Using Bond Dissociation Energies to calculate Enthalpy of formation of Methane

[manohman]

So say we have a reaction

C(s) + 2 H2(g) --> CH4 (g)

And we are given bond dissociation energies for C-H and for H-H bonds.


Why can't we just do Bond dissociation energy of reactants - Bond dissociation energy of products?


Hess's law says we can do Enthalpy of Bonds Broken - Enthalpy of Bond Formed


So why cant we do 2*H-H bond - 4*C-H bond enthalpy = enthalpy of the reaction.


I m guessing some of it has to do with the Carbon being in solid form but why should it matter? There is no bond? IF Carbon were in Gas form would the above method work?

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