0:02 so here's a more complicated kind of
0:06 hess's law problem so what we want to do
0:08 is we want to figure out what the heat
0:11 of En or the enthalpy
0:15 is uh for this reaction over here so
0:17 let's first look at this and and kind of
0:20 given these three fundamental reactions
0:22 so first of all we look at these
0:24 reactions and we know that we have
0:27 Florine gas on the reactant side there's
0:30 only one equation that has Florine gas
0:32 in it so in this case we're going to
0:36 have to go ahead and reverse this
0:39 one and when we reverse this reaction we
0:41 end up getting I'll write this over again
0:42 again
0:50 2f2 + O2 yield uh 20 F2 and our Delta H
0:55 knot for 2 equals -
0:58 49.4 but we also know that we only need
1:01 one Florine over here we have two here
1:04 so we have to take this whole thing and
1:07 cut it in half uh on here so if we end
1:09 up cutting this in half we end up
1:12 getting Florine
1:17 plus2 yields o F2 and our Delta H KN of
1:19 I I now equals
1:21 equals
1:23 um oh and the other thing is we also
1:25 have to change a sign up here so we have
1:28 to change a sign of this and then we end
1:30 up cutting it in half is we end up
1:35 getting a positive 2 uh uh 24.7
1:43 KJ so we reversed it and cut it in half
1:45 so reversing it changes the sign and
1:49 then we can cut our Delta H in half and
1:51 that is purely because it's a state
1:53 function then we look at this reaction
1:57 we now know that we have um
2:00 O2 and kind of uh thinking about this we
2:04 know we want cf3 as a product we look
2:07 over here the cf3 is there but we need
2:09 to reverse that reaction so when we go
2:11 ahead and reverse
2:17 that reaction we end up getting uh 1 12
2:23 cl2o plus 3es o F2 ends up yielding CF
2:30 uh CL F3 + O2 gas
2:32 and our Delta H since we didn't change
2:35 anything uh we reversed it it now
2:38 becomes a negative to
2:40 to
2:45 36.2 K on here and then we can go ahead
2:48 and take the third the other uh reaction
2:56 four then we can look at our final our
2:59 reaction three and we can take that
3:03 reaction as is except we need to cut
3:08 that in half so we end up getting um two
3:12 clf plus O2 yeld
3:17 cl2o plus o f 2 if we want to get rid of
3:19 some of those kinds of reaction we know
3:22 that we have to cut this in
3:25 half so our Delta
3:28 H now equals we did not change the
3:31 direction so so we can still keep it as positive
3:33 positive
3:37 24.0 we divided by that by two we end up getting
3:38 getting 107
3:40 107 k
3:42 k
3:45 107.0 then what we can do is we can go
3:47 ahead and take all these reactions and
3:50 add them up if you actually add them all
3:52 up you will find out that you get
3:58 clf plus F2 yield
4:01 clf3 and then our Del H now it's going
4:05 to be the sum of all the reactions after
4:06 we make the
4:09 manipulations of all those reactions so
4:11 this Delta H now equals
4:14 equals 104.5
4:16 104.5
4:20 K so that is a more difficult kind of
