Steam Distillation

Lief · Mon Mar 28, 2005 4:42 am

Steam Distillation. Distillation of a Pair of Immiscible Liquids.
Elementary Practical Organic Chemistry Part I Small Scale Preparation
by Arthur I. Vogel; pages 15-20

http://rapidshare.de/files/1029351/SteamDistillation.djvu.html

Lief · Mon Mar 28, 2005 6:40 am

Distillation with water:

http://rapidshare.de/files/1030304/DistillationWater.pdf.html

IndoleAmine · Dreamreader Deluxe

Interesting thing you dug out there, Lief..

- I thought about this one a bit...

- and came to strange conclusions.

(ready for a real long brain-twister? Very Happy

Here we go...)

To explain what I realized/discovered, first lemme state here that in my understanding

a) the sum of the partial vapor pressures of two immiscible compnds. equals the total vapor pressure or "pressure of the whole system", and the liquid boils only if the sum of all partial vapor pressures at least equals the outer pressure, or is greater

b)the outer pressure is reversely proportional to the inner pressure, meaning more outer pressure can be considered less inner pressure (or commonly "a stronger vacuum"), which always causes lower bp, and therefore

c) the amount of both compnds. in the total amnt of vapor at various temps. is directly proportional to the ratio between the partial vapor pressures of the two immiscible compounds, since they both have the same amnt. of molecules per liter when being gaseous (-> ideal gas law; all gaseous substances have the same molar density at the same temp.)

Now secondly, I want to remark that the shown table indicates that at a temp. somewhere between 95-100°C, the distillate will contain slightly more than 18.61199 percent of bromobenzene by moles - this would be the bromobenzene content at 751mmHg ( the point where this happens is exactly where the sum of the partial pressures equals 761mmHg, to be precise Wink

) ...

118mmHg / 634mmHg = 0.1861199 or ~18.612 molar percent bromobenzene/water

Now assuming that lower total pressure (or vacuum) will result in lower bp of the azeotropic mixture (this most certainly will always the case Wink

), one can safely assume that a vacuum of 38mmHg will result in the bromobenzene/water mixture boiling at 30°C according to the table, no?

Interestingly, I just calculated the theoretical bromobenzene content in the vapor phase at the given total vapor pressures/boiling points, and came up with strange results....

bromobenzene content in vapor phase at various temperatures; given in molar percent/l

30°C 18.75
40°C 18.18182
50°C 18.47826
60°C 18.79195
70°C 18.88412
80°C 18.59155
90°C 18.66667
95°C 18.61199
100°C 18.55263

Thats everything else but a linear rise/decrease in bromobenzene content, would I say (not even a logarithmic one by any means) - but it should be, assuming therer is ANY regularity behind the vapor composition of immiscible substances..

(IF they would follow ANY rule, the concentration of bromobenzene would either rise or fall with temp., logarithmic or linear depending on how fast the single vapor pressures of bromobenzene and water change with temp., or better said: depending on how steep their individual deltaH evporation constant graph is....)

this means that Vogel's is wrong here, either in giving false values (maybe rounded?), or... Shocked

...one of the rules about vapor pressures of immiscible liquids at various temps. is simply - WRONG...... Confused

(dunno which one tho Sad

- you hear me Nicodem? Very Happy

)

i_a

IndoleAmine · Dreamreader Deluxe

Also please note the difference between "vapor pressure" and "pressure": they work adversely (vapor pushing up and pressure pushing vapor back), and if teh vapor pressure is bigger a substance will evaporate, and if the inner, atmospherical pressure is bigger it won't - maybe I didn't express it clearly enough... Wink

i_a

Lief · Thu Mar 31, 2005 4:33 am

When the vapor pressure of the liquid you are distilling equals the atmospheric pressure then the liquid boils.

Dalton's law and Raoult's law seems to apply here, too.

for example:

http://library.thinkquest.org/12596/dalton.html

http://www.wwnorton.com/chemistry/concepts/chapter12/ch12_6.htm