Problem 8.57 [ENDORSED]

[Leah Savage 2F]

For problem 57, which says: Determine the reaction enthalpy for the hydrogenation of ethyne to ethane, C2H2(g) 2 H2(g) S C2H6(g), from the following data: Hc (C2H2, g) 1300. kJ·mol 1, Hc (C2H6, g) 1560. kJ·mol 1, Hc (H2, g) 286 kJ·mol 1.

Why is this problem solved in the solution manual by using the reactions of each product/reactant with oxygen? Why can't you just subtract the sum of enthalpies of reactants from products?

[Chem_Mod]

You are totally correct. You could just use enthalpies of formation to solve the problem. However, this problem assumes you have no enthalpy of formation data at hand. The only data you have are the enthalpies of combustion for each component. You can solve this by setting up as a Hess's Law style problem.

It would be interesting to compare your answer to that obtained using enthalpies of formation (if you can find them in the Appendix or another reputable source). The results should be the same as long as the data was taken at the same temperature and pressure conditions.

[Leah Savage 2F]

So when there are enthalpies of combustion, do you always have to use Hess's Law? What other cases would you use Hess's Law in?

[Gurkriti Ahluwalia 1K]

I have looked over multiple discussion questions on this question and there are two things I am still very confused about.
First, how does this question assume there is no enthalpy of formation?
Secondly, if we do apply Hess's Law using combustion reactions, how do we eliminate the appearance of Oxygen in the products? In other words, how is it possible to perform a combustion reaction without the use of oxygen?