If my thermodynamics prof had given me this question I'd have written him a paper on "Why he should get a life and set some sensible bloody questions"
(Fortunately he had a big thing about engines so we had loads of questions on that - still hated it tho
)
Anyway, I know that thermodynamics is ALL about making asumptions, but this takes the biscuit. For starters you have to decide on which version of hell to use, though going for "The infinate planes of hell" would make it interesting.
Still. I'd be tempted to go for the simple answer - Souls are the fuel for the fires of hell, which give off heat, thus exothermic. Job done
Doubt that would get many marks though.....