0:04 our final application of hess's law is a
0:06 classic kind of way of thinking about it
0:10 is that if we can look at our Delta H
0:13 knots we can actually calculate that
0:16 based on heat of formations of our
0:18 products and our reactants so we're
0:20 going to take our balanced chemical
0:22 reactions and we're going to multiply
0:25 the sto geometry by the heat of the
0:28 formation of the products minus the heat
0:32 of formation of the reactants so you
0:34 will hear this uh sound a lot like
0:37 products minus reactants and it will
0:40 come up so we can get the standard
0:42 enthalpy of change for this following
0:45 reaction by looking these up at in terms
0:49 of appendix G and looking up these
0:53 values and if we look up the heat of
0:56 formations uh for these values we can
1:00 look this up for this uh this uh no two as
1:01 as
1:05 33.2 K Water just watch out there are
1:07 two choices so make sure you pick the
1:11 one with the uh the state of matter on
1:12 there this isus
1:19 K or
1:23 83 nitric acid is aquous look that up in
1:32 207.49 nitrogen monoxide is looking that
1:35 up in the book is
1:40 9.25 K so our Delta
1:43 H not for this
1:46 reaction we're going to sum up all of
1:49 the products multiplied by the
1:53 stochiometric ratios minus the sum of
1:55 all of these values multiplied by its
1:59 sto metric ratios so we start off as this
2:00 this
2:03 um the products let's start off with no
2:07 there's one n and that's 90.2
2:09 90.2
2:13 kles plus there's two Nitric acids and
2:16 the heat of formation of nitric acid is
2:34 there are three nitrogen
2:36 dioxides and those are 33.2
2:38 33.2
2:41 K and we're going to add to that the
2:59 28583 1 k
3:01 and if you end up calculating that make
3:04 sure you're using your brackets properly
3:08 because this minus sign that's over
3:12 here this minus sign over here is
3:16 actually for this whole thing over uh
3:18 this whole summation so make sure you're
3:20 using your brackets properly and if you
3:23 end up calculating this this equals minus
3:25 minus 138.4
3:27 138.4
3:31 K and that is your heat of reaction for
3:34 your entire reaction using products
