Getting a "feeling" for heats of solution

The dissolution reactions of two salts are explored; the differing heats of solution are considered at the molecular level.

Ingredients: copper chloride (hot pack), ammonium nitrate (cold pack)

Procedure: A complete recipe follows.

1. Mix the solid salt and liquid water.

2. Feel the heat added to the solution, both positive and negative.

3. Determine if the depression in temperature, in the reaction of the cold pack, is greater or less than the increase in temperature, in the reaction of the hot pack.

Understanding: The dissolution of salts can be either an exothermic process (adding heat to the surroundings) or endothermic process (absorbing heat from the surroundings). In our experiment, we find that the dissolution of calcium chloride is exothermic

CuCl2(s) → Cu2+(aq) + 2 Cl-(aq)

while the dissolution of ammonium nitrate is endothermic

NH4NO3(s) → NH4+(aq) + NO3-(aq)

Why is the dissolution of cupric chloride exothermic while the dissolution of ammonium nitrate is endothermic? How can we understand these differences in terms of intermolecular forces and interactions at the atomic level?

We can think of the dissolution of the salt as (1) the separation of the ions that compose the salt and (2) the solvation of those ions in water. The first step in the process will always cost energy. It will cost energy to pull the copper cations away from the chloride anions, just as it will cost energy to pull the ammonium cations away from the nitrate anions.

As the cupric ions are divalent cations, we reason that it will cost more energy to separate the cupric ions from the chloride ions than to separate the ammonium ions from the nitrate ions. Indeed it does. So how is it that dissolving the cupric chloride salt in water adds heat to the surroundings?

The answer comes in the second step in our process, the solvation of the separate ions in water. Water molecules are polar, and there is a strong attraction of water molecules to both cations and anions through the charge-dipole intermolecular interaction.

The solvation of the ammonium and nitrate ions lowers the energy of the system through favorable charge-dipole solvation of the ions. However, there is not quite enough lowering of energy to make the overall process of separating the ions and solvating the ions energetically favorable. So the process requires heat to be added from the surroundings and is endothermic, with a heat of reaction of +28.05 kJ.

The solvation of the cupric and chloride ions lowers the energy of the system to a greater extent, due to the more favorable interaction of the divalent cupric ion with the dipolar water molecules. Very favorable! That favorabble solvation energy leads to a process that is exothermic, with a heat of reaction of -81.35 kJ.

Deeper consideration of the heats of solution

Question: Mixing water with most salts is exothermic. When one mole of sodium chloride is added to 50 mL of water, the temperature decreases by 3C. When one mole of potassium nitrate is added to 50 mL of water, the temperature decreases by 12C. When one mole of lithium chloride is added to 50 mL of water, the temperature increases by 70C.

Use your knowledge of intermolecular forces to explain these results.

You can check your answers here.