Edexcel International A Level Chemistry Lattice Energy Exam Questions

About Edexcel International A Level Chemistry Lattice Energy

Lattice energy in Unit 4 of Edexcel International A Level Chemistry takes the ionic bonding ideas from Unit 1 and the Hess's law cycles from Unit 2 and puts them together in Born-Haber cycles. You define lattice energy and the enthalpy changes that make up the cycle, then calculate lattice energies or other missing values. Comparing experimental values from Born-Haber cycles with theoretical values from the ionic model shows the degree of covalent character caused by polarisation. You also use enthalpies of hydration and solution to explain solubility. Examiners like full Born-Haber cycle construction, calculations and explanations of trends in lattice energy.

What you need to know

• Lattice energy The enthalpy change when one mole of an ionic solid is formed from its gaseous ions under standard conditions, for example Na+(g) + Cl-(g) gives NaCl(s). It is always exothermic.
• Component enthalpy changes Enthalpy of atomisation (forming one mole of gaseous atoms from the element), first and successive ionisation energies, and first and second electron affinities. The second electron affinity is endothermic because an electron is added to a negative ion.
• Born-Haber cycles By Hess's law, enthalpy of formation = atomisation of metal + ionisation energies + atomisation of non-metal + electron affinities + lattice energy.
• Factors affecting lattice energy Lattice energy is more exothermic for smaller ions and ions with higher charges, because of stronger electrostatic attraction.
• Experimental versus theoretical Theoretical values assume perfectly spherical ions. If the experimental (Born-Haber) value is more exothermic, the compound has some covalent character due to polarisation of the anion by the cation.
• Hydration and solution Enthalpy of hydration is the enthalpy change when one mole of gaseous ions dissolves to form an infinitely dilute solution. Enthalpy of solution = -lattice energy + sum of hydration enthalpies.

Common mistakes in Lattice Energy

• Writing lattice energy equations with the solid on the left. Edexcel defines it as forming the lattice from gaseous ions.
• Missing the factor of 2 when a compound contains two of an ion, for example in MgCl2 you need two atomisations and two electron affinities of chlorine.
• Using the enthalpy of dissociation of Cl2 instead of the atomisation of chlorine. Atomisation is for one mole of Cl atoms, half the bond enthalpy.
• Saying the second electron affinity of oxygen is exothermic. It is endothermic because of repulsion between the electron and the O- ion.
• Explaining a difference between experimental and theoretical values without mentioning polarisation and covalent character.

Try three Lattice Energy exam-style questions