An instant cold pack purchased in a pharmacy contains a...

An instant cold pack purchased in a pharmacy contains a packet of solid ammonium nitrate surrounded by a pouch of water. When the packet of $\mathrm{NH}_{4} \mathrm{NO}_{3}$ is broken, the solid dissolves in water and a cooling of the mixture occurs because the solution process for $\mathrm{NH}_{4} \mathrm{NO}_{3}$ in water is endothermic. Explain, in terms of what happens to the molecules and ions, why this mixing occurs spontaneously.

Key Concepts

Solvation and Ion-Dipole Interactions

Solvation occurs when solvent molecules surround and interact with solute particles, typically through ion-dipole interactions in the case of ionic compounds. Water molecules, having a significant dipole moment, align themselves around the ions, stabilizing them in solution. This process aids in breaking the ionic lattice and facilitates the increase in entropy required for the dissolution process to be spontaneous.

Enthalpy

Enthalpy refers to the heat content of a system. In the dissolution of an ionic compound, breaking the ordered ionic lattice into separate ions requires energy, making the process endothermic (positive ?H). However, the energy absorbed is outweighed by the increase in entropy under the right conditions.

Entropy

Entropy measures the disorder or randomness in a system. When an ionic solid dissolves in water, the ions separate and disperse throughout the solvent, greatly increasing the system's disorder. This increase in entropy is a major driving force for the dissolution process, even when the process absorbs heat.

Gibbs Free Energy

Gibbs free energy is the thermodynamic potential that predicts whether a process will occur spontaneously. It is defined by the relationship ?G = ?H - T?S, where ?H is the change in enthalpy and ?S is the change in entropy. Even if a process is endothermic (positive ?H), it can still be spontaneous if the increase in entropy (T?S) is large enough to make ?G negative.

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