Enthalpy of Solution

Sodium hydroxide dissolution is exothermic, meaning heat is released during the process. The attractive interactions between solute and solvent particles release energy, causing heat to transfer from the solution to surrounding water. This energy release increases the water temperature, making it feel warm to the touch.

Solution formation occurs in three steps: first, solute particles separate, requiring energy input to overcome attractive forces. Second, solvent particles separate, also requiring energy. Third, solute and solvent particles mix, releasing energy as new attractive interactions form. The net enthalpy change depends on the balance between these three steps.

Hess's Law states that the net enthalpy change equals the sum of enthalpy changes from each step. If the sum of separation enthalpies exceeds the mixing enthalpy, dissolution is endothermic. If mixing enthalpy exceeds separation enthalpies, dissolution is exothermic. When values are equal, no heat is released or absorbed.

Ammonium chloride dissolution is endothermic, absorbing heat from surroundings to break apart solute and solvent particles. This heat absorption causes the solution temperature to decrease. Ammonium nitrate exhibits similar behavior and is used in instant cold packs for treating injuries by absorbing heat from the affected area.

Solution formation is a physical change where the solute dissolves but retains its identity. Evaporating the solution recovers the original solute unchanged. In contrast, chemical reactions alter reactant properties permanently. For example, copper hydroxide dissolved in hydrochloric acid forms copper chloride, which cannot be recovered by evaporation.

Exothermic dissolution releases heat and favors spontaneous solution formation, though it does not guarantee it. Endothermic dissolution absorbs heat but can still be spontaneous due to increased disorder in the system. Entropy increase from matter dispersal can overcome unfavorable enthalpy changes, allowing endothermic solutions to form spontaneously.

The net enthalpy change determines dissolution type. When attractive interactions between solute and solvent particles release more energy than required to separate particles, dissolution is exothermic. When separation requires more energy than mixing releases, dissolution is endothermic. The relative magnitudes of these competing enthalpy values control the overall process.