Bond Enthalpy & Enthalpy of a Reaction

Question

N2 + 3H2 --> 2NH3

a. What change in energy is required to make 1.00 mol of NH3?

b. What change in energy is required to make 1.00 gram of NH3?

c. What change in energy is required to use up 2.34g N2?

Bond Enthalpies:

N2 = 941 kJ, H2 = 436kJ, N-H = 391 kJ

Answer 1

N₂ + 3H₂ ==> 2NH₃

Calculate bond energy for N2, 3H2 and 2NH3

N₂: 941 kJ

3H₂: 3x436 = 1308 kJ

2NH₃: 391x3 = 1173 for each NH₃ x 2NH₃ = 2346 kJ

(a) products - reactants = 2346 - (941 + 1308) = 97 kJ for 2 mols = 48.5 kJ for 1.00 mol NH₃

(b) 1.00 g NH₃ x 1 mol NH₃ / 17 g = 0.0588 mols x 48.5 kJ/mol = 2.85 kJ

(c) 2.34 g N₂ x 1 mol N₂ / 28 g = 0.0836 mols N₂ x 48.5 kJ/mol NH₃ x 1 mol N₂ / 2 mol NH₃ = 2.03 kJ

(be sure to check all of the math)