4.1 - Lattice Enthalpy

Question 1

Construct a Born-Haber cycle for sodium fluoride and calculate the lattice enthalpy using the data below.

First ionisation energy of sodium = +496
Enthalpy of atomisation of sodium = +107
Bond enthalpy of fluorine = +158
Electron affinity of fluorine = -333
Enthalpy of formation of sodium fluoride = -574

Answer 1

Lattice enthalpy of sodium fluoride = +923 kJ mol-1

Bond enthalpy of fluorine is per two atoms of fluorine produced (as fluorine is diatomic), therefore divide by two.

Full working out on page 19 of Lattice Enthalpy booklet

Question 2

Magnesium fluoride and magnesium chloride are both soluble in water. Magnesium chloride has an enthalpy of solution of -155 kJ mol-1.

Given that the enthalpies of hydration of magnesium ions and chloride ions are -1920 kJ mol-1 and -364 kJ mol-1 respectively, calculate the lattice enthalpy of magnesium chloride.

Answer 2

Lattice enthalpy of magnesium chloride = +2493 kJ mol-1

Working out on page 18 of Lattice Enthalpy booklet

Question 3

Complete a Born-Haber cycle for magnesium fluoride and, using the data given below, calculate the lattice enthalpy for magnesium fluoride

Standard enthalpy of formation for magnesium fluoride = -1123
First electron affinity of fluorine = -348
Atomisation enthalpy of fluorine = +79
First ionisation enthalpy of magnesium = +736
Second ionisation enthalpy of magnesium = +1450
Atomisation enthalpy of magnesium = +150

Answer 3

Lattice enthalpy for magnesium fluoride = +2921 kJ mol-1

Working out in full on pages 17 and 18 of Lattice Enthalpy booklet

Question 4

Using diagrams, show how magnesium and fluoride ions interact with water molecules in an aqueous solution of magnesium fluoride

Answer 4

Diagrams on page 18 of Lattice Enthalpy booklet