Heat of Solution

In the previous lesson, we learned how heat can be absorbed or released in a phase change. We saw how to express the amount of heat absorbed/released as a value in J/g or J/mol that we labeled ΔH_fus or ΔH_vap and referred to as the "heat of fusion" or "heat of vaporization."

The process of dissolving a compound also absorbs or releases heat, which can be measured in the exact same way. The only real difference is that we refer to this heat as the "heat of solution" and use the symbol ΔH_sol. Also, while heats of fusion and vaporization are always positive, it is possible for a heat of solution to be negative (though most of them are indeed positive, meaning heat is absorbed in the process of dissolving most substances).

We can do the same type of calculations with ΔH_sol as we did with phase changes. For example, say we want to know how much heat is absorbed in dissolving 25.0 g of NaBr, which has a heat of solution of 7.25 kJ/mol. We would convert grams to moles of NaBr and then multiply by the heat of solution, giving us 1.76 kJ of heat as our answer.

One component of your lab this week will be to determine the heat of solution of an ionic compound.