Enthalpies of Combustion

[GiTsticker]

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What is the value of deltaH for this reaction?

C4H4 + 2H2 > C4H8

Enthalpies of Combustion:
C4H4 -2341 kJ/mol
H2 -286 kJ/mol
C4H8 -2775 kJ/mol

a) -158kJ/mol
b) -128kJ/mol
c) 128kJ/mol
d) 158kJ/mol

I'm having problems because this isn't a combustion reaction but I'm given enthalpies of combustion. Can I convert to enthalpies of formation somehow? Thanks in advance.

 

[sweetcinuhstix]

It's sum of products minus sum of reactants, if i'm not mistaken.

Don't forget to times the H2 by 2.

 

[GiTsticker]

Doesn't work out...

DeltaH = (Products) - (Reactants)
= (-2775) - (-2341 + -286(2))
= (-2775) - (-2913)
= +138

None of the answer choices match. The only place I feel I could have gone wrong was by using enthalpies of combustion with a non-combustion reaction. Any ideas?

 

[NeoAkira]

Doesn't work out...

DeltaH = (Products) - (Reactants)
= (-2775) - (-2341 + -286(2))
= (-2775) - (-2913)
= +138

None of the answer choices match. The only place I feel I could have gone wrong was by using enthalpies of combustion with a non-combustion reaction. Any ideas?

Is the answer C? Because if it is then it's possible that whatever book you're using has a typo in the answers.

 

[Ryltar]

I posted this ACS problem earlier. Yes, there is a typo in the answer key. I believe combustion is the reverse of formation, so multiply the enthalpies of combustion by negative one to get the enthalpies of formation and then solve normally. I got -138kJ/mol when I solved.