Question

The bond enthalpy of H2O and OH are 502 KJ mol^-1 and 427 KKJ mol^-1. Then what is the average bond enthalpy?

(a) 502 KJ mol^-1

(b) 464.5 KJ mol^-1

(c) 427 KJ mol^-1

(d) 75 KJ mol^-1

The question was posed to me in exam.

Question is from Bond Parameters in division Chemical Bonding and Molecular Structure of Chemistry – Class 11

Answer 1

Right answer is (b) 464.5 KJ mol^-1

The explanation is: We know that H2O(g) → H(g) + OH(g); ΔH1 = 502 kJ mol^-1 and OH(g) → H(g) + O(g); ΔH2 = 427 kJ mol^-1. So the average bond enthalpy is given by their mean that is 427 + 502/2 = 464.5 KJ mol^-1. This method is used for polyatomic molecules like water.