0:02 so how do we calculate these lattice energies
0:04 energies
0:07 we can use hess's law and do a classic cycle
0:08 cycle
0:10 which we call the born haber cycle and
0:11 what this
0:14 is is a cyclic kind of series of reactions
0:15 reactions
0:18 that we know most of the energies that
0:19 are involved
0:22 except for some of the uh except for one
0:23 of the values
0:26 and we can calculate one of those values
0:28 uh in there so we are first going to
0:31 look at our compound
0:34 um thinking about the formation of
0:38 csf and what is the delta h
0:42 lattice of this reaction
0:45 so just remembering uh what the lattice
0:46 energy is
0:49 it should be csf um
0:53 forming as a solid to get cs
0:56 plus one in the gaseous state plus fluorine
0:57 fluorine
1:00 minus 1 in the gaseous state
1:03 so we can write a series of reactions
1:04 that look like
1:06 this and kind of thinking about the the
1:08 reactions that are
1:11 important to us um so we're going to
1:14 start off at the bottom with csf
1:16 and then you'll notice on the series of
1:18 reactions that one of them looks very familiar
1:19 familiar
1:21 that one thing that looks very familiar is
1:22 is
1:26 the formation of csf delta h
1:29 not formation and that delta h naught
1:31 formation if you look this up in the
1:32 backyard book
1:37 is minus 553.5
1:41 kilojoules and if we write that reaction
1:43 uh for it so remember we are going to
1:45 form a mole of that
1:46 we're going to start off with cesium solid
1:48 solid
1:52 plus one half of a molecule of fluorine
1:55 gaseous so that is already here we
1:57 already see that right here
2:00 so that is this reaction down here this delta
2:00 delta
2:03 h formation not however
2:06 we want to go the opposite direction we
2:07 want to go
2:10 this direction we want to stay cesium
2:14 fluoride and breaking it apart
2:17 to form cesium solid and fluorine gas
2:19 half a mole of fluorine gas
2:22 so that ends up being negative of our delta
2:22 delta
2:25 h not a formation and that's going to equal
2:26 equal
2:28 we're going to change the sign of that
2:33 553.5
2:38 kilojoules per mole of this reaction
2:40 and then we can look at these other
2:42 reactions and most of them can be found
2:45 in tables so this first one is the heat
2:46 of sublimation
2:49 we are going to look at the cesium solid
2:50 going to cesium
2:53 gas so it will sublime into a gas form
2:56 and how much energy does that take
2:58 the next thing we want to think about is
2:59 ionization energy
3:03 so remember our ionization energy uh
3:06 periodic trends it's the energy it takes
3:06 to remove
3:09 one electron from its outer core shell
3:11 so taking it moving it off
3:14 cesium has one that will remove and this
3:15 is the energy
3:18 to remove one of those electrons now we
3:20 have to think about taking fluorine gas
3:22 so we want to do fluorine gas
3:25 and we want to break this apart so we
3:27 want to break that apart
3:29 of that fluorine gas how much energy
3:31 does that take
3:33 and since we're only looking at half a
3:35 mole of fluorine gas
3:37 we're going to cut that dissociation
3:38 energy in half
3:41 so half of that dissociation energy equals
3:42 equals
3:46 79.4 kilojoules per mole
3:48 so we're at this point right now in our
3:50 born haber cycle
3:53 the next step uh is we're interested in forming
3:54 forming
3:57 the anion fluorine gas so there's an
3:59 electron affinity
4:01 and how much energy it takes to attract
4:02 an electron
4:04 so when we attract an electron we're
4:06 going to uh
4:08 drop in our energy so that's our
4:09 electron affinity
4:13 energy for that reaction on here
4:16 so uh then we're left with this reaction
4:19 and this cesium plus going to fluorine minus
4:20 minus
4:24 will form cesium f and that is actually the
4:24 the
4:28 opposite of this reaction over
4:32 here so our delta h lattice
4:36 oops different color
4:45 going from cesium uh over here to the
4:48 cesium plus and the fluorine gas
4:50 but what we actually want to calculate
4:56 is the opposite of this so that's our
4:58 negative delta h
5:01 of our lattice for that reaction
5:04 okay so that means on red we are going down
5:05 down
5:09 in energy so the born haber cycles means
5:12 we started at cesium fluoride remember
5:16 according to the state function if we start
5:16 start
5:18 and end at the same point we haven't
5:20 done anything
5:23 so the sum of all of our delta h's have
5:24 to equal
5:28 zero for our reactions
5:30 and when we have an l delta h equals
5:31 zero we can add all them
5:35 up our only unknown is this value over here
5:36 here
5:39 we are able to look up everything so
5:41 that means our plus
5:45 553.5 our delta h formation
5:54 plus our ionization energy
5:57 plus half of our dissociation energy
6:00 which is 79.4
6:07 328.2 kilojoules
6:18 our sorry minus our delta h lattice
6:21 so that is this value right here
6:24 we set this all equal to zero that means
6:26 our delta h lattice
6:30 equals 756.9
6:34 kilojoules per mole
6:37 of reaction and that is our reaction
6:38 over here
6:42 so this is our cesium f forms
6:46 cesium plus in the gaseous state
6:50 plus fluorine fluoride in the gaseous state
6:51 state
6:55 so that is how we calculate
6:58 one of the values using the born haber cycle
