Chemistry - Thermochemistry Question with Solution | TestHub
Statement 1: For all the salts completely soluble in water, the magnitude of sum of enthalpies of hydration of ions is greater than the magnitude of lattice enthalpy of the salt.
Statement 2: If the magnitude of sum of enthalpies of hydration of ions is less than the lattice enthalpy of the salt, the salt is completely insoluble in water.
Options:
Answer:
Solution:
The enthalpy of solution, Δ Hsol, is given by the equation:
Δ Hsol = Δ Hhydration - Δ Hlattice
A salt that dissolves completely and easily does so because the hydration enthalpy of its ions outweighs the lattice enthalpy holding the crystal together. This makes dissolution energetically favorable, which is correctly stated in Statement 1.
Statement 2, however, goes too far. When the hydration enthalpy is slightly smaller than the lattice enthalpy, dissolution is endothermic but can still be driven by a positive entropy change. For example, NaCl is freely soluble even though its Δ Hsol is approximately +4 kJ/mol. Therefore, the salt is not "completely insoluble" in such cases. Thus, Statement 2 is incorrect.
The answer is (c).
