What is the ΔH change when a salt crystal dissolves in water?

When a salt crystal dissolves in water, the overall enthalpy change (ΔH) can vary depending on the nature of the salt and its interactions with water. Generally, most salts dissolve in water in an endothermic process, resulting in a positive ΔH value. This means that the dissolution absorbs heat from the surroundings, leading to a cooling effect in the solution.

The process involves the breaking of ionic bonds in the salt lattice, requiring energy input, which leads to the positive ΔH. While the formation of hydration shells around the separated ions releases energy and is exothermic, the endothermic process of breaking the ionic bonds typically outweighs this effect for many common salts. Thus, when sodium chloride, for example, dissolves, the positive value for ΔH signifies that energy is absorbed overall during the dissolution process.

In cases where certain salts might exhibit slightly different behavior, understanding the specific interactions and energies involved is key. However, for most salts, the dissolution process is primarily characterized by a positive ΔH, which reflects the energy absorbed when ionic interactions are disrupted and the crystal lattice is dissolved in water.