Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: aqualev on March 12, 2009, 03:02:53 PM
-
How does a simple and fractional distillation graph tell me about the purity of my substance?
Fractional distillation:
^temp
|..____... 110C, (bp of toluene), temp constant. volume increasing.
|.|....this section is temp rising and no volume collected
|.|
|.|...
|___|. 81C..(bp of cyclohexane)
|.
------------------> volume
Simple distillation:
^temp
|...____... this would be 110C (toluene)
|../... this is temp rising as remaining cyclohexane is boiling off
|__/.|.
------------------> volume
I can see that with simple distillation the temperature rises gradually and with fractional distillation our temperature levels off. But how does this tell me anything about the purity of my product? in other words, what does purity have to do with a sharp temperature rise or gradual rise?
-
OK, the fractional distillation has a sharp transition between two things boiling off, and the simple distillation has a gradual temperature rise, with some boiling happening at all times. Is that what you observed? Can you make some predictions on the purity?
-
From what I understand a pure substance boils at a steady temperature. the first 10 ml from my fractional distillation was collected during a fairly stable temperature range (73 C) but than began increasing steadily to 100 C at 20 mL. since the higher boiling point was closer to that of toluene, I'd assume that my boiling flask contained mostly toluene (correct me if I'm wrong) but I'm not sure exactly what my condensed distillate would be.
The simple distillation, on the other hand, boiled first at 82 C and went up gradually to 100 C. since the temperate wasn't constant I'd assume that the separation was not the best. But I'm still confused about this whole process.