Lattice Enthalpy
Questions
The enthalpy change when one mole of a solid ionic compound is broken up to form its gaseous ions under standard conditions.
Sodium chloride. Chloride ions are smaller than bromide ions (yet have the same charge) and therefore have a larger charge density. Therefore, there is a greater electrostatic attraction between the sodium ions and the chloride ions, resulting in a more exothermic lattice enthalpy of formation.
Lithium chloride. Lithium ions are smaller than sodium ions (yet have the same charge) and therefore have a larger charge density. Therefore, there is a greater electrostatic attraction between the lithium ions and the chloride ions, resulting in a more exothermic lattice enthalpy of formation.
Aluminium hydroxide. Aluminium ions are more highly charged (3+ vs 2+) and are smaller. They therefore have a larger charge density. There is therefore a greater electrostatic attraction between the aluminium ions and the hydroxide ions, resulting in a more exothermic lattice enthalpy of formation.
The theoretical value is calculated based on an assumption that the interaction is purely ionic. However, lithium iodide has some covalent character. This is due to the small cation polarizing the large anion, resulting in a distorted anion (whereas the ionic model assumes that the ions are perfectly spherical).
Smaller. Lithium iodide experiences a larger difference because it has more covalent character than sodium iodide. This is due to the fact that lithium is a smaller ion than sodium. The smaller lithium ion has a larger charge density than the sodium ion, which distorts the anion more. This results in greater covalent character.
Sodium iodide is more likely to be soluble. This is because covalent character generally decreases solubility in polar solvents like water.
| AgBr | NaBr | |
|---|---|---|
| ΔHLat (experimental) / kJ mol-1 | +890 | +733 |
| ΔHLat (calculated) / kJ mol-1 | +758 | +732 |
Sodium bromide (NaBr). Perfectly ionic lattice means that there is (almost) no covalent character (ions are spherical). You can tell that this is the case because the experimental (+733) and theoretical (+732) values for the lattice enthalpy are almost identical.
From the perspective of experimental: 14.83%
From the perspective of theoretical: 17.41%
The bromide ion is distorted, resulting in significant covalent character. The silver ion is small and has a high charge density relative to its size (poor shielding by d-electrons), which causes it to polarize the anion more strongly than a Group 1 metal like sodium.
| Enthalpy | Value / kJ mol-1 |
|---|---|
| Enthalpy of formation of Al2O3 | -1675.5 |
| Atomisation enthalpy of Al | 326.4 |
| Atomisation enthalpy of O | 249.2 |
| First ionisation enthalpy of Al | 578 |
| Second ionisation enthalpy of Al | 1817 |
| Third ionisation enthalpy of Al | 2745 |
| First electron affinity of O | -141.1 |
| Second electron affinity of O | 789 |
Comment: The experimental value (-15300) is more exothermic than the theoretical value (-14912). This is consistent with the fact that aluminium oxide has significant covalent character due to the small, highly charged Al3+ ion polarizing the oxide ion.
The experimental value is the true value. It is derived from real physical data (Born-Haber cycle) which accounts for all interactions, including covalent character, whereas the theoretical model assumes purely ionic bonding.