Answer to Question #21796 in Physical Chemistry for Angus Coyne

Question

To my understanding La Chateilers principal suggests that compounds which react exothermically with water will have decreased solubility with temperature. However, both KCL and KI react exothermically with water- but they have INCREASED solubility with watet? Why is this so? thank you!

Answer 1

If the heat given off in the dissolving reaction is less than the heat required to break apart the solid, the net dissolving reaction is endothermic (energy required). The addition of more heat facilitates the dissolving reaction by providing energy to break bonds in the solid. This is the most common situation where an increase in temperature produces an increase in solubility for solids.The use of first-aid instant cold packs is an application of this solubility principle. A salt such as ammonium nitrate, potassium chloride or potassium iodide is dissolved in water after a sharp blow breaks the containers for each. The dissolving reaction is endothermic - requires heat. Therefore the heat is drawn from the surroundings, the pack feels cold.