What does Unit 3 of WJEC A-Level Chemistry cover?

Unit 3, Physical and Inorganic Chemistry, is the first A2 unit. It covers redox and standard electrode potentials, redox reactions and titrations, the chemistry of the p-block and d-block (including transition-metal complexes), chemical kinetics and rate equations, enthalpy, entropy and Gibbs free energy, the equilibrium constants Kc and Kp, and acid-base equilibria with pH, Ka and buffers.

How is Unit 3 examined in WJEC A-Level Chemistry?

Unit 3 is an A2 written paper at higher demand. Expect electrode-potential and feasibility calculations, redox-titration arithmetic, Born-Haber and free-energy calculations, rate-equation deductions, Kc and Kp work, and pH, Ka and buffer calculations, alongside the descriptive chemistry of the p-block and d-block. A calculator and a periodic table are provided.

How do I use electrode potentials to predict feasibility?

Work out the cell EMF as the standard electrode potential of the positive electrode minus that of the negative electrode. A positive cell EMF means the reaction is thermodynamically feasible. Remember that feasibility says nothing about the rate, which may still be very slow.

What decides whether a reaction is feasible in terms of free energy?

The sign of the Gibbs free-energy change, where free energy equals enthalpy change minus temperature times entropy change. A reaction is feasible when the free-energy change is negative or zero. When the enthalpy and entropy changes have the same sign, the temperature decides feasibility, switching at the temperature equal to the enthalpy change divided by the entropy change.

How are orders of reaction found in Unit 3?

Orders are found experimentally, not from the equation. Change one concentration at a time and see how the rate responds: no change is zero order, a proportional change is first order, and a squared change is second order. The species in the rate equation reveal the rate-determining step.

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WJEC A-Level Chemistry Unit 3 Physical and Inorganic Chemistry: a deep dive on redox and electrode potentials, the p-block and d-block, kinetics, free energy and equilibria

A deep-dive WJEC A-Level Chemistry guide to Unit 3. Covers redox and standard electrode potentials, redox titrations, the chemistry of the p-block and d-block, chemical kinetics and rate equations, enthalpy, entropy and free energy, and the equilibrium constants Kc, Kp, Ka and pH.

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What Unit 3 actually demands
Redox and electrochemistry
The p-block and d-block
Kinetics, free energy and equilibria
Check your knowledge

What Unit 3 actually demands

Unit 3 is the heavyweight physical and inorganic unit. Examiners want confident calculations across electrochemistry, thermodynamics, kinetics and equilibria, plus precise descriptive chemistry of the p-block and d-block. Almost every topic carries a calculation, so accuracy and units matter.

This guide ties together the eight topics of the unit, each with a matching dot-point page.

Redox and electrochemistry

Standard electrode potentials measure the tendency to be reduced relative to the standard hydrogen electrode. The cell EMF predicts feasibility, and redox titrations with manganate(VII) and thiosulfate apply the same electron-transfer ideas quantitatively.

The p-block and d-block

The p-block covers halogen trends, displacement and disproportionation. The d-block covers transition metals: variable oxidation states, complex ions and ligands, the origin of colour, catalysis and ligand substitution.

Kinetics, free energy and equilibria

Kinetics introduces rate equations, orders and the rate-determining step. Thermodynamics adds Born-Haber cycles, entropy and Gibbs free energy, with $\Delta G = \Delta H - T\Delta S$ deciding feasibility. Equilibria quantify the position with Kc and Kp, and acid-base equilibria bring pH, Ka, buffers and titration curves.

Check your knowledge

Attempt these under timed conditions, then check against the solutions.

State the standard electrode potential of the standard hydrogen electrode. (1 mark)
A cell has $E^\circ_{cell} = +0.46$ V. State whether the reaction is feasible. (1 mark)
State the colour change at the end point of a manganate(VII) titration. (1 mark)
Explain why transition-metal complexes are coloured. (3 marks)
Doubling a concentration quadruples the rate. State the order. (1 mark)
Write the equation for Gibbs free energy and the condition for feasibility. (2 marks)
State what changes the value of an equilibrium constant. (1 mark)
Calculate the pH of $0.0100$ mol dm $^{-3}$ hydrochloric acid. (2 marks)

Sources & how we know this

WJEC A-level Chemistry specification — WJEC (2015)