Two hydrated salts, magnesium sulfate heptahydrate and ammonium nitrate, are each tested separately in identical, well-insulated coffee-cup calorimeters. In each trial, 5.00 g of the solid is quickly added to 100.0 g of distilled water initially at 25.0 °C. The specific heat capacity of the aqueous solution may be assumed to be the same as that of pure water.
After complete dissolution and thermal equilibration, the maximum temperature change recorded for each salt is listed below.
Numerical Data Table (all masses in g, temperatures in °C):
Salt added | Initial temperature | Final temperature
Magnesium sulfate heptahydrate | 25.0 | 22.8
Ammonium nitrate | 25.0 | 21.1
(a) Identify for each salt whether the dissolution process is exothermic or endothermic. Justify your answer using the experimental temperature data.
(b) Without doing any calculations, compare the magnitudes of the molar enthalpies of dissolution for the two salts. State which salt releases or absorbs the larger amount of energy per mole and explain your reasoning qualitatively.
(c) Propose one particulate-level explanation for the sign of the enthalpy change observed for ammonium nitrate.