CCEA A Level Chemistry 4.1 Lattice Enthalpy Exam Questions

About CCEA A Level Chemistry 4.1 Lattice Enthalpy

Section 4.1 opens Unit A2 1: Further Physical and Organic Chemistry. You need to define lattice enthalpy, construct Born-Haber cycles for Group I and Group II halides and oxides, and calculate any missing value algebraically. The cycles combine enthalpy of formation, enthalpy of atomisation, ionisation energies and electron affinities, so they draw on atomic structure (1.2) and energetics (2.8). You also need to define and calculate the enthalpy change of solution of an ionic compound from lattice and hydration enthalpies. Examiners regularly ask you to draw a labelled cycle, calculate a lattice enthalpy and explain differences in values using ionic charge and radius.

What you need to know

• Lattice enthalpy Lattice enthalpy relates one mole of an ionic solid to its separate gaseous ions. Forming the lattice from gaseous ions is exothermic; breaking it into gaseous ions is endothermic with the same magnitude. Learn the definition and sign convention used in your course and apply it consistently.
• Steps in a Born-Haber cycle Enthalpy of atomisation of the metal and of the non-metal (per mole of gaseous atoms formed), first (and second) ionisation energies, first (and second) electron affinities, enthalpy of formation, and the lattice enthalpy.
• Electron affinity The first electron affinity is the enthalpy change when one mole of gaseous atoms each gains an electron to form one mole of gaseous 1- ions; it is exothermic. The second electron affinity of oxygen is endothermic because an electron is added to a negative ion.
• Size and charge Lattice enthalpy is larger in magnitude for smaller ions and ions with higher charges, because the electrostatic attraction is stronger. MgO has a much larger lattice enthalpy than NaCl.
• Enthalpy of solution The enthalpy change when one mole of an ionic solid dissolves in water to give a very dilute solution. Delta H solution = lattice dissociation enthalpy + sum of the hydration enthalpies of the ions.

Common mistakes in 4.1 Lattice Enthalpy

• Using the bond enthalpy of Cl2 for atomisation. Atomisation forms one mole of atoms, so use half the Cl-Cl bond enthalpy.
• Forgetting the second ionisation energy for Group II metals. Mg needs both first and second ionisation energies to form Mg2+.
• Saying the second electron affinity of oxygen is exothermic. It is endothermic because of repulsion between the electron and the O- ion.
• Mixing up the sign of lattice enthalpy between formation and dissociation. Check the direction of the arrow in the cycle before adding values.
• Using a value for Cl2 instead of 2Cl for MgCl2. Two moles of chlorine atoms and two electron affinities are needed.

Try three 4.1 Lattice Enthalpy exam-style questions