Bond dissociation enthalpies of hydrogen(g) and nitrogen(g) are 436.0 kJ/mol and 941.8 kJ/mol and enthalpy of formation of NH3(g) is -46kJ/mol .What is the enthalpy of atomisation of NH3? What is the average bond enthalpy of N - H bond?

  1. 1171.9 kJ/mol 
  2. 1170.9 kJ/mol 
  3. 1168 kJ/mol 
  4. 1268  kJ/mol 

Average bond enthalpy of NH bond

1/2N2(g) + 3/2H2(g)-->NH3(g) 

Standard. enthalpy of reaction = -46kJ/mol

1/2 an N2 bond is 941.8/2 = 470.9 kJ/mol

3/2 H2 bonds = 3×436/2 = 654 kJ/mol

Total energy of reaction = (total energy of bonds broken) - (total energy of bonds formed)

Total energy = -46.3 kJ/mol

Therefore, -3(N-H) + 1124.5 = -46.3

3(N-H) = 1124.5 + 46.3 = 1170.8

(N-H) = 1170.8/3 = 390.2667 ≈ 390 kJ/mol

Enthalpy of atomization of ammonia is 1170.9 KJ/mol.

  • 16
What are you looking for?