Average C-H bond energy in methane

[Janiceleong26]

Homework Statement

"By means of a balanced chemical equation, including state symbols, illustrate the term average C-H bond energy in methane."
Ans: CH4 (g) -> C(g) + 4H(g)
Average bond energy = +x/4 kj/mol

Why C (g) ? Why is the state of C gas?

 

[Borek]

To avoid including unrelated C(g) → C(s) reaction energy in the C-H bond.

Bond energy is the energy required to dissociate the bond itself, if the final product would be C(s) you would not deal with just a bond dissociation.

 

[Janiceleong26]

To avoid including unrelated C(g) → C(s) reaction energy in the C-H bond.

Bond energy is the energy required to dissociate the bond itself, if the final product would be C(s) you would not deal with just a bond dissociation.

Sorry I'm abit lost..what do you mean to avoid unrelated C(g)-> C(s) ?
So does it mean that, naturally, when CH4 is broken.. The carbon formed is in gas state? But I thought it is more stable for C to exists as C(graphite) ?

 

[Borek]

No, it doesn't mean carbon exists in the gas state (at least not in typical for us temperatures and pressures). But if you would use the enthalpy of the reaction CH₄(g) → C(s) + 4H(g) to calculate energy of the C-H bond, you would include energy of converting carbon from gas state into the solid state (C(g) → C(s), actually just a reversed sublimation), making the calculated energy much higher than it really is.

Please remember bond energy has nothing to do with the standard states of the elements involved. It is not only a problem with carbon, hydrogen in standard state doesn't exist as H(g), but as H₂(g). But when talking about the bond energy all we care about is the amount of energy required to break the bond, we don't care about what happens to products. And when all you do with gaseous CH₄ is breaking all four bonds, what you get is a gaseous mixture of carbon and hydrogen atoms. Yes, they will react/condense after that, but these are separate processes that we don't care about when determining the bond energy.

 

[Janiceleong26]

No, it doesn't mean carbon exists in the gas state (at least not in typical for us temperatures and pressures). But if you would use the enthalpy of the reaction CH₄(g) → C(s) + 4H(g) to calculate energy of the C-H bond, you would include energy of converting carbon from gas state into the solid state (C(g) → C(s), actually just a reversed sublimation), making the calculated energy much higher than it really is.

Please remember bond energy has nothing to do with the standard states of the elements involved. It is not only a problem with carbon, hydrogen in standard state doesn't exist as H(g), but as H₂(g). But when talking about the bond energy all we care about is the amount of energy required to break the bond, we don't care about what happens to products. And when all you do with gaseous CH₄ is breaking all four bonds, what you get is a gaseous mixture of carbon and hydrogen atoms. Yes, they will react/condense after that, but these are separate processes that we don't care about when determining the bond energy.

I see, thanks thanks.