Lattice Enthalpy, Hydration Enthalpy And Enthalpy Of Solution - Practice Questions & MCQ

Lattice Enthalpy

The lattice enthalpy of an ionic compound is the enthalpy change which occurs when one mole of an ionic compound is formed from its ions in gaseous state.

$\mathrm{Na}^{+}(\mathrm{g})+\mathrm{Cl}^{-}(\mathrm{g}) \longrightarrow \mathrm{NaCl}(\mathrm{s}), \Delta \mathrm{H}=-788 \mathrm{kJmole}^{-1}$

Heat of Hydration

The enthalpy change during hydration of one mole of any gaseous ion is called heat of hydration.

$\begin{aligned} & \mathrm{Na}^{+}(\mathrm{g}) \longrightarrow \mathrm{Na}^{+}(\mathrm{aq}), \Delta \mathrm{H}=\Delta \mathrm{H}_{\mathrm{hyd}_{\mathrm{Na}^{+}}} \\ & \mathrm{Cl}^{-}(\mathrm{g}) \longrightarrow \mathrm{Cl}^{-}(\mathrm{aq}), \Delta \mathrm{H}=\Delta \mathrm{H}_{\mathrm{hyd}_{\mathrm{Cl}-}}\end{aligned}$

Heat of Solution

It is change in enthalpy when one mole of a solid solute is dissolved in excess of solvent.

$\mathrm{NaCl}(\mathrm{s}) \longrightarrow \mathrm{Na}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq}), \Delta \mathrm{H}=\Delta \mathrm{H}_{\mathrm{sol}_{\mathrm{NaCl}}}$

Solubility of an Ionic Compound in water

When an ionic compound dissolves in a solvent, the ions leave their ordered positions on the crystal lattice. These are now more free in solution. But solvation of these ions (hydration in case solvent is water) also occurs at the same time. The enthalpy of solution of any ionic solid, in water is, therefore, determined by the selective values of the lattice enthalpy and enthalpy of hydration of ions. This will be more clear with the help of the diagram given below:

Thus, the enthalpy of solution, enthalpy of hydration and the lattice energy can be related as

$\Delta_{\text {sol }} \mathrm{H}^0=\Delta_{\text {lattice }} \mathrm{H}^0+\Delta_{\text {hyd }} \mathrm{H}^0$

For most of the ionic compounds, $\Delta_{\text {sol }} \mathrm{H}^0$ is positive and the dissociation process is endothermic. Therefore the solubility of most salts in water increases with rise of temperature. If the lattice enthalpy is very high, the dissolution of the compound may not take place at all.