The dissolution of a salt AB in water occurs according to the process:
AB(s)→A+(aq)+B−(aq)
Relevant thermodynamic data (all values in kJ mol−1):
• Lattice energy of AB: ΔHlattice=+770
• Hydration enthalpy of A+: ΔHhyd(A+)=−420
• Hydration enthalpy of B−: ΔHhyd(B−)=−340
Answer the following about the energetic profile of the dissolution.
(a) Construct a qualitative energy diagram (energy vs. reaction coordinate) for the dissolution process, labeling reactants, products, and intermediate separated gaseous ions. Indicate all relevant energy changes on the diagram.
(b) Using the data provided, calculate the enthalpy change of solution ΔHsoln for AB.
(c) State whether the dissolution is endothermic or exothermic and predict the qualitative temperature change (if any) that would be observed in the water during dissolution of a small amount of AB. Justify your prediction.
(d) Suppose a student repeats the experiment but observes a smaller temperature change than predicted. Propose two plausible experimental factors that could account for this discrepancy, and explain the effect of each on the measured temperature change.