uemura (Hive Bee)
11-28-01 13:11
No 241585
      Boiling Point Measurement  Bookmark   

This time some small questions for the experts in physical chemistry. Say you are destilling a substance under normal atmospheric pressure and you perform two measurements:

1) Temperature inside the boiling liquid in the destillation flask. Let's say you get 263 DegC. You have enough liquid in the flask and you have heated up very carefully to avoid overheating and the destillation is going on in a settled state. 

2) Temperature of the vapour before entering the cooler (using the thermometer in the standard bridge from flask to the Liebig cooler). Say you measure 250DegC.

Q1: Is it correct that the temp of a boiling liquid defines the boiling point of the substance?
Q2: Where is the correct 'entry' to measure the boiling point.
Q3: How can there bee a temperature difference of 13 DegC supposing that both sensors have accuraccies within 1-2 DegC?

Thanks for attention
Carpe Diem
 
 
 
 
    Rhodium
(Chief Bee)
11-28-01 14:47
No 241603
      Re: Boiling Point Measurement  Bookmark   

The vapor temp is the correct one. The reason the liquid temp may be higher is that it might be contaminated with higher-boiling substances.
 
 
 
 
    b159510
(Professional Student)
11-28-01 15:16
No 241607
      Re: Boiling Point Measurement  Bookmark   

I am no expert, but have completed college p-chem. You are distilling an impure liquid of mutually soluble chemicals? The pot temp will be higher than the head temp due to decreased vapor pressure in the flask. The vapor pressure at any temp will be reduced by a constant ammount if one of the chemicals is not volatile. If you are distilling a pure volatile liquid let me know and I will modify my response. I'll just say that the properties of molecules in the liquid phase differ from those in the vapor phase and additional energy is needed to overcome these forces. I like to say 'bubble pressure' so I'll throw that out there too....lol

Q1: Is it correct that the temp of a boiling liquid defines the boiling point of the substance?..
The boiling point is the temp at which the vapor pressure of the liquid is the same as the pressure pushing down on it.

Q2: Where is the correct 'entry' to measure the boiling point
At the point of equllibrium, where equal number of molecules are condensing and vaporizing. I think that would be the 'true' boiling point.

Q3: How can there bee a temperature difference of 13 DegC supposing that both sensors have accuraccies within 1-2 DegC?

I think I answered that but let me know if you need a more specific response, I'll see if I can help....later

Back to the Primitive
 
 
 
 
    lugh
(Hive Bee)
11-28-01 15:39
No 241610
      Re: Boiling Point Measurement  Bookmark   


How can there bee a temperature difference of 13 DegC supposing that both sensors have accuraccies within 1-2 DegC?




When matter changes state, heat is either given off or required as an input. This is how your refrigerator, engine in your car etc. works. In this particular case, you are dealing with the "heat of vaporization" for the compound(s) that you are distilling smile

 
 
 
 
    uemura
(Hive Bee)
11-28-01 15:51
No 241612
      Re: Boiling Point Measurement  Bookmark   

Thanks b159510! OK. Uemura should be more precise.

The question came up when he tried his first epoxide thermal rearrangment smile

The liquid in the flask was the epoxide which should get converted into the ketone. In this respect the liquid is indeed a mixture of epoxide, converted ketone and perhaps some side product.

What confused Uemura was the fact that his sensor in the destillation flask showed a temp of 263 degree (for, say 5min), the temp in the head was only 250. Now the anetholeketone has a literature boiling point of 260DegC not 250DegC. This is a bit confusing since the mercury thermometer in the head is accurate within 1-2 DegC. Are the 10Deg within an error range due to current atmospheric pressure, non-linearity of mercury thermometer (this one is from -20 to 270)?
Carpe Diem
 
 
 
 
    uemura
(Hive Bee)
11-28-01 15:56
No 241613
      Re: Boiling Point Measurement  Bookmark   

lugh,
thanks, uemura realises that the boiling liquid should be hotter than the vapour because heat=energy is consumed to push out the molecules. He just explained his confusion in the previous reply.
Carpe Diem
 
 
 
 
    Chromic
(Hive Addict)
11-28-01 16:37
No 241632
      Re: Boiling Point Measurement  Bookmark   

You measure the temperature at the still head just as everyone has said.. your thermal rearrangement is likely done by the time you hit 260C, you could just distill under vacuum?

The reason why the temperature of the liquid is higher is due to the distillation being a non-equilibrium process (and a high difference in Gibb's energy, or mixture fugacity of the components, or of chemical potential of the components -- three things saying the same thing) is required to give enough kinetic energy for the process to be run at an appreciable speed. Basically, the liquid has to be super-heated...

The other explanation, less mathematical/theoretical, is that the liquid is probably contaminated by miscible, non-volatile components.
 
 
 
 
    uemura
(Hive Bee)
11-28-01 16:52
No 241638
      Re: Boiling Point Measurement  Bookmark   

Hi Chromic,

likely you are right and the conversion took place. Uemura measured the liquid temperature to see if there is some kind of 'violent' rxn which is mentioned in the Hoering paper. There was none , once the liquid started boiling at 245DegC, it went up very smoothly during 30mins to 265DegC and at the same time the ketone? came over with a head bp between 240 and 250DegC.

Perhaps Uemura should put the destillate and the residue together, reflux for half an hour and then doing a vacuum destillation.
Carpe Diem
 
 
 
 
    b159510
(Professional Student)
11-28-01 17:21
No 241647
      Re: Boiling Point Measurement  Bookmark   

Your situation is a little more complex, being a reaction and not just a distillation of known chemicals and known molar fractions. I won't comment on the specifics of your situation, but- Are the 10Deg within an error range due to current atmospheric pressure...this is possible, even likely at high altitude, but you didn't say what altitude you are at.

non-linearity of mercury thermometer .. also possible, but such a large discrepancy is unlikely. However, if combined with the altitude question it may be the cause. I notice about a 8 °C lowering in BP at 1700 meters above sea level. That usually corresponds to an atmospheric pressure of 640 torr around here.

Back to the Primitive
 
 
 
 
    Chromic
(Hive Addict)
11-28-01 17:50
No 241658
      Re: Boiling Point Measurement  Bookmark   

I'm sure you formed the anethole ketone by the time you got the vapours to 260C. I heated until the vapours hit 240C and sure enough, anethole ketone was present, and the epoxide was gone.

Any word on asarone epoxide yet?
 
 
 
 
    uemura
(Hive Bee)
11-28-01 19:03
No 241671
      Re: Boiling Point Measurement  Bookmark   

b159510:
- Altitude is 800meter over sea,
- yesterday deep pressure system around. Did unfortunately not register the barometer reading Uemuras baro/vacuummeter provides.
The mercury thermometer seems to be accurate at least up to 130DegC because here Uemura has experiences with the destillation of substances with known bp. In the higher range this doesn't apply because normally he always destilles under reduced pressure.

Chromic:
you say your expoxide also started to boil around 240DegC. Have you seen any evidence for a 'heavy' or 'violent' first stage of the rearrangment or went it smoothly as with Uemuras run?
Asarone: Uemura is keen on it. However he would have to do the rearrangment of only 8ml in a 250ml flask (because of the electric heating mantle which is fully controllable) and in his destillation apparatus he normally also looses 1-2 ml of substance. He thinks more of using another 20ml asarone and carefully repeating the epoxidation at lower temp and destilling of the solvents ander reduced pressure to see if the yield gets better. Will report on it of course

Carpe Diem
 
 
 
 
    Chromic
(Hive Addict)
11-28-01 19:31
No 241674
      Re: Boiling Point Measurement  Bookmark   

It was a smooth increase in temperature, and the solution only boiled lightly as the temperature increased. Perhaps with a larger amount of epoxide it could be a noticably exothermic reaction, but in my experience it was not.
 
 
 
 
    uemura
(Hive Bee)
11-28-01 22:19
No 241735
      Re: Boiling Point Measurement  Bookmark   

Nice to hear that. BTW: neither vigorous happened for the asarone epoxide rearrangement. See Post 241734 (uemura: "Asarone Epoxide Rearrangment", Novel Discourse) smilesmile

Have a good night
 
 
 
 
    Osmium
(Stoni's sexual toy)
11-29-01 02:04
No 241832
      Re: Boiling Point Measurement  Bookmark   

Once you do the 1 mol batch size it will be exothermic.
In that case you can slow it down by doing the whole thing under a vacuum. The reflux will limit reaction temperature. The whole reaction will take longer, but might help in limiting decomposition.
 
 
 
 
    Chromic
(Hive Addict)
11-29-01 04:02
No 241879
      Re: Boiling Point Measurement  Bookmark   

When I ran the NMR, and it looked like there might have been some unconverted epoxide... it looks like I either needed to increase the reflux time (maybe to 30 minutes or so) or distill it as the reference says. The PMMA.HCl made from the ketone, had a perfect spectra (didn't ingest it).
 
 
 
 
    terbium
(Old P2P Cook)
11-29-01 07:21
No 241968
      Re: Boiling Point Measurement  Bookmark   

This is a bit confusing since the mercury thermometer in the head is accurate within 1-2 DegC. Are the 10Deg within an error range due to current atmospheric pressure, non-linearity of mercury thermometer (this one is from -20 to 270)?
Is this a partial immersion thermometer or a total immersion thermometer?
 
 
 
 
    uemura
(Hive Bee)
11-29-01 09:31
No 241983
      Re: Boiling Point Measurement  Bookmark   

Chromic

When I ran the NMR, and it looked like there might have been some unconverted epoxide



This would fit with Uemuras observation that a sample from destilled 'ketone' from the anethole epoxide did not react well with bisulfite, still lot of oily substance in.
BTW: Höring mentioned reflux but didn't say how long, in the Houben-Weyl they only say 'destillation'


Carpe Diem
 
 
 
 
    uemura
(Hive Bee)
11-29-01 09:36
No 241984
      Re: Boiling Point Measurement  Bookmark   

terbium,

Is this a partial immersion thermometer or a total immersion thermometer?



It is a classical lab thermometer to be mounted in the head of the destillation apparatus. Should therefore be a partial immersion thermometer. The sensor inside the flask is an electronic sensor from 0-450DegC from which only the first 10mm of the stainless-steel stick is the active part.


Carpe Diem