Standard enthalpy change of solution

The standard enthalpy change of solution, ΔHsolo, is the change in enthalpy when one mole of a solute dissolves in a solvent to form an infinitely dilute solution under standard conditions. This means that we need to dissolve the solute in excess solvent until there is no change in the energy absorbed or released by the system.

Some examples are:

KOH(s)\rightarrow KOH(aq)\; \; \; \; \; \; \; \Delta H_{sol}^{\: o}=-57.6\: kJmol^{-1}

HCl(g)\rightarrow HCl(aq)\; \; \; \; \; \; \; \Delta H_{sol}^{\: o}=-74.8\: kJmol^{-1}

Since KOH(aq) and HCl(aq) are fully dissociated in water, we can also write the above equation as:

KOH(s)\rightarrow K^+(aq)+OH^-(aq)\; \; \; \; \; \; \; \Delta H_{sol}^{\: o}=-57.6\: kJmol^{-1}

HCl(g)\rightarrow H^+(aq)+Cl^-(aq)\; \; \; \; \; \; \; \Delta H_{sol}^{\: o}=-74.8\: kJmol^{-1}

ΔHsolo can be positive or negative. Compounds with large positive ΔHsolo are relatively insoluble.


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