Instant heat packs and cold packs use compounds that have a negative or positive
ID: 939409 • Letter: I
Question
Instant heat packs and cold packs use compounds that have a negative or positive enthalpy of solution, Hsol, to produce a temperature change. The packs consist of a pouch of water and a dry chemical, CaCl2 for heat packs, or NH4NO3 for cold packs. The pack is activated by breaking the seal of the water pouch, which dissolves the dry chemical and forms a solution that either increases or decreases in temperature. Predict the signs of Hsol and Ssol for the following solutions.
NH4NO3(s) + H2O(l) ---> NH4NO3(aq)
CaCl2(s) + H2O(l) ---> CaCl2(aq)
Explanation / Answer
Dissolving ammonium nitrate(NH4NO3) in water is endothermic. The energy released by solvation of the ammonium ions and nitrate ions is less than the energy absorbed in breaking up the ammonium nitrate ionic lattice and the attractions between water molecules.
NH4NO3(s) + H2O(l) ---> NH4NO3(aq) Hsol = +25.7 kJ/kg
Dissolving calcium chloride(CaCl2) is exothermic, as more energy is released during solvation than is used in breaking up the solute and solvent.
CaCl2(s) + H2O(l) ---> CaCl2(aq) Hsol = 240 kJ/kg