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Cambridge IAL Chemistry study materials
Cambridge IAL Chemistry study resources

Cambridge IAL Chemistry: Physical Chemistry

Cambridge IAL Chemistry · Work through your exam, one topic at a time.

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Physical Chemistry

Physical chemistry in Cambridge IAL Chemistry tests quantitative and explanatory reasoning: moles and stoichiometry, enthalpy changes, reaction rates, equilibria, acids and bases, redox and electrochemistry. Questions often combine a calculation with a justification, so show units, state symbols and the model you are using.

Core rules

  • Amount of substance: n = m/M = cV/1000 for cm3; use the balanced equation to convert between species and identify the limiting reagent before calculating yield.
  • Enthalpy: ΔH is negative for exothermic. In Hess cycles, formation enthalpies go from elements; combustion enthalpies go to CO2 and H2O. Bond breaking is endothermic (+), bond making exothermic (−).
  • Equilibria: Kc depends only on temperature; changing concentration or pressure shifts position but not K. For gases, Kp uses partial pressures and mole fractions.
  • Kinetics: rate equations are experimental; order is not read from stoichiometry. Arrhenius: ln k = ln A − Ea/RT.
  • Electrochemistry: Ecell = Ereduction − Eoxidation using standard electrode potentials; a positive Ecell indicates a feasible reaction under standard conditions. Standard conditions are 298 K, 1 mol dm−3 and 100 kPa.

Worked example

Calculate ΔH for CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g) using mean bond enthalpies: C–H 413, Cl–Cl 243, C–Cl 346, H–Cl 431 kJ mol−1. Bonds broken: one C–H and one Cl–Cl = 413 + 243 = +656 kJ mol−1. Bonds formed: one C–Cl and one H–Cl = 346 + 431 = 777 kJ mol−1. ΔH = energy in − energy out = 656 − 777 = −121 kJ mol−1. The negative sign means the reaction is exothermic overall; the calculation assumes gaseous species and mean bond values, so it is an estimate.

Traps

Mixing up ΔHf and ΔHc directions in Hess cycles; omitting state symbols; using the wrong stoichiometric ratio; treating K as changed by concentration; quoting Ecell without reversing the oxidation half-equation; and rounding intermediate values too early. For pH, remember strong acid [H+] = c, weak acid needs Ka, and buffers need the Henderson–Hasselbalch form or an ICE table.