An outdoor medical kit must include a dissolvable solid that, when added to 250 g of water at 15 °C, absorbs approximately 50 kJ of heat to reduce the temperature of the water but must not cool it below 5 °C. The following candidate solids are available.
Compound ΔHdissolution (kJ\,mol−1) Molar Mass (g\,mol−1)
NH4NO3 +26 80.0
KNO3 +35 101.1
CaCl2 –82 110.9
(a) For each compound, calculate the minimum mass that must dissolve to absorb 50 kJ of heat (use absolute values where appropriate).
(b) Using the specific heat capacity 4.18Jg−1∘C−1, determine the expected temperature change of the 250 g of water for each compound if that minimum mass is dissolved, assuming no heat loss to the surroundings and neglecting the heat capacity of the solid.
(c) Identify which compound(s), if any, meet both project criteria. Justify your selection with quantitative calculations and a discussion of how particle-level interactions dictate whether the process is endothermic or exothermic.
(d) Propose one modification—other than changing the compound—that could allow the kit to meet the cooling requirement more effectively, and explain the chemical reasoning behind your suggestion.