What is the standard enthalpy change of solution defined as?

• A. The enthalpy change when one mole of solute is dissolved in solvent to form an infinitely dilute solution.
• B. The enthalpy change when a solid dissolves in water.
• C. The enthalpy change when one mole of water is formed.
• D. The enthalpy change when a gas is formed.

Answer: (A)

The standard enthalpy change of solution is the energy change when exactly one mole of solute dissolves in a solvent to form a solution that is infinitely dilute, under standard conditions. The infinite dilution part matters because it ensures there are no interactions between solute particles, so the measured energy comes from solute–solvent interactions only. This makes the value a standard, per-mole measure that can be compared across different processes.

Other scenarios aren’t the standard enthalpy of solution: dissolving a solid in water at an unspecified concentration isn’t necessarily at infinite dilution or standard conditions; forming water from elements is an enthalpy of formation, and a gas being formed relates to different processes altogether.