Question

The standard enthalpy of formation of NH₃ is -46.0 kJ mol^-1 . If the enthalpy of formation of H₂ from its atoms is -436 kJ mol^-1 and that of N₂ is -712 kJ mol^-1 , the average bond enthalpy of N-H bond in NH₃ is.

a) -1102 kJ mol^-1

b) -964 kJ mol^-1

c) +352 kJ mol^-1

d) 1056 kJ mol^-1

Answer 1

Correct option is c) +352 kJ mol^-1

Enthalpy of formation of NH₃ = -46 kJ mol^-1

B.E. (N-H) = + 352 kJ mol^-1

There are 3 N-H bonds in NH₃ , so average bond enthalpy will be (B.E. of NH₃)/3.