Question

How do you calculate lattice energy?

I have to place the following in order of decreasing magnitude of lattice energy, but I don't know how to do it.

KF MgS RbI

Is there a formula or some simple way of knowing which has the greatest energy?

2. I also have the same question, but with regard to vapor pressure. Given NF3, NH3, and BCl3, how do I figure out their vapor pressures? Thanks.

Answer 1

For 1, the smaller the ions the greater the lattice energy so RbI would be lowest and KF the highest (of those two). Mg and S are divalent and the attractive forces are higher than for monovalent cations and anions. So MgS I would expect to be the highest of the three.

Answer 2

For 2, the boiling point are
NH3 = -33
NF3 = -129
BCl3 = 12.5
You would expect vapor pressure to track (in reverse) the boiling point; i.e., higher b.p. gives lower v.p.
BCl3 has the highest molar mass and the IM forces are greatest; therefore b.p. is highest and vapor pressure the least.
NH3 has lowest molar mass BUT has hydrogen bonding so expect higher b.p. and lower v.p.
NF3 has no H bonding and a b.p. of -129 so vapor pressure should be the highest.