NaCl and KF have the same crystal structure. The only difference between the two is the distance that separates cations and anions. (b) Use the ionic radii given in Figure 7.8 to estimate the Na─Cl and K─F distances.

Video transcript

Hi everyone for this problem we're told the lattice energy of lithium fluoride is 1030 kg joules per mole. While that of potassium bromide is 671 kg joules promote, given that both lithium fluoride and potassium bromide have the same rock salt structure. And ionic charges determine which compounds have the greater ionic separation. Okay, so we want to know which one has the greater ionic separation. In order to figure that out. We need to recall our lattice energy formula which is our lattice energy is equal to the negative charge one times charge two over R where r represents our ionic separation. So when we compare the two we can see that lattice energy and ionic separation have an inverse relationship. So that means what occurs to one, the opposite is going to occur to the other. And here we're specifically being asked which will have the greater ionic separation. And so if we have a high ionic separation, we're going to have a low lattice energy. And so now let's compare both of the lattice energies that were given. Were told that lithium fluoride has a lattice energy of 1030 and our potassium bromide has a lattice energy of 671. So our potassium bromide has a lower lattice energy. So that means it's going to have the higher the greater ionic separation because of the inverse relationship. So our final answer here is that our potassium bromide has the higher ionic separation. Okay, so potassium bromide goes higher ionic separation and that is the end of this problem. I hope this was helpful.