A student dissolves exactly 0.10 mol of solid MgCl2 in 100 mL of water at 25 °C and observes that the temperature of the solution rises by 4 °C. When the procedure is repeated with 0.10 mol of solid NaCl under identical conditions, the temperature increases by only 0.5 °C.
Assume that the heat capacity of the solution is the same as that of pure water (4.18 J g−1 °C−1) and that the mass of the solution equals the mass of the water added.
(a) Describe two structural features of the Mg2+ and Na+ ions that account for the difference in the magnitude of the temperature change observed when their chlorides dissolve.
(b) Explain, in terms of competing energetic factors, why dissolution of MgCl2 releases more heat per mole than dissolution of NaCl.
(c) Predict which salt—MgCl2 or NaCl—has the larger molar solubility in water at 25 °C. Justify your prediction using your reasoning from part (b).