Enthalpy of Solution

Dissolving a solute in a solution is either an exothermic or an endothermic process.

When sodium hydroxide dissolves in water, heat is transferred from the solution to the surrounding water causing the temperature of the water to increase. This is an exothermic process.

In endothermic processes, such as dissolving ammonium chloride in water, heat is absorbed by the solution causing the temperature of the water to decrease.

At constant pressure, the heat released or absorbed is called the enthalpy change.

Solution formation has three steps, each associated with a corresponding enthalpy change.

Step one is the separation of the solute particles. This requires an input of energy to overcome the attractive forces between the solute particles.

Step two is the separation of the solvent particles. This is also an endothermic step since energy is required to disrupt the attractive forces between the solvent particles.

Step three occurs when the solute and solvent particles mix. This step is exothermic because the attractive interactions between solute particles and solvent particles release energy.

For a stepwise process, Hess’s Law states that the net enthalpy change is the sum of the ent