[nicoliutw]

My question look like this:
1/2 C2H4 (g) ---> C (s) + H2 (g) delta H = -26.2 KJ (it is not under standard condition)
What is the enthalpy of formation of C2H4 (g) in Kj/mol?

I know to calculate enthalpy of formation, you take the product minus the reactant. But in this case, how do I know heat of both product and reactant? All we know here is it yield release 26.2Kj.

[SirRoderick]

Delta H = Enthalpy
You have the enthalpy for dissociation of 1/2 C2H4. You need the enthalpy for forming 1 C2H4.

Think on that.

[nicoliutw]

Reverse the thermochemical equal. so it would need absorb 26.2kj heat.
So I just do 26.2kj *2?