Heat of formation of ethane is -19.46 k cal. Bond energies of H-H ,C-H and C-C bonds are 104.18, 99.0 and 80.0 k cal respectively. Calculate the heat of atomization of graphite in k.cal

The balanced chemical reaction for the formation of ethane is
2C(s)+3H_2​(g)⟶C₂​H₆​(g);Δ​H_f = −19.46Kcal  Let the heat of atomisation is x kcal.
Bond energies  of reactants = 2x+3×(H−−H)  =2x+3×104.18  
=2x+312.54
 Bond energy of products = (C−−C)+6(C−−H) 
=80+6×99  =674
The enthalpy change for the formation of ethane is the difference between the bond energy of reactants and the bond energy of products.
 ΔH=2x+312.54−674  

2x = 342

∴x=171kcal
Hence, the heat of atomisation of graphite (and the value of x) is 171kcal