so how do we calculate these lattice energies we can use hess's law and do a classic cycle which we call the born haber cycle and what this is is a cyclic kind of series of reactions that we know most of the energies that are involved except for some of the uh except for one of the values and we can calculate one of those values uh in there so we are first going to look at our compound um thinking about the formation of csf and what is the delta h lattice of this reaction so just remembering uh what the lattice energy is it should be csf um forming as a solid to get cs plus one in the gaseous state plus fluorine minus 1 in the gaseous state so we can write a series of reactions that look like this and kind of thinking about the the reactions that are important to us um so we're going to start off at the bottom with csf and then you'll notice on the series of reactions that one of them looks very familiar that one thing that looks very familiar is the formation of csf delta h not formation and that delta h naught formation if you look this up in the backyard book is minus 553.5 kilojoules and if we write that reaction uh for it so remember we are going to form a mole of that we're going to start off with cesium solid plus one half of a molecule of fluorine gaseous so that is already here we already see that right here so that is this reaction down here this delta h formation not however we want to go the opposite direction we want to go this direction we want to stay cesium fluoride and breaking it apart to form cesium solid and fluorine gas half a mole of fluorine gas so that ends up being negative of our delta h not a formation and that's going to equal we're going to change the sign of that so now it's positive 553.5 kilojoules per mole of this reaction and then we can look at these other reactions and most of them can be found in tables so this first one is the heat of sublimation we are going to look at the cesium solid going to cesium gas so it will sublime into a gas form and how much energy does that take the next thing we want to think about is ionization energy so remember our ionization energy uh periodic trends it's the energy it takes to remove one electron from its outer core shell so taking it moving it off cesium has one that will remove and this is the energy to remove one of those electrons now we have to think about taking fluorine gas so we want to do fluorine gas and we want to break this apart so we want to break that apart of that fluorine gas how much energy does that take and since we're only looking at half a mole of fluorine gas we're going to cut that dissociation energy in half so half of that dissociation energy equals 79.4 kilojoules per mole so we're at this point right now in our born haber cycle the next step uh is we're interested in forming the anion fluorine gas so there's an electron affinity and how much energy it takes to attract an electron so when we attract an electron we're going to uh drop in our energy so that's our electron affinity energy for that reaction on here so uh then we're left with this reaction and this cesium plus going to fluorine minus will form cesium f and that is actually the opposite of this reaction over here so our delta h lattice oops different color so this is our delta h lattice going from cesium uh over here to the cesium plus and the fluorine gas but what we actually want to calculate for this is the opposite of this so that's our negative delta h of our lattice for that reaction okay so that means on red we are going down in energy so the born haber cycles means we started at cesium fluoride remember according to the state function if we start and end at the same point we haven't done anything so the sum of all of our delta h's have to equal zero for our reactions and when we have an l delta h equals zero we can add all them up our only unknown is this value over here we are able to look up everything so that means our plus 553.5 our delta h formation plus our sublimation energy plus our ionization energy plus half of our dissociation energy which is 79.4 minus our electron affinity 328.2 kilojoules minus our negative our sorry minus our delta h lattice so that is this value right here we set this all equal to zero that means our delta h lattice equals 756.9 kilojoules per mole of reaction and that is our reaction over here so this is our cesium f forms cesium plus in the gaseous state plus fluorine fluoride in the gaseous state so that is how we calculate one of the values using the born haber cycle