[escapeartistq]

Prove that 2C₈H₁₈(l) + 25O₂(g) -> 16CO₂(g) + 18H₂O(g)

is exothermic, admiting that all gases behave like ideal gases.

I tried this:

?U = ?H + W
W = -RT?n
?n = (16+18) - 25 = 9  

so W < 0

How do you prove that AH < 0 as asked?

[savoy7]

Use Hess's law to determine the heat of reaction.

http://energy.sdsu.edu/testcenter/testhome/Test/solve/basics/tables/tablesComb/formation.html
this site has some heats of formations that you can use to determine the heat of reaction.

[escapeartistq]

Thanks, for your suggestion.  Now I see it was a stupid question because I thought there was a way to show that AH < 0 without recurring to Hess's law.