This content explains how to calculate the enthalpy change (ΔH) for a target chemical reaction using Hess's Law by manipulating and combining a series of given thermochemical equations.
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so here's a more complicated kind of
hess's law problem so what we want to do
is we want to figure out what the heat
of En or the enthalpy
is uh for this reaction over here so
let's first look at this and and kind of
given these three fundamental reactions
so first of all we look at these
reactions and we know that we have
Florine gas on the reactant side there's
only one equation that has Florine gas
in it so in this case we're going to
have to go ahead and reverse this
one and when we reverse this reaction we
end up getting I'll write this over again
again
2f2 + O2 yield uh 20 F2 and our Delta H
knot for 2 equals -
49.4 but we also know that we only need
one Florine over here we have two here
so we have to take this whole thing and
cut it in half uh on here so if we end
up cutting this in half we end up
getting Florine
plus2 yields o F2 and our Delta H KN of
I I now equals
equals
um oh and the other thing is we also
have to change a sign up here so we have
to change a sign of this and then we end
up cutting it in half is we end up
getting a positive 2 uh uh 24.7
KJ so we reversed it and cut it in half
so reversing it changes the sign and
then we can cut our Delta H in half and
that is purely because it's a state
function then we look at this reaction
we now know that we have um
O2 and kind of uh thinking about this we
know we want cf3 as a product we look
over here the cf3 is there but we need
to reverse that reaction so when we go
ahead and reverse
that reaction we end up getting uh 1 12
cl2o plus 3es o F2 ends up yielding CF
uh CL F3 + O2 gas
and our Delta H since we didn't change
anything uh we reversed it it now
becomes a negative to
to
36.2 K on here and then we can go ahead
and take the third the other uh reaction
four then we can look at our final our
reaction three and we can take that
reaction as is except we need to cut
that in half so we end up getting um two
clf plus O2 yeld
cl2o plus o f 2 if we want to get rid of
some of those kinds of reaction we know
that we have to cut this in
half so our Delta
H now equals we did not change the
direction so so we can still keep it as positive
positive
24.0 we divided by that by two we end up getting
getting 107
107 k
k
107.0 then what we can do is we can go
ahead and take all these reactions and
add them up if you actually add them all
up you will find out that you get
clf plus F2 yield
clf3 and then our Del H now it's going
to be the sum of all the reactions after
we make the
manipulations of all those reactions so
this Delta H now equals
equals 104.5
104.5
K so that is a more difficult kind of
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