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

Title: The effect of coordination number on boiling/melting point?
Post 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)
Title: The effect of coordination number on boiling/melting point?
Post by: lightarrow on 14/11/2007 22:20:38
Quote from: 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)
From my book of General Chemistry (Lezioni di Chimica _Seconda edizione_ Bertini - Mani):

U = -kNAM*e2/r

U = lattice energy
k = 8.98*109 J m C-2
NA = 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)
"    CaF2            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.
 
Title: The effect of coordination number on boiling/melting point?
Post by: BenUK on 14/11/2007 22:58:38
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  [?]
Title: The effect of coordination number on boiling/melting point?
Post by: lightarrow on 15/11/2007 17:44:48
Quote from: BenUK on 14/11/2007 22:58:38
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 = Ssolid - Sliquid.

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.
Title: The effect of coordination number on boiling/melting point?
Post by: BenUK on 15/11/2007 18:43:40
Thanks for your help lightarrow.
Title: The effect of coordination number on boiling/melting point?
Post by: lightarrow on 16/11/2007 16:37:09
Quote from: BenUK on 15/11/2007 18:43:40
Thanks for your help lightarrow.
You're welcome!