Calcium sulfate (CaSO₄) is only sparingly soluble in water even though the…
CHEMISTRY · Multi-part · AP · Chemistry · Thermodynamics and Electrochemistry · Thermodynamics and Electrochemistry
Problem
Calcium sulfate (CaSO₄) is only sparingly soluble in water even though the dissolution produces two highly charged ions (Ca²⁺ and SO₄²⁻). A computational chemist complains that predicting ΔG∘ for this dissolution is "nearly impossible" with routine quantum-chemical methods.
(a) Describe two factors specific to CaSO₄(aq) that make an accurate calculation of the lattice enthalpy especially challenging.
(b) Explain how long-range restructuring of the liquid-water hydrogen-bond network around doubly charged ions complicates estimation of the hydration entropy.
(c) Ion pairing between Ca²⁺ and SO₄²⁻ is observed in dilute solutions. Discuss how ignoring ion pairing could lead to significant error in a theoretical prediction of ΔG∘.
(d) Suggest one qualitative change in solvent (e.g., temperature or cosolvent addition) that could experimentally test the importance of water-network restructuring, and predict how it would affect the measured ΔG∘.