A-Level Chemistry: Physical Chemistry Practice

A-Level Chemistry — Physical Chemistry Practice

18 MCQ practice problems. Select answer, submit, review explanation.

What These Questions Test

These problems test your ability to apply physical chemistry concepts to quantitative and qualitative scenarios. You will calculate enthalpy changes, determine reaction orders, analyse equilibrium systems, and work with pH.

Typical question types:

• Energetics: Calculate $\Delta H$ from bond enthalpies, Hess’s law cycles, or calorimetry data. Use $\Delta H = -mc\Delta T$ for solution calorimetry. Interpret Born-Haber cycles.
• Kinetics: Determine the order of reaction from rate-concentration data or graphs. Calculate the rate constant with correct units. Use the half-life method: for first-order reactions, $t_{1/2} = \frac{\ln 2}{k}$ (constant half-life).
• Equilibrium: Calculate $K_{c}$ or $K_{p}$ from equilibrium concentrations or partial pressures. Predict the effect of changes on equilibrium position. Relate $K$ to $\Delta G$ via $\Delta G = -RT\ln K$.
• pH calculations: Calculate pH of strong acids, weak acids (using $K_{a}$), and buffer solutions. For buffers: $\text{pH} = \text{p}K_{a} + \log\frac{[\text{base}]}{[\text{acid}]}$. Interpret titration curves.
• Redox: Assign oxidation states. Write and balance half-equations. Identify the oxidising and reducing agents. Relate redox reactions to electrode potentials.

Approach Strategy

1. Draw the energy diagram. For energetics problems, sketch the enthalpy profile. Label $\Delta H$, $E_{a}$, and the activated complex. This prevents sign errors.
2. Check the order of reaction first. Before calculating $k$, determine whether the reaction is zero, first, or second order. Use the method of initial rates or the half-life test.
3. Write the $K_{c}$ expression before calculating. Identify all species in the equilibrium. Exclude solids and liquids. Check that the units of $K_{c}$ are consistent with the expression.
4. Use the buffer equation carefully. The Henderson-Hasselbalch equation $\text{pH} = \text{p}K_{a} + \log\frac{[A^{-}]}{[HA]}$ requires the ratio of conjugate base to acid, not the other way round.

Intuition

Physical chemistry is about understanding energy and speed at the molecular level. Enthalpy tells you whether a reaction is energetically favourable. Kinetics tells you how fast it will happen. Equilibrium tells you how far it will go. A reaction can be thermodynamically favourable ($\Delta G < 0$) but kinetically slow (high $E_{a}$) — like a ball sitting on top of a hill that could roll down but does not because of friction.

For pH, think of the logarithmic scale as a way to compress a huge range of hydrogen ion concentrations into manageable numbers. A change of 1 pH unit means a tenfold change in $[H^{+}]$.

Common Mistakes

1. Wrong units for the rate constant. The units of $k$ depend on the overall order. Zero order: $\text{mol dm}^{-3}\text{s}^{-1}$. First order: $\text{s}^{-1}$. Second order: $\text{mol}^{-1}\text{dm}^{3}\text{s}^{-1}$. Getting the units wrong in most cases means you have the wrong order.
2. Forgetting that $K_{c}$ is temperature-dependent. Changing concentration or pressure does NOT change $K_{c}$. Only temperature changes $K_{c}$.
3. Confusing strong and weak acids. A strong acid fully dissociates; a weak acid partially dissociates. The pH of a strong acid is lower than a weak acid of the same concentration.
4. Misidentifying the limiting reagent. In titration calculations, the limiting reagent is the one that runs out first. Always calculate moles of both reactants and compare with the stoichiometric ratio.

Cross-References

• A-Level Chemistry SL/HL: Core chemistry content covering atomic structure, bonding, and organic chemistry.
• A-Level Physics: Thermodynamics, energy transfers, and kinetic theory.
• A-Level Mathematics: Logarithms, exponentials, and calculus used in kinetics.
• IB Chemistry: IB chemistry curriculum for comparison.
• University Chemistry: Advanced physical chemistry and thermodynamics.