Ask A Scientist Chemistry Archive

name         Paola
status       other
grade        other
location     IL

Question -   I am searching for the enthalpy of sublimation of FeCL3
anhydrous.
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Paola-
     My CRC says FeCL3 dissociates at 315C, just above it's melting point.
Doesn't say what it changes to, but I suspect it looses a chlorine atom 
and becomes FeCl2,
which melts at a higher temperature (~670C), and sublimes too.

Are you presuming you will have a closed vessel with some high vapor 
pressure of Cl2 to keep the FeCl3 from dissociating?

    Sorry, I do not have your enthalpy number handy.
I would like to make it from the difference between enthalpy of the solid 
and enthalpy of the vapor,
but if FeCl3 has a "dissociation pressure of Cl2" greater than one atmosphere,
then the enthalpy of the vapor will not be a commonly available number,
will not be part of  the standard thermodynamic data for binary compound 
species.
You'd have to track the energy changes FeCl3(s) -> FeCl2(s) -> FeCL2(g),
plus somehow find equilibrium data for FeCl2(g) + 1/2Cl2(g) <--> FeCl3(g).

    I cannot tell from your short letter whether you know all this and 
 still need it,
or whether the number is actually meaningless to you because
in your practice you'll only have FeCl2 vapor not FeCL3 vapor.
You should try to find a dissociation pressure vs. temperature curve for 
FeCl3,
and think about that.
Maybe someone has done a study which is in some old journal.

Unfortunately I do not think any thermo data for FeCl3 will exist above 315C.
There the FeCl3 (L) phase terminates, becoming not FeCl3(g) but FeCl2(s).
All this because the "standard conditions" assume a partial-pressure limit 
of 1 atmosphere.

hoping I am not blowing smoke-
Jim Swenson
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