Three different ionic compounds—KNO₃, LiBr, and Na₂S—are independently dissolved in large quantities of water at 25 °C. For each dissolution, the student records the temperature change (ΔT) of the solution when 0.100 mol of the compound is added to 100.0 g of water.
Compound ΔT (°C)
KNO₃ –7.5
LiBr +11.2
Na₂S +3.4
(Assume the specific heat capacity and density of the solutions are identical to those of pure water.)
(a) Calculate the enthalpy change, ΔH (kJ mol⁻¹), for dissolution of each compound.
(b) Rank the dissolutions from the process that absorbs the most energy to the process that releases the most energy on a per-mole basis. Provide both the ranking order and the corresponding ΔH values with correct signs.
(c) Propose a molecular-level explanation for why LiBr(aq) is markedly more exothermic than Na₂S(aq), even though both processes release heat.
(d) Predict qualitatively how the ΔT for dissolving 0.050 mol of KNO₃ under identical conditions would compare with the original ΔT. Justify your prediction.