our final application of hess's law is a classic kind of way of thinking about it is that if we can look at our Delta H knots we can actually calculate that based on heat of formations of our products and our reactants so we're going to take our balanced chemical reactions and we're going to multiply the sto geometry by the heat of the formation of the products minus the heat of formation of the reactants so you will hear this uh sound a lot like products minus reactants and it will come up so we can get the standard enthalpy of change for this following reaction by looking these up at in terms of appendix G and looking up these values and if we look up the heat of formations uh for these values we can look this up for this uh this uh no two as 33.2 K Water just watch out there are two choices so make sure you pick the one with the uh the state of matter on there this isus 285.0 3 K or 83 nitric acid is aquous look that up in your in the chapter of your book - 207.49 nitrogen monoxide is looking that up in the book is 9.25 K so our Delta H not for this reaction we're going to sum up all of the products multiplied by the stochiometric ratios minus the sum of all of these values multiplied by its sto metric ratios so we start off as this um the products let's start off with no there's one n and that's 90.2 kles plus there's two Nitric acids and the heat of formation of nitric acid is a - 207.49 there are three nitrogen dioxides and those are 33.2 K and we're going to add to that the water there's only one of those minus 28583 1 k and if you end up calculating that make sure you're using your brackets properly because this minus sign that's over here this minus sign over here is actually for this whole thing over uh this whole summation so make sure you're using your brackets properly and if you end up calculating this this equals minus 138.4 K and that is your heat of reaction for your entire reaction using products minus the reactants