The bond enthalpies of H2,X2 an HX are in the ratio of 2 : 1 : 2. If enthalpy of formation of HX is −50 kJ mol−1, the bond enthalpy of X2 is
100 kJ mol−1
200 kJ mol−1
300 kJ mol−1
400 kJ mol−1
12H2+12X2→HX,Let x=enthalpy of X−X bondΔHf(HX)=−50=12x−x=−x2 or, x=50×2=100 kJ mol−1
The standard enthalpy of formation of NH3 is 46.0 kJ mol−1. If the enthalpy of formation of H2 from its atom 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