1· It is necessary that you add boiling stones to the distillation flask. Explai

Question

1· It is necessary that you add boiling stones to the distillation flask. Explain why. Boiling points in the labs at WVU tend to be slightly lower than expected. Why do you think this is? 2. When doing a vacuum distillation, it is important that you apply the vacuum first and then begin heating. Why would it be problematic to heat first and then apply the vacuum? 3. Use the interactive nomograph (link above) to determine the following: a) 4. Boiling Point of dimethylsulfoxide (bp 189 °C at 760 mmHg) at 0.4 mmHg. b) Reduced pressure necessary to lower the boiling point of HMPA (bp 233 °C at 760 mmHg) to 100 °C Look up the physical data for each of the following compounds and determine whether crystallization or distillation would be better suited for purification. All compounds can be found at www.sigmaaldrich.com 5. Experiment 5-Distillation Pg. 8

Explanation / Answer

1) Boiling stones or boiling chips must always be added while performing distillation of a compound for two reasons:

a) Boiling stones offer a more controlled boiling and prevents the liquid from bumping into the condenser. Sever bumping of the liquid inside the condenser may cause accidents.

b) Boiling stones do not allow superheating of the liquid. Superheating is a phenomenon where a liquid boils beyond its boiling temperature without conversion of the liquid into vapour.

2) I will say that the only thing that can lower boiling point is a lower pressure. Possibly while performing the experiment, the atmospheric pressure was lower than 1 atmosphere and thus the observed boiling point was slightly lower than the expected value.

3) Vacuum distillation works on the principle that the boiling point of a compound (usually vacuum distillation is used for compounds with very high boiling points) is significantly lowered when the vapour pressure above the compound is lowered. Hence, it is imperative that the pressure inside the distillation flask is lowered first and then the mixture is heated. This lowers the boiling point of the highest boiling compound in the mixture and results in efficient separation.

If we attempt to perform vacuum distillation the other way round, i.e, heat first and then apply vacuum, then think about the problems? First, the temperature has to be raised significantly higher (i.e, you have to heat at a very high temperature) which can cause decomposition of the compound. This heating must be performed under atmospheric pressure or higher. Second, if you suddenly lower the pressure from the atmospheric pressure to a lower value, then the flask may crack leading to a severe accident. Remember that this distillation flask contains heated liquid at a very high temperature. Hence, in vacuum distillation, we must always lower the pressure and then heat the mixture.

4) I do not have access to the link (you didn’t copy-paste the same); I am sorry I cannot look up at it.

5) (a) Toluene can be separated by distillation from a mixture of toluene (bp 110-111°C) with another liquid that readily dissolves toluene.

(b) Acetonitrile (bp 81-82°C) can be separated from a mixture of acetonitrile and water (bp 100°C) by distillation.

(c) Anthracene is a white powder (bp 340°C) and is most easily separated by crystallization (or recrystallization – the technique is also used to purify organic solids). Anthracene can be easily crystallized from DMF.

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