Posted by Danya on .
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.

Chem/ Calculating Lattice Energy, Vapor Pressure 
DrBob222,
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.

Chem/ Calculating Lattice Energy, Vapor Pressure 
DrBob222,
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.