7th edition 4D.15

[Alyssa Bryan 3F]

Determine the reaction enthalpy for the hydrogenation of ethyne to ethane, C2H2 (g) + 2H2 (g) = C2H6 (g), from the following data: ΔHc(C2H2, g) = -1300 kJ/mol, ΔHc(C2H6, g) = -1560 kJ/mol, ΔHc(H2, g) = -286 kJ/mol. Why is the answer -312 kJ/mol and not +312 kJ/mol?

[Alexandra Albers 1D]

I believe that the way to solve this is that you add together the delta H of the first reaction (-1300 kJ) with the reversed delta H of the second reaction (+1560 kJ) because your original reaction involves the creation of C2H6 and not its breakdown. You have to therefore reverse the reaction and also its delta H I believe. Then you add to that the delta H of the third reaction but multiply it by 2 to account for the presence of the coefficient of H2. All of that should add up to -312 kJ/mol.