Four aqueous processes are investigated under standard conditions. For each process, the energy change for one mole of material is either measured or observed indirectly.
Process W: Dissolving 1.0 mol of NH₄NO₃(s) in sufficient water lowers the solution temperature by 6.3 °C.
Process X: Hydration of 1.0 mol of CaCl₂(s) raises the solution temperature by 10.5 °C.
Process Y: Vaporization of 1.0 mol of H₂O(l) at 100 °C and 1 atm.
Process Z: Conversion of 1.0 mol of super-cooled liquid H₂O at –10 °C to ice at –10 °C.
(The specific heat capacities of the solutions may be assumed equal to that of pure water, 4.18 J g⁻¹ °C⁻¹. The density of all solutions may be taken as 1.00 g mL⁻¹.)
(a) For each process, state whether it is exothermic or endothermic, citing the experimental or contextual evidence provided.
(b) Rank the four processes from the greatest magnitude of energy released to the greatest magnitude of energy absorbed per mole. Justify your ranking qualitatively; numerical calculations are not required.
(c) Choose one pair of adjacent processes from your ranking and describe, at the particulate level, a mechanistic reason for the difference in the magnitude of their energy changes.