Does Hess' law always require canceling out all parts of equation to get the delta H of the reaction?

Question

Does Hess' law always require canceling out all parts of equation to get the delta H of the reaction?

What am I doing wrong on this Hess' Law problem?

Question: What is the delta H of the reaction KS=K+S

1) K+S=KS Delta H = +32.5 kJ

2) KS+K=K2S Delta H = +38.2 kJ

Step 1: Switch sides fo equation 1 and change the delta H to negative:

KS=K+S Delta H = -32.5 kJ

After flipping sides on the first equation, it gives the reaction I'm looking for KS=K+S.

Now I don't have anything to cancel out the second equation with the first.

Is the answer really just -32.5 kj or is this a trick question and I need to look at it differently?

Thank you!

Answer 1

It really is that simple. If the reverse of a given reaction provides the reaction of interest, the the ∆G for the reaction of interest is the same numerical value but with the opposite sign. So, you'd be correct in your answer of -32.5 kJ. I'm not sure that this would actually be considered an example of Hess' law, but that's just a technicality.