Chemistry · Thermodynamics · NEET
It is the enthalpy (heat) change when you add MORE solvent to a solution that already has solute dissolved in it. The amount of solute stays the same; only the amount of solvent goes up. In NCERT, HCl.25aq + 15aq goes to HCl.40aq, and the heat change (-0.76 kJ/mol) is the enthalpy of dilution.
Enthalpy of solution is when 1 mole of solute (e.g. HCl gas or NaCl solid) FIRST dissolves in solvent. Enthalpy of dilution is when you take that ALREADY dissolved solution and add EXTRA solvent. Solution = solute meets solvent for the first time. Dilution = extra water added to a solution that already exists.
Yes. It is the difference between the enthalpy of solution at the two concentrations. If HCl.25aq has ΔH = -72.03 kJ/mol and HCl.40aq has ΔH = -72.79 kJ/mol, then enthalpy of dilution = (-72.79) - (-72.03) = -0.76 kJ/mol. You subtract the less-dilute solution's ΔH from the more-dilute one's ΔH.
Write the dilution as: HCl.25aq + 15aq goes to HCl.40aq. This is like taking equation (S-2) 'HCl(g)+40aq' and subtracting equation (S-1) 'HCl(g)+25aq'. The HCl(g) cancels, leaving the dilution step. So ΔH_dilution = ΔH(40aq) - ΔH(25aq) = -72.79 - (-72.03) = -0.76 kJ/mol.
It can be either. It depends on the solute and how concentrated the solution was. For HCl it is a small negative value (-0.76 kJ/mol), meaning a little heat is given out. NCERT calls it 'the heat withdrawn from the surroundings when additional solvent is added', but the sign is not fixed for every substance.
Because the ion-ion and ion-water interactions change as the solution gets more dilute. In a concentrated solution ions are close and interact strongly; adding water separates them. So the heat change depends on how concentrated you started and how much solvent you add. NCERT states this directly: it 'is dependent on the original concentration of the solution and the amount of solvent added'.
As you keep adding solvent, the enthalpy of solution stops changing and reaches a limiting value. That limit is the enthalpy of solution at infinite dilution, where ions are so far apart they no longer interact. For HCl, this limiting value is ΔH = -74.85 kJ/mol (HCl.∞aq).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is reported per mole of solute (kJ/mol), so it is treated as a molar quantity. Its value depends on concentration and how much solvent is added, not on being intensive or extensive in the simple sense.
For diluting HCl.25aq to HCl.40aq (adding 15 mol water), the enthalpy of dilution is -0.76 kJ/mol, found from (-72.79) - (-72.03).
It appears rarely on its own but is important to know so you do not confuse it with enthalpy of solution, hydration, or lattice enthalpy, which are all tested. Understanding the difference protects you from trap options.
Because the main heat change (breaking the lattice and hydrating ions) already happened during dissolving. Adding more water only slightly changes the ion interactions, so the extra heat change is small.
