A commercially available instant cold pack consists of a thin plastic bag filled with…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A commercially available instant cold pack consists of a thin plastic bag filled with water that surrounds a separate inner pouch containing solid ammonium nitrate. When the inner pouch is ruptured, the ammonium nitrate dissolves completely and the temperature of the mixture drops from 25 °C to 8 °C.
(a) Identify the types of interparticle interactions that are (i) disrupted and (ii) newly formed during the dissolution of ammonium nitrate in water. Describe how the relative strengths of these interactions account for the observed temperature decrease.
(b) Using macroscopic energy conservation, explain why the temperature drop indicates the sign of the enthalpy change for the dissolution process.
(c) A manufacturer considers replacing ammonium nitrate with calcium chloride in the same cold-pack design. It is known that the magnitude of the lattice energy of CaCl2 is about 2.25 × 103 kJ mol−1 greater than that of NH4NO3, while the magnitude of the hydration enthalpy of Ca2+ and Cl− ions combined is about 2.60 × 103 kJ mol−1 greater than that for NH4+ and NO3−. Predict qualitatively whether a calcium-chloride pack would feel colder, warmer, or show little temperature change relative to the ammonium-nitrate pack. Justify your prediction.