Commercial cold packs and hot packs are available for...

Commercial cold packs and hot packs are available for treating athletic injuries. Both types contain a pouch of water and a dry chemical. When the pack is struck, the pouch of water breaks, dissolving the chemical, and the solution becomes either hot or cold. Many hot packs use magnesium sulfate, and many cold packs use ammonium nitrate. Write reactions to show how these strong electrolytes break apart when they dissolve in water.

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Key Concepts

Electrolyte Dissociation

Electrolyte dissociation refers to the process in which a soluble ionic compound splits into its constituent ions when dissolved in water. This complete ionization occurs because water is a polar solvent that stabilizes the separated ions through hydration, leading to the formation of aquated ions that are no longer associated in a solid lattice.

Dissolution Energy Changes

The energy change accompanying the dissolution of an electrolyte depends on the balance between lattice energy and hydration energy. In some cases, such as with certain cold packs, the dissolution is endothermic, meaning it absorbs heat from the surroundings and produces a cooling effect. Conversely, an exothermic reaction, which releases heat upon dissolution, is beneficial for producing a warming effect in hot packs.

Representation of Dissolution Reactions

Writing the dissolution reaction for a strong electrolyte involves showing the solid ionic compound breaking apart into its constituent ions in aqueous solution. For example, when magnesium sulfate dissolves in water, it dissociates completely into magnesium ions and sulfate ions, and similarly, ammonium nitrate dissociates into ammonium and nitrate ions. These net ionic equations help reveal the mechanism by which the ionic compounds interact with water molecules.

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