Fractional Distillation of an Ethanol-Water Mixture objectives Learn how to buil

Question

Fractional Distillation of an Ethanol-Water Mixture objectives Learn how to build a fractional distillation apparatus Carry out a fractional distillation procedure using an aqueous ethanol solution. Determine whether a sample of the ethanol-water azeotrope has been obtained by evaluating the density of the distillate. Reference Material The PowerPoint presentation for this experiment is provided on eCampus. Zubrick: pp 164-170; 178-183. The distillation will be carried out using an apparatus similar to that depicted in Figure 19.10 on page 181 Also see setup for simple distillation (Figure 19.1; page 166) for comparison. There are several types of fractionating columns in common laboratory use; we will be using a Vigreux column. Procedure The apparatus for fractional distillation is assembled using a 250 mL round-bottom flask with thermometer sidearm and 100 ml of 20% ethanol solution along with a few boiling chips. A round clamp on the neck of the flask is used to hold it securely in place so that no support is required from below. A Lab Jack in an extended position holds a large Thermowell under the flask. A thermometer extends into the distillation head and another is placed into the solution via the sidearm of the flask. The Vigreur column should be perfectly vertical. Water to the condenser is carefully turned on to create a low flow of water from the outlet tube and the Powerstat is tumed on at approximately 50% of full voltage (35% for Powermites) to heat the ethanol solution. Meanwhile, about 10 receiving test tubes are numbered and arranged in order in a small test tube rack. The tubes are calibrated at 1.5 and 3.0 mL by addition of carefully measured amounts of water and indicating those volumes on the tubes with a marker. Additional tubes may be required as the distillation proceeds. For recording data in the notebook, a table is used with the following headings: tube number, olume of distillate (0 mln 1.5 mL, 3.0 ml, etc.), pot temperature and head temperature 0 mL" is taken when the first drop of distillate is obtained. The reading at to be used is held in an Erlenmeyer flask and its position is adjusted so that The receiving tube the outlet of the curved adapter is just inside the tube. Once the mixture boils and vapors are seen

Explanation / Answer

1.

Boiling chips are used in the distillation flask for two reasons: they will prevent superheating of the liquid being distilled and they will cause a more controlled boil, eliminating the possibility that the liquid in the distillation flask will bump into the condenser.

2.

A Vigreux column is a glass distillation column used from fractional distillation. The inside of the column contains many rows of downward facing 'spikes' that condenses the distilling vapor as it passes through the column. This column is designed to face upwards so that the condensate falls back into the heated flask. The repeated vaporization and condensation processes that occurs as the condensate passes up the column allows for a more accurate distillation of mixtures by narrowing the boiling point window of condensate that makes it to the top of the column. This is the basic priciple behind fractional distillation and how the Vigreux column works.

3.

Volatile impurities are the first compounds to distil. This first fraction, called the fore-run, is generally collected separately. When the temperature has risen to the desired level and has been recorded, place a fresh receiver under the adapter to collect the main fraction. Hence to get pure compound (fractions), fractions are collected in multiple tubes instead of collecting the distilate in a single receiving vessel.

4.

Simple distillation would have been a worse choice. The reason that fractional distillation gives better separation between the liquids is because the glass beads in the fractionating column provide "theoretical plates" on which the refluxing liquid can condense, re-evaporate, and condense again, essentially distilling the compound over and over.

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