Solid ammonium nitrate dissolves endothermically in water according to the process
NH4NO3(s) → NH4+ (aq) + NO3− (aq)
Standard enthalpies of formation (kJ mol−1): NH4NO3(s) –365.6; NH4+(aq) –132.5; NO3−(aq) –207.0.
A cold pack contains a sealed pouch with 500 g of liquid water (initially 25 °C) and a separate compartment containing NH4NO3(s). When the pouch is ruptured, the salt dissolves and cools the water.
(a) Calculate the standard enthalpy change, ΔH°dissolution, for dissolving one mole of NH4NO3(s).
(b) Assuming no heat loss to the surroundings and that the solution’s heat capacity is the same as that of pure water (4.18 J g−1 °C−1), determine the minimum mass of NH4NO3(s) required to lower the temperature of the 500 g of water from 25 °C to 15 °C.
(c) Discuss two practical factors that could cause the actual temperature drop achieved in a commercial cold pack to be smaller than the value predicted in part (b).