Textbook 4D.15

[Ziyi Meng 2K]

Hi,

I was able to get the correct answer, but can someone tell me why it is negative?
Because -1560-(-1300+(2*-286)) is equal to 312, not -312?
Also, why is units kJ per mole and not just kJ?

[Reza Hemmati 3L]

Hi!

First off, the units are kJ/mole and not kJ bc you calculated the molar enthalpies for 1 mole of ethyne, so if you doubled the molar amounts the energy change would be doubled. (Plus, all of the units of the values you added are in kJ/mol)
As for the negative value, the values listed are the enthalpies of combustion, not the enthalpies of formation. (It took me a while to notice that) I'm not sure how that affects the final answer, but that is probably why the values are different. (Also, the enthalpy of formation of a pure substance like H2 is 0).

Hope this helps!