Q13.29 P - Chemistry: Molecular Nature Of Matter And Change

Question

An ionic compound has a highly negative ${\mathbf{\Delta H}}_{\mathbf{soln}}$ in water. Would you expect it to be very soluble or nearly soluble or nearly insoluble in water? Explain in terms of enthalpy and entropy changes.

Step-by-Step Solution

Answer

If a compound has highly negative $\Delta H_{solution}$ in water, it would be very soluble in water.

$∆G=∆H-T∆S$, $∆G$ must be negative for solubility.

Step 1: The condition for the solubility.

Two factors that determine whether a solute dissolves in a solvent or not are the heat of solution  $\left({\mathbf{\Delta H}}_{\mathbf{solution}}\right)$and entropy change (S). The solution process involves the interplay of these two factors, which gives us $\Delta G$.

$∆G=∆H-T∆S$, $\Delta G$ must be negative for solubility.

If $\Delta H$ is highly negative, and $\Delta S$is highly positive(because) entropy increases on dissolution). Then $\Delta G$ must be negative, which means the reaction is highly feasible and the ionic compound is highly soluble in water.

Step 2: The reason for the high solubility of an ionic compound with highly negative ΔH solution .

When an ionic compound dissolves in water, the crystal breaks down and ions mix with water molecules leading to an increase in entropy along with enthalpy change. 

$∆G=∆H-T∆S$

As $\Delta H$ is highly negative, and $\Delta S$ is highly positive, then $\Delta G$ must be negative, which means the reaction is highly feasible.

Hence, ionic compounds with highly negative $\Delta H_{solution}$ is highly soluble in water.