OxfordAQA International A Level Chemistry 3.1.8 Thermodynamics Exam Questions

About OxfordAQA International A Level Chemistry 3.1.8 Thermodynamics

Thermodynamics is an A2 physical chemistry topic examined in Unit 3. You need to construct Born-Haber cycles to calculate lattice enthalpies, using definitions of enthalpy of atomisation, ionisation energy, electron affinity and bond enthalpy, and compare experimental values with those from a perfect ionic model to show covalent character. You also need enthalpy of hydration and enthalpy of solution cycles. The second part covers entropy and the Gibbs free energy equation ΔG = ΔH - TΔS, including finding the temperature at which a reaction becomes feasible. It builds on energetics from Unit 1 and links to electrode potentials and equilibria.

What you need to know

• Lattice enthalpy Lattice enthalpy can be defined as lattice formation (gaseous ions to one mole of solid, exothermic) or lattice dissociation (solid to gaseous ions, endothermic). Always state which definition you use.
• Born-Haber cycles The cycle links enthalpy of formation with enthalpies of atomisation, ionisation energies, electron affinities and lattice enthalpy. Use Hess's law to calculate the unknown value.
• Covalent character If the Born-Haber value is larger than the value from a perfect ionic model, the bonding has some covalent character, because the cation distorts (polarises) the anion.
• Hydration and solution Enthalpy of solution = lattice dissociation enthalpy + sum of enthalpies of hydration of the ions. Hydration enthalpies are more exothermic for smaller ions with higher charge.
• Entropy Entropy (S) measures disorder. Gases have much higher entropy than liquids and solids. ΔS = sum of S products - sum of S reactants, in J K-1 mol-1.
• Gibbs free energy ΔG = ΔH - TΔS. A reaction is feasible when ΔG is zero or negative. The temperature at which it becomes feasible is T = ΔH / ΔS when ΔG = 0, converting ΔS to kJ K-1 mol-1.

Common mistakes in 3.1.8 Thermodynamics

• Mixing units in ΔG = ΔH - TΔS. ΔH is usually in kJ mol-1 and ΔS in J K-1 mol-1, so divide ΔS by 1000.
• Forgetting to double the atomisation enthalpy or electron affinity when there are two atoms or ions, such as in MgCl2.
• Saying a feasible reaction will happen quickly. Feasibility says nothing about rate; the activation energy may be high.
• Using the wrong sign for the first electron affinity. The first electron affinity is usually exothermic; the second is endothermic.

Try three 3.1.8 Thermodynamics exam-style questions