A student dissolves 5.00 g of NH4NO3 (M = 80.04 g/mol) in 100.0 g of water in a coffee-cup calorimeter. Initial water temperature = 22.0 C. After dissolution and complete mixing, the minimum temperature is 18.6 C. Specific heat of solution = 4.18 J/(g*C). Total mass of solution = 105.0 g. Calorimeter heat capacity is negligible.
Despite being endothermic, NH4NO3 dissolves spontaneously in water at 25 C. The thermodynamic explanation is:
- A
the solution releases heat to the surroundings
- Bcheck_circle
the entropy of dissolution is positive and large enough that T*delta S > delta H
- C
the reaction is actually exothermic
- D
the calorimeter is poorly insulated
Explanation
delta G = delta H - T*delta S < 0 requires delta S > delta H / T. The disordering of the ionic lattice into solvated ions provides large positive delta S.