DH calculations?

Question

Calculate DH° for the reaction C4H4(g) + 2H2(g) C4H8(g), using the following data:
DH°combustion for C4H4(g) = -2341 kJ/mol

DH°combustion for H2(g) = -286 kJ/mol

DH°combustion for C4H8(g) = -2755 kJ/mol


A. -128 kJ
B. -158 kJ
C. 128 kJ
D. 158 kJ
E. none of these

Just by doing the math, I am getting 158 kJ. But I am unsure. Is this one of those problems where I am supposed to change the sign? Please help.

Answer 1

When you use enthalpies of combustion, it's reactants minus products, and you get -158 in this case. You are probably doing products minus reactants, which is what you do for enthalpies of formation.