What is the standard enthalpy change of hydration of anhydrous salt defined as?

• A. The enthalpy change when one mole of a hydrated salt is formed from one mole of the anhydrous salt under standard conditions.
• B. The enthalpy change when water hydrates a salt.
• C. The enthalpy change when a salt dissolves.
• D. The enthalpy change when a gas dissolves.

Answer: (A)

The key idea is the heat change that accompanies forming a hydrated solid from its anhydrous form and water. Standard enthalpy change of hydration for anhydrous salt is defined as the energy change when one mole of the hydrated salt is formed from one mole of the anhydrous salt under standard conditions (298 K, 1 bar). For example, CuSO4(s) + 5 H2O(l) → CuSO4·5H2O(s) has this enthalpy change. This process is about binding water molecules into the solid lattice to create the hydrate, not about dissolving the salt or involving gas dissolving.