Chemistry - Thermochemistry Question with Solution | TestHub
Statement 1: Enthalpy changes are positive when Na2SO4·10H2O, CuSO4·5H2O and salt like NaCl, KCl, etc., which do not form hydrates are dissolved in water. But enthalpy changes are negative when anhydrous salts capable of forming hydrates are dissolved in water.
Statement 2: The difference in the behaviour is due to large difference in the molecular masses of hydrated and anhydrous salts. The substance with large molecular mass usually show positive enthalpy changes on dissolutions.
Options:
Answer:
Solution:
Dissolving a hydrate (e.g., Na₂SO₄·10H₂O, CuSO₄·5H₂O) or a salt like NaCl or KCl, whose ions are only weakly hydrated, is an endothermic process (
Δ H > 0
), meaning the solution cools. Conversely, anhydrous salts that can hydrate (e.g., Na₂SO₄, CuSO₄) release a large amount of hydration enthalpy upon dissolving (
Δ H < 0
), causing the solution to warm. Therefore, Statement 1 is correct.
The primary reason for these enthalpy changes is the hydration enthalpy of the ions, not any difference in molecular masses. Thus, Statement 2 is incorrect. The answer is (c).
