What does the hydration enthalpy of anhydrous salt describe?

What does the hydration enthalpy of anhydrous salt describe?

• 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 one mole of hydrated salt dissolves in water.
• C. The enthalpy change when a salt dissolves.
• D. The enthalpy change when water is added to the solid.

Answer: (A)

Hydration enthalpy of anhydrous salt describes the energy change when water molecules join the salt to form a hydrated solid. For one mole of salt, this is the enthalpy change when a hydrated salt is formed from one mole of the anhydrous salt under standard conditions. This is why forming the hydrated salt from the anhydrous form is the best description. Dissolving the salt in water (or dissolving a hydrated salt) involves additional steps like breaking the lattice and solvation of ions, so those processes are not the hydration enthalpy itself. Adding water to the solid is related, but the standard phrasing is specifically the formation of the hydrate from the anhydrous salt.

Hydration enthalpy of anhydrous salt describes the energy change when water molecules join the salt to form a hydrated solid. For one mole of salt, this is the enthalpy change when a hydrated salt is formed from one mole of the anhydrous salt under standard conditions. This is why forming the hydrated salt from the anhydrous form is the best description. Dissolving the salt in water (or dissolving a hydrated salt) involves additional steps like breaking the lattice and solvation of ions, so those processes are not the hydration enthalpy itself. Adding water to the solid is related, but the standard phrasing is specifically the formation of the hydrate from the anhydrous salt.