Boiling toluene/water azeotrope mixture?

[djh101]

Can somebody explain what happened in this experiment? The mixture of toluene/water was heated to its boiling point (without a condenser) of about 84C. Over the course of under a few minutes, the boiling point increased rapidly and then remained constant at about 96C (more or less- it jumped up and down a lot but stayed in the same range).

The constant final temperature is because the toluene was all vaporized and just water remained, correct? But since the mixture was an azeotrope mixture, shouldn't the concentration have remained constant (and the toluene not completely vaporized before the water)? Was this supposed to happen or was it probably due to measurement error when preparing the mixture?

 

[chemisttree]

Were you measuring the temperature of the pot or of the vapors?

 

[djh101]

Temperature was measured of the liquid.

 

[chemisttree]

Then that's your problem. Azeotropes are measured under equilibrium conditions, best approximated as the temperature of the condensate in your system. The pot was not in thermal equilibrium with the vapors so your data wanders around a bit. Was the bulb of the thermomer resting on the bottom of the flask... where the temperature is at it's highest?

 

[djh101]

Well the problem is that it jumped to 96C (near the temperature of water) instead of staying at 85C (the given BP of a toluene-water azeotrope mixture). And yes, the thermometer was near the bottom. It was a microscale lab, though (3mL total solution), and if I had to guess I would think the most likely cause of an abnormally high temperature would be bad measurements (but were it not azeotropic, would the solution still reach a non-azeotropic constant temperature?).

 

[chemisttree]

Toluene/water is a two phase system. You were measuring boiling in the water phase.

 

[djh101]

That makes sense. Thank you.