Hess's Law can be applied to calculate the standard enthalpy change of a reaction by using the standard heats of formation of the products and reactants, based on their stoichiometric coefficients.
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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
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
K or
83 nitric acid is aquous look that up in
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
this
um the products let's start off with no
there's one n and that's 90.2
90.2
kles plus there's two Nitric acids and
the heat of formation of nitric acid is
there are three nitrogen
dioxides and those are 33.2
33.2
K and we're going to add to that the
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
minus 138.4
138.4
K and that is your heat of reaction for
your entire reaction using products
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