Chemistry - Thermochemistry Question with Solution | TestHub
The thermal effects of reactions in liquid NH3 at -33°C were measured by observing the quantity of liquid NH3 vaporized by the process of interest. The heat of vaporization of NH3 at -33°C is 320.0 cal/g. When 0.98 g of NH4Br was dissolved in 20 g of liquid NH3, 0.25 g of NH3 was vaporized. (Br = 80)
The molar heat of solution of NH4Br in liquid NH3 at this concentration is
Options:
Answer:
Solution:
The molar mass of NH₄Br is calculated as 14 + 4 + 80 = 98 g/mol.
Therefore, 0.98 g of NH₄Br corresponds to 0.0100 mol.
The heat absorbed during vaporization is 0.25 × 320 = 80 cal. This heat is supplied by the dissolution process, indicating that the dissolution is exothermic.
The enthalpy change (Δ H) for the dissolution is calculated as:
Δ H = (-80 cal)/0.0100 mol = -8000 cal/mol = -8.0 kcal/mol
The correct answer is (c).
