22. Which one of the following processes has a negative value for AH°? (a) CO2(s) + CO2(g)
(b) C(s) + O2(g) + CO2(g)
(c) 2H2O(1) 2H2(g) + O2(g)
(d) H2O(l) → H2O(g)

C

Explanation:

A negative deltaH means that the reaction has to give up heat in order to happen. You have to treat deltaH as a reactant. So the question is do you need to add heat to the reactants to make the products. If you do, deltaH is plus.

Heat is required to make a solid go to a gas. deltaH is plus. A is not the answer.

A lot of heat is required for B (something like 400 Kj / mole. Like A, deltaH is plus and B is not the answer.

C: The liquid has to give up heat in order for the this reaction to take place. C is the answer.

D requires heat. It is not the answer.