When NH4NO3 dissolves in water, the temperature of the solution decreases. What describes the enthalpy change, entropy change, and which change drives the process?

• A. Delta H is positive and Delta S is positive and the process is driven by the entropy change.
• B. Delta H is negative and Delta S is negative and the process is driven by enthalpy change.
• C. Delta H is positive and Delta S is negative and the process is driven by entropy change.
• D. Delta H is negative and Delta S is positive and the process is driven by enthalpy change.

Answer: (A)

Dissolving ammonium nitrate in water is endothermic, so the enthalpy change is positive. The solid lattice breaks apart and the ions mix into the water, increasing disorder, so the entropy change is also positive. Whether the process occurs spontaneously depends on ΔG = ΔH − TΔS. With ΔH > 0 and ΔS > 0, the dissolution can be driven by the entropy term when the temperature is high enough (TΔS > ΔH), making ΔG negative. The observed cooling of the solution shows heat is absorbed from the surroundings, reinforcing that the driving factor for dissolution in this case is the increase in entropy.