OCR A-Level Chemistry A: Module 3 (Periodic table and energy) overview

What this module demands

Module 3 of OCR A-Level Chemistry A joins two strands: the patterns of the periodic table and the physical chemistry of energy and rates. It rewards precise explanation of trends from electron structure, confident calculation of enthalpy changes, and a clear grasp of how equilibria respond to change.

This guide walks through Module 3 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.

The periodic table (3.1)

Periodicity (3.1.1) explains the structure of the table (periods, groups, and the s, p and d blocks) and the trends in atomic radius, first ionisation energy and melting point across Periods 2 and 3. The two ionisation-energy dips reveal sub-shell and orbital structure.

Group 2 and the halogens (3.1.2) contrasts the Group 2 metals as reducing agents (more reactive down the group) with the halogens as oxidising agents (less reactive down the group), covering reactions with water, halogen-halide displacement, disproportionation of chlorine, and the silver nitrate halide test.

Qualitative analysis (3.1.3) sets out the test-tube tests for carbonate, sulfate, halide and ammonium ions, and the crucial order (carbonate, then sulfate, then halide) that avoids false positives.

Physical chemistry (3.2)

Enthalpy changes (3.2.1) introduces standard enthalpy definitions, calorimetry with $q = mc\Delta T$, average bond enthalpies, and Hess's law for formation and combustion cycles. Sign conventions and the bonds-broken-minus-bonds-made rule are where marks are won.

Reaction rates (3.2.2) uses collision theory to explain the effects of concentration, pressure, surface area and temperature, interprets the Boltzmann distribution against the activation energy, and explains homogeneous and heterogeneous catalysis.

Chemical equilibrium (3.2.3) covers dynamic equilibrium, Le Chatelier's principle, the equilibrium constant $K_c$, and the compromise conditions of industrial processes such as the Haber process.

How this module is examined

A typical OCR profile for Module 3:

• Explanation questions. Ionisation-energy trends and dips, atomic radius, Group 2 and halogen reactivity, and why a temperature rise speeds a reaction.
• Calculations. Calorimetry ($q = mc\Delta T$ then per mole), average bond enthalpies, Hess's-law cycles, and evaluating $K_c$.
• Practical and analytical. Ion-test sequences with ionic equations, and the disproportionation and displacement of halogens.
• Applied questions. Le Chatelier predictions and the compromise conditions for the Haber and contact processes.

Check your knowledge

A mix of explanation and calculation across Module 3. Attempt them under timed conditions, then check against the solutions.

1. Explain why first ionisation energy generally increases across a period. (2 marks)
2. Write an equation for the disproportionation of chlorine in cold dilute sodium hydroxide. (1 mark)
3. State the colours of the precipitates formed when silver nitrate is added to solutions of chloride, bromide and iodide ions. (2 marks)
4. Calculate the heat released when $100\ \text{g}$ of water rises by $20.0\ ^{\circ}\text{C}$. (1 mark)
5. Using the Boltzmann distribution, state why increasing temperature increases the rate of reaction. (2 marks)
6. State and explain the effect of increasing pressure on the position of the equilibrium $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$. (2 marks)