Stoichiometric amounts of nitrogen gas and hydrogen gas react in a calorimeter to produce 5.00 g of ammonia gas. The calorimeter temperature rises 0.42°C. The calorimeter and water have a combined heat capacity of 32.16 kJ/K. Calculate the heat of formation of ammonia, ΔH_f°, in kJ/mol. The formation reaction for ammonia is: 0.5N₂(g) + 1.5H₂(g) → NH₃(g).

Question

Quizplus · Accepted Answer

The heat of formation (ΔH_f°) of ammonia can be calculated using the heat absorbed by the calorimeter and the amount of ammonia produced. The heat absorbed by the calorimeter is given by the product of the temperature change and the heat capacity of the calorimeter and water, which is 0.42°C * 32.16 kJ/K = 13.5072 kJ. This is the heat released by the formation of 5.00 g of ammonia. The molar mass of ammonia (NH3) is 14.01 (N) + 3*1.01 (H) = 17.03 g/mol. Thus, 5.00 g of NH3 corresponds to 5.00 g / 17.03 g/mol = 0.2936 mol. The heat of formation per mole of NH3 is then 13.5072 kJ / 0.2936 mol = 46.01 kJ/mol. Since the reaction is exothermic (releases heat), the heat of formation is negative, making the correct answer −46 kJ/mol.

Stoichiometric Amounts of Nitrogen Gas and Hydrogen Gas React in a Calorimeter