Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: BenUK on 14/11/2007 20:12:24

Hello,
I have a question that hopefully a more knowledgable chemist may be able to help me with. I'm currently writing a piece for college about the properties of ionic compounds and i was wondering if the coordination number of a compound had any effect on the boiling or melting point?. I thought it might be that the higher the coordination number the higher the b/m point but i couldnt seem to find the information in any of the books i've got or the websites i tried.
Thanks alot [:)]
(also first post so hello everyone)

Hello,
I have a question that hopefully a more knowledgable chemist may be able to help me with. I'm currently writing a piece for college about the properties of ionic compounds and i was wondering if the coordination number of a compound had any effect on the boiling or melting point?. I thought it might be that the higher the coordination number the higher the b/m point but i couldnt seem to find the information in any of the books i've got or the websites i tried.
Thanks alot [:)]
(also first post so hello everyone)
From my book of General Chemistry (Lezioni di Chimica _Seconda edizione_ Bertini  Mani):
U = kN_{A}M*e^{2}/r
U = lattice energy
k = 8.98*10^{9} J m C^{2}
N_{A} = total number of cations (= number of anions) in the lattice
M = Madelung's constant which depends on the kind of lattice = on how ions are reciprocally located in space: it also depends on the coordination number
r = distance from the centres of two near oppositely charged ions (= cation's radius + anion's radius)
Some values for M:
kind of lattice M
like NaCl 1.7475
" CsCl 1.7627
" ZnS 1.6413 (wurtzite)
" CaF_{2} 5.0388 (fluorite)
Clearly lattice energy U is related to melting enthalpy and not exactly on melting point; to know it we should also know lattice entropy.
I hope to have written something useful, even if it doesn't answer your question.

Thanks for taking the time to supply this information. It's much appreciated [:)]
If i understand correctly, the formula would suggest that as a higher coordination number is responsible for an increased lattice energy this would result in an increased boiling/melting point. i think [?]

Thanks for taking the time to supply this information. It's much appreciated [:)]
If i understand correctly, the formula would suggest that as a higher coordination number is responsible for an increased lattice energy this would result in an increased boiling/melting point. i think [?]
Yes, if you can assume the same variation of entropy between solid and liquid:
in the transition solid > liquid the free energy changes of ΔG = ΔH  TΔS; where ΔH is the enthalpy of fusion, T is the (constant) temperature at which the process happen (= melting point) and ΔS = S_{solid}  S_{liquid}.
ΔH is > 0 so the process becomes spontaneous (= happens) only when T is high enough to make ΔG from positive to negative, that is ΔG = 0. But this means ΔH = TΔS and so T = ΔH/ΔS.
So, if you can assume the same ΔS, the more the coordination number, the higher U and so ΔH and so the higher the melting point T.

Thanks for your help lightarrow.

Thanks for your help lightarrow.
You're welcome!