What does Module 5 of OCR Chemistry A cover?

Module 5 (Physical chemistry and transition elements) is the most calculation-heavy module. 5.1 covers reaction rates (orders, rate equations, the rate-determining step and the Arrhenius equation), the equilibrium constants Kc and Kp, and acids, bases and buffers (pH, Ka, buffers and titration curves). 5.2 covers lattice enthalpy and Born-Haber cycles, entropy and Gibbs free energy, and redox and electrode potentials. 5.3 covers the transition elements (electron configurations, complex ions, ligand substitution, colour and catalysis).

How is Module 5 assessed in OCR Chemistry A?

Module 5 is assessed in Paper 1 (Periodic table, elements and physical chemistry) and the synoptic Paper 3. It carries the heaviest mathematical load: deducing orders and rate equations, evaluating Kc and Kp, calculating pH of strong and weak acids and buffers, completing Born-Haber and entropy and Gibbs cycles, and using standard electrode potentials to predict feasibility. Transition-element questions test electron configurations, ligand substitution with colour changes, and catalysis.

Which Module 5 equations must I know?

The rate equation $\text{rate} = k[A]^m[B]^n$ and the Arrhenius equation, the Kc and Kp expressions, $\text{pH} = -\log_{10}[\text{H}^+]$ with $K_w$ and $K_a$, the buffer expression $\text{pH} = \text{p}K_a + \log_{10}([\text{A}^-]/[\text{HA}])$, the Born-Haber and Hess cycles for lattice enthalpy, $\Delta S = \Sigma S_{\text{products}} - \Sigma S_{\text{reactants}}$ and $\Delta G = \Delta H - T\Delta S$, and $E^{\circ}_{\text{cell}} = E^{\circ}_{\text{positive}} - E^{\circ}_{\text{negative}}$.

What is the most common mistake in Module 5?

Using concentrations from the wrong time when deducing orders, forgetting that pure solids and liquids are left out of Kc and Kp, mixing up the buffer ratio, getting the sign or units wrong in Gibbs free energy ($\Delta S$ in joules, $\Delta H$ in kilojoules), subtracting electrode potentials the wrong way round, and forgetting that a catalyst speeds the approach to equilibrium without changing Kc or the yield.

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OCR A-Level Chemistry A: Module 5 (Physical chemistry and transition elements) overview

A deep-dive overview of Module 5 of OCR A-Level Chemistry A (H432): rates and rate equations, the equilibrium constants Kc and Kp, acids, bases and buffers, lattice enthalpy and Born-Haber cycles, entropy and Gibbs free energy, redox and electrode potentials, and the transition elements.

Generated by Claude Opus 4.816 min readH432/5Updated 2026-06-02

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What this module demands
Rates, equilibrium and acids (5.1)
Energetics and electrochemistry (5.2)
Transition elements (5.3)
How this module is examined
Check your knowledge

What this module demands

Module 5 of OCR A-Level Chemistry A is the quantitative heart of the course. It extends the rates, equilibrium and energetics of Module 3 into rate equations, equilibrium constants and thermodynamic cycles, adds acid-base and electrochemistry calculations, and finishes with the rich chemistry of the transition elements. It rewards fluent calculation, careful units and signs, and precise explanation of trends and reactions.

This guide walks through Module 5 in specification order and sets out the exam patterns OCR repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Rates, equilibrium and acids (5.1)

Rates of reaction (5.1.1) introduces orders, the rate equation $\text{rate} = k[A]^m[B]^n$ , the rate constant and its units, concentration-time and rate-concentration graphs, the rate-determining step, and the Arrhenius equation linking the rate constant to activation energy and temperature.

Equilibrium constants (5.1.2) covers $K_c$ (in concentrations) and $K_p$ (in partial pressures and mole fractions), how to calculate them from equilibrium amounts, and the effect of temperature (changes the constant) versus catalysts (no effect).

Acids, bases and buffers (5.1.3) covers the Bronsted-Lowry model and conjugate pairs, $\text{pH} = -\log_{10}[\text{H}^+]$ , $K_w$ , $K_a$ and $\text{p}K_a$ for weak acids, buffer action and pH, titration curves, and indicator choice.

Energetics and electrochemistry (5.2)

Lattice enthalpy and Born-Haber cycles (5.2.1) defines lattice enthalpy, builds Born-Haber cycles to find it, and uses enthalpies of solution and hydration, relating their magnitudes to ionic charge and radius.

Entropy and free energy (5.2.2) introduces entropy as a measure of disorder, $\Delta S = \Sigma S_{\text{products}} - \Sigma S_{\text{reactants}}$ , and the Gibbs free energy equation $\Delta G = \Delta H - T\Delta S$ to decide feasibility and the temperature at which a reaction becomes feasible.

Redox and electrode potentials (5.2.3) covers redox half-equations, standard electrode potentials measured against the standard hydrogen electrode, cell notation and $E^{\circ}_{\text{cell}}$ , predictions of feasibility, and storage and fuel cells.

Transition elements (5.3)

Transition elements (5.3.1) defines a transition element, gives the electron configurations (including the chromium and copper exceptions), and covers complex ions, ligands and shapes, ligand substitution with colour changes, precipitation reactions, the origin of colour from d-orbital splitting, and catalysis.

How this module is examined

A typical OCR profile for Module 5:

Rate calculations. Deduce orders and the rate equation from data, find the rate constant and its units, and use the Arrhenius equation graphically.
Equilibrium calculations. Evaluate $K_c$ and $K_p$ from equilibrium amounts and predict the effect of changes.
Acid-base calculations. pH of strong and weak acids, $K_a$ and buffers, and interpreting titration curves to choose an indicator.
Thermodynamics. Complete Born-Haber and Hess cycles, and use $\Delta G = \Delta H - T\Delta S$ for feasibility.
Electrochemistry and transition metals. Use $E^{\circ}$ values to predict reactions, and recall ligand-substitution colour changes and catalysis.

Check your knowledge

A mix of calculation and explanation across Module 5. Attempt them under timed conditions, then check against the solutions.

State the units of the rate constant for a reaction that is second order overall. (1 mark)
Write the expression for $K_c$ for $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g)$ . (1 mark)
Calculate the pH of $0.0100\ \text{mol dm}^{-3}$ hydrochloric acid. (1 mark)
State how the magnitude of lattice enthalpy changes as ionic radius increases, with ionic charge constant. (1 mark)
Give the sign of $\Delta G$ for a feasible reaction. (1 mark)
State the colour change when excess ammonia is added to aqueous copper(II) ions. (2 marks)

Sources & how we know this

OCR A-Level Chemistry A (H432) specification — OCR (2015)