Eduqas A-Level Chemistry: Physical Chemistry overview

What this topic demands

The physical chemistry of the Eduqas Physical and Inorganic section is the most quantitative part of the A-level. It makes the qualitative AS ideas of energetics, kinetics and equilibrium fully numerical, and the examiners reward accurate calculation, correct units and a clear grasp of the difference between feasibility and rate.

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

Electrochemistry

Redox and standard electrode potential (PI1.1) defines the standard hydrogen electrode and standard electrode potentials, builds electrochemical cells, calculates the EMF as the more-positive minus the more-negative electrode potential, and uses the sign of the EMF to predict feasibility, with the caution that feasible does not mean fast.

Kinetics

Chemical kinetics (PI3) introduces the rate equation rate $= k[\text{A}]^m[\text{B}]^n$, with orders found experimentally from concentration-time graphs (constant half-life means first order) and initial-rate data. The rate-determining step controls which species appear in the rate equation, so the rate equation tests a proposed mechanism, and the Arrhenius equation links the rate constant to activation energy.

Thermodynamics

Enthalpy changes for solids and solutions (PI4.1) covers lattice enthalpy and Born-Haber cycles (a Hess's-law cycle through atomisation, ionisation energy, electron affinity and formation), plus the enthalpies of solution and hydration that decide solubility.

Entropy and feasibility of reactions (PI4.2) introduces entropy as disorder, the entropy change from the change in moles of gas, and Gibbs free energy $\Delta G = \Delta H - T\Delta S$ as the feasibility criterion, explaining why temperature can switch a reaction on.

Equilibria

Equilibrium constants (PI5.1) writes and calculates $K_c$ and $K_p$ with their units, and establishes that only temperature changes the constant.

Acid-base equilibria (PI5.2) covers the pH scale, $K_w$, $K_a$ and $pK_a$, the pH of strong and weak acids and bases, buffer action and calculation, and titration curves with indicator choice.

How this topic is examined

A typical Eduqas profile for the physical chemistry:

• Electrochemistry. Cell EMF, overall equations and feasibility predictions from electrode potentials.
• Kinetics. Determining orders and the rate constant from data, writing rate equations, and linking to the rate-determining step.
• Thermodynamics. Born-Haber lattice-enthalpy calculations, entropy-change predictions and Gibbs free-energy and feasibility-temperature calculations.
• Equilibria. $K_c$ and $K_p$ calculations, and pH, $K_a$ and buffer calculations with titration-curve interpretation.

Check your knowledge

A mix of calculation and explanation questions across the physical chemistry. Attempt them under timed conditions, then check against the solutions.

1. Calculate the EMF of a cell made from $\text{Zn}^{2+}/\text{Zn}$ ($-0.76\ \text{V}$) and $\text{Cu}^{2+}/\text{Cu}$ ($+0.34\ \text{V}$). (1 mark)
2. A reaction is first order in A and second order in B. Write the rate equation and state the overall order. (2 marks)
3. State the condition, in terms of $\Delta G$, for a reaction to be feasible. (1 mark)
4. Define lattice enthalpy (formation). (1 mark)
5. State the only factor that changes the value of an equilibrium constant. (1 mark)
6. Calculate the pH of $0.0500\ \text{mol dm}^{-3}$ hydrochloric acid. (1 mark)