Enthalpy of solution
The enthalpy of solution (ΔsolnH) of a substance is the enthalpy change on the dissolving 1 mol of that substance in sufficient solvent so that particles of solute are well separated and do not interact with one another.
For dissolving an ionic solid in water:
MX(s)
M+(aq) + X–(aq)
Forming a solution containing an ionic solid in water can be envisaged as the steps given below.
Thus a cycle can be constructed that allows calculation of the enthalpy of solution from the lattice enthalpy and the hydration enthalpies for its ions.
MX(s)
M+(aq) + X–(aq)As a consequence of dissolving, the attractive forces between anion and cation in the lattice are replaced by attractive forces between each of the anion and cation and water. The complex formed between water and the cation (K+ in this case) is shown at the right.
Water also interacts with the anion through hydrogen bonding.
Water also interacts with the anion through hydrogen bonding.
Forming a solution containing an ionic solid in water can be envisaged as the steps given below.
| Step 1: Separating ions in the lattice | M+(g)+ X–(g) MX(s) | lattice dissociation enthalpy (ΔLH) endothermic (separating charged particles) | |
| Step 2: Forming complex of the ions with the solvent. | M+(g) H2OM+(aq) | X–(g)H2O X–(aq) | hydration enthalpy (ΔhydH) exothermic (dispersing charge over a larger volume) |
Thus a cycle can be constructed that allows calculation of the enthalpy of solution from the lattice enthalpy and the hydration enthalpies for its ions.