Question

Is the dissolution of KNO3 exothermic or endothermic and what does this mean?

Answer 1

Energy is required to separate KNO3 into ions. When KNO3 dissolves in water to form K+ ions and NO3- ions water molecules surround the ions and help separate them and in the process absorb heat from the surrounding water, cooling it down.
What could possibly make the this process occur since the products, the ions, are at a higher thermal energy after they are separated? Enthalpy, the heat energy, is not the only factor. There is also Gibbs free energy, DG.
DG = DH - TDS

... where DG is the Gibbs free energy, DH is the enthalpy change, T = absolute temp, DS = entropy change.

For a process to occur spontaneously (without outside intervention), like the dissolution of KNO3, the total energy of the "products" must be lower than the "reactants". What makes the dissolving of KNO3 spontaneous is the increase in entropy.

So the bottom line is that the increase in entropy of the resulting ions "drives" the reaction to occur and as it occurs energy is transferred from the surrounding water molecules to the KNO3 (heat) as it separates into ions. Therefore, the process is both spontaneous and endothermic.