Many reactions cannot be carried out in a single step, but instead require multiple steps to get to the final product. We're going to say Hess's law states that the enthalpy of reaction of an overall reaction is the sum of the enthalpies, the standard enthalpy values of these multiple steps. So here we have a partial reaction one and a partial reaction 2. By combining them together they help to give me my overall reaction down here.
Now what we do is this Xenon difluoride is a reactant, so it comes down. Then we're going to say here that this fluorine, which is a solid, I'm not solid. A product cancels out with one of these fluorines, which is a reactant. Remember, they can't exist on both sides. OK, this causes an imbalance. These are called reaction intermediates and they will cancel one another out. So this cancels out with one of these, leaving us with one behind.
This xenon here is a product, and this one here is a reactant. Again, it can exist as both there's one and one on each, so they both cancel out each other entirely. What's left behind comes down South. This F2 comes down to give me this reactant, and then this Xenon triford comes down to give me this final product.
To find the overall enthalpy of reaction for this overall reaction, you add up each of these partial standard entropy values. So when you add 123 and you subtract it, subtract 262. That's how we come up with this new delta H of reaction. That's a -139 kilojoules. This in essence is what Hessel's law tries to do. It takes partial reactions and from them helps us to determine the overall reaction and associated with it in overall enthalpy of reaction.
