Hess' Law Chemistry

Question

When using Hess’s Law to calculate rnthalpy of a rxn, what does a negative or positive (delta)H mean? When do you add or subtract and why?

Answer 1

A negative ∆H means the reaction is exothermic (giving off heat from the system to the surroundings).

A positive ∆H means the reaction is endothermic (heat absorbed by the system from the surroundings).

When using Hess's Law, you add the ∆H values of the reactions being combined. The sign of ∆H will change if you have to reverse the reaction, but you will still ADD all the values at the end.

For example, if you have A + 2B ==> C and ∆H is (+), when you reverse it to C ==> A + 2B, the ∆H value will be the same BUT with a (-) number.