Enthalpy of Solution & Hydration (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about enthalpy of solution and hydration.

The two are linked through lattice enthalpy: dissolving an ionic solid means reversing its lattice enthalpy to free the gaseous ions, then releasing hydration enthalpy as those ions dissolve.

Enthalpy of solution and enthalpy of hydration are connected through lattice enthalpy — reversing the lattice enthalpy of an ionic solid gives gaseous ions, and hydrating those gaseous ions releases the hydration enthalpy, so the enthalpy of solution is the sum of the two.

This video first covers what enthalpy of solution and enthalpy of hydration actually are, then covers how Hess's Law cycles are used to calculate one of these values from the other two.

The standard enthalpy change of solution is the enthalpy change when one mole of an ionic substance dissolves in enough water to form an infinitely dilute solution — this can be exothermic or endothermic. The standard enthalpy change of hydration is the enthalpy change when one mole of a specified gaseous ion dissolves in enough water to form an infinitely dilute solution — this is always exothermic. Hydration enthalpy comes from ion-dipole attractions: the delta negative oxygen of water is attracted to positive ions, and the delta positive hydrogens are attracted to negative ions. Enthalpy of solution equals the reverse of the lattice formation enthalpy, plus the hydration enthalpy.

Gaseous ions can form ions in aqueous solution by two routes: directly, or indirectly via the ionic solid. Hess's Law says both routes give the same total enthalpy change, so the enthalpy of hydration equals the lattice enthalpy plus the enthalpy of solution. Each ion has its own hydration enthalpy, so for a compound with more than one ion — such as magnesium chloride — the hydration enthalpies of each ion are added together, with the appropriate coefficient for how many of that ion are present.

This exam board uses the term 'lattice energy' for both lattice formation and lattice dissociation, so 'reverse lattice enthalpy' here means the reverse of the lattice formation enthalpy. When working through these energy cycles, work step by step: label each enthalpy change clearly, show your working, and use brackets to separate the enthalpy values from the arithmetic.

Enthalpy of solution and enthalpy of hydration are both defined per mole, forming an infinitely dilute solution — hydration is always exothermic, while solution can be either. The two are connected through lattice enthalpy, and Hess's Law cycles let you calculate any one of solution, hydration, or lattice enthalpy from the other two.