–concentration graphs and from initial-rate data, identify a first-order reaction by its constant half-life, and use the rate equation to deduce the rate-determining step and a mechanism. The rate equations for halogenoalkane hydrolysis are the evidence for SN1 and SN2. Activation energy comes from an Arrhenius plot — the equation is given. Core Practicals 9a/9b (iodine–propanone by titration, and a clock reaction) and 10 (activation energy) are examined here and in Unit 6.

Entropy explains why endothermic reactions can happen: ΔS_total = ΔS_system + ΔS_surroundings, with ΔS_surroundings = −ΔH/T, and a reaction is feasible when ΔS_total is positive. You calculate the temperature at which feasibility begins and distinguish thermodynamic from kinetic stability. The second half is lattice energy: Born–Haber cycles, comparing experimental and theoretical lattice energies as evidence of covalent character, and enthalpy of solution from lattice energy and hydration enthalpies.

Kc and Kp expressions for homogeneous and heterogeneous systems, calculated with units from experimental data. The examiner's favourite question is the one students most often get wrong: temperature changes K, while concentration, pressure and catalysts do not — they change position, not the constant. Link it to entropy through ΔS_total = R ln K.

Brønsted–Lowry acids and bases, pH from [H⁺] and back, Ka and pKa for weak acids, Kw for strong bases, and the assumptions behind each calculation. Titration curves for every strong/weak combination, choosing an indicator from the curve, and reading Ka from the half-neutralisation point. Buffers: how they work, calculating their pH, and preparing one at a given pH. Core Practical 11 finds Ka for a weak acid.

The largest topic. Optical isomerism and racemic mixtures, with optical activity used as evidence for SN1 versus SN2 and for nucleophilic addition to carbonyls. Aldehydes and ketones: Fehling's, Tollens', LiAlH₄ reduction, the HCN/KCN nucleophilic addition mechanism with curly arrows, 2,4-DNPH and the iodoform test. Carboxylic acids and their derivatives: acyl chlorides with water, alcohols, ammonia and amines; ester hydrolysis; polyesters. Analysis closes the topic: high-resolution mass spectra, ¹³C NMR environments, ¹H NMR with the (n+1) rule, and Rf values in chromatography with HPLC and GC–MS applications.