1. (3 pts) During fractional distillation, liquid can be seen dripping from the

Question

1. (3 pts)  During fractional distillation, liquid can be seen dripping from the fractionating column back into the distilling flask. What effect does this returning condensate have on the composition of the vapor? On the composition of the liquid in the distilling flask?
  
2. (2pts) Based upon your answer to the above question, tell me why better results are obtained when fractional distillation is carried out slowly...collecting only 10 drops or so a minute? Use the proper terminology.

3. (3 pts) In an Org II lab experiment the procedure calls for distilling the liquid product in order to isolate it from the reaction solvent. The difference in their boiling points is about 75°C, with the solvent having the lower boiling point. As the lower boiling component condenses on the thermometer bulb, the temp shoots up to 64°C and distillate is collected. This continues for a while within a 64-66°C range before no additional distillate is noted. At this point, the thermometer reading actually begins to drop! Sometimes it goes as low as 45-50°C, despite the fact that the higher boiling component is still in the distillation flask. Many students panic....sure that something is drastically wrong. The TAs will counsel them to be patient. Sure enough, after a period of continued heating, vapor again condenses on the thermometer bulb and the temp shoots up to well above 125°C for the next fraction. Can you explain why it wasn’t necessary to panic? Why did the temp drop for a short period of time between fractions?

4. (2 pts) This quirky business of azeotropes may be a pain in the neck for corporate distilleries, but there are times when chemists... and even bootleggers...could find it quite useful. Give an example of how an azeotrope could be advantageous.

Explanation / Answer

1. during fractional distillation, liquid drops back into the distilling flask. This happens as the components separates at wach of the fractionating column end to get purer fraction of one component (lower boiling) into the vapor phase and other component (higher biling) back into th flask. The composition of vapor becomes pure and that os liquid becomes richer in higher boiling fraction.

2. A slow distillation ensures better separation of the components of the liquid during distillation and thus is preferred more.

3. During distillation of solvent (low boiling) from higher boiling compound, the temperature drops after all the solvent is collected as the difference in boiling point for th hgher component is large. It takes additional heat (higher temperature) to heat the remaining distillate in the flask before the temperature rises again. This is the general principle.

4. When doing a reaction where we get water as a side product, one may run the reaction in say toluene and the water formed can easily be removed while the reaction is running by formation of azeotrope on a side nexk (dean-stark apparatus) and collecting the water when the system cools down. This method helps the reaction to not proceed in the reverse direction due to the water generated.

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