0:03 so one of the advantages thinking about
0:06 our enthalpy as a state function is that
0:09 we can start to add things together uh
0:12 between the different reaction so that
0:15 concept is from hess's law and this is
0:17 totally due to the fact that it's a
0:22 state function for enthalpy so it the
0:25 process of enthalpies can be written as
0:28 a series of independent stepwise
0:31 reactions and the enthalpy change for
0:34 all of it is the sum of all the enthalpy
0:37 changes in all of the
0:40 reactions so here's an example of
0:42 thinking about hess's law so we want to
0:45 determine the enthalpy of formation of
0:49 iron Tri chloride so thinking about how
0:51 we want to calculate that and we are
0:54 given these two steps and what we can do
0:56 is we can do some sort of manipulations
0:59 of this so first of all we need to go
1:02 ahead and WR the reaction for the
1:06 formation of iron chloride so we know
1:08 that the enthalpy of formation for the
1:10 reaction is we are going to form one
1:13 mole of iron chloride and we are going
1:17 to form this from the elements at its
1:20 standard state so we know that we have
1:25 iron as a solid in here and we know
1:29 chlorine uh exists as a datomic gas but
1:32 how many chlorines do we actually need
1:34 so if we think about this we only need
1:37 three so if we think about our leish
1:39 structure looks something like this we
1:44 only need three out of the four so
1:47 that's three Hales of a
1:50 chlorine of chlorine
1:53 molecules so how do we actually
1:55 calculate this so this is now our Delta
1:58 H knot this is written as a formation we
2:02 are given these two fundamental steps um
2:05 uh up here so one and two what can we do
2:09 to actually manipulate them and add them
2:11 together and in this case it's actually
2:13 relatively straightforward we notice
2:16 that we have a product of fe2 over here
2:19 and we have one of them on this side of
2:22 the equation and lo and behold if we
2:25 take both of these equations and we just
2:28 add them up we see that we have an iron
2:31 on this side we have have an iron over
2:34 here we have one chloride plus a half of
2:37 a chloride this gives us our three Hales
2:41 chlorine and um our iron over here and
2:43 if we look on the product side if we add
2:46 this up this iron uh D chloride ends up
2:48 being canceled on both sides and we have
2:51 iron Tri chloride so for this we can
2:53 take our hess's law that our heat of
2:56 formation is going to be the sum of
2:59 these two over there and if we add up
3:04 the some of these we end up getting -3 99.5
3:06 99.5
3:09 K so that is the final answer for this
3:12 reaction so that is a fairly
3:15 straightforward kind of hess's law where
3:17 you can just kind of add it up the only
3:22 tricky part was go was writing this kind
3:25 of um enthalpy of formation and that was
3:28 just by the definition of what a
3:31 formation reaction actually
3:34 is and the next part is we will go
3:36 through something a little bit more complicated
