CHEMISTRY · Single correct · A Level · Chemistry · Bonding · Bonding
Problem
Two crystalline salts, W and X, are compared. Their ionic characteristics are shown below.
• Salt W: cation +1 (radius 152 pm); anion −1 (radius 220 pm)
• Salt X: cation +2 (radius 72 pm); anion −1 (radius 181 pm)
Based on these data, which salt is most likely to show a larger increase in molar solubility as temperature rises, and why?
(a) Salt X, because its cation’s very high charge density gives an extremely negative (exothermic) hydration enthalpy, so adding heat drives the dissolution forward according to Le Châtelier’s principle.
(b) Salt X, because its very small ions make its lattice energy low, leaving the process strongly endothermic and therefore more temperature dependent.
(c) Salt W, because its relatively low lattice energy makes dissolution slightly endothermic, so higher temperature shifts the equilibrium toward dissolved ions.
(d) Salt X, since its large lattice energy and hydration enthalpy nearly cancel, leaving the entropy term to dominate; the higher temperature amplifies the entropy contribution and increases solubility.