Organic Chemistry

Distillation

Andrea Pausma

October 1, 2001

I. Introduction

The process of distillation has been used by humans for years to create alcoholic beverages. Distillation is the process of boiling a pair of liquids with different boiling points and then condensing the vapors above the boiling liquid in an attempt to separate them. One might suspect that the mixed two liquids of different boiling points could be separated simply by raising the temperature to the lower boiling point of the two liquids. However, this is not the case. The two liquids "boil" together at some temperature between their two boiling points.

Raoult's law states that the vapor pressure of one liquid is equal to the product of the vapor pressure of the pure liquid and the mole fraction of that liquid in the liquid. The total vapor pressure is simply the sum of the partial pressures of the two liquid components. Dalton's law states that the mole fraction of one liquid in the vapor is equal to the partial pressure of the liquid divided by the total pressure. These laws can help explain the process of fractional distillation.

When a mixture of ethanol and water is heated, it will boil at a temperature between 78.3 C (the boiling point of pure ethanol) and 100 C (the boiling point of pure water). In fractional distillation, the vapor will condense on a surface. The condensate will then evaporate again and then condense on another surface. This process will continue until the percentage of ethanol in the mixture continues to get larger as the percentage of water decreases. The more "surfaces" that the vapor settles on, the higher percentage of ethanol one will collect. However, one will never collect pure ethanol. Ethanol and water form an azeotrope at 78.15 C. An azeoptrope is a mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition. The azeoptrope of ethanol and water will be 95% ethanol and 5% water.

The purpose of this experiment is to compare the processes of distillation and fractional distillation to discover which procedure enables a more pure sample of ethanol to be collected from an ethanol/water mixture.

II. Procedure

For simple distillation, I added 4 mL of a 10-20% ethanol-water mixture to a 5 mL round-bottomed long-necked flask. I joined the flask to a distilling head fitted with a thermometer through a rubber connector. The thermometer bulb was lowered below the side arm of the distilling head. A 30-mL beaker was put below the distilling head. The entire apparatus was put in a heated sand bath. The apparatus was closely monitored during boiling so that the distillation rate was no faster than 2 drops per minute.

After a couple milliliters of sample were collected, the sample was poured onto a watch glass. The sample was then lit by a match to test how much of it burned. Any remaining sample was washed down the drain.

For fractional distillation, I added 4 mL of a 10-20% ethanol-water mixture to a 5-mL short-necked flask. I joined the flask to a distilling column that was packed with thickly with some steel wool. This was then joined to a distilling head with a thermometer through a rubber connector. The procedure was the same as for the simple distillation. A representative sample was then burned and the results of the fractional and simple distillation were compared. Any remaining sample was washed down the drain.

III. Results

During the simple distillation, the sample started boiling around 74 C. The temperature continued to rise, and at 90 C, the heating was terminated to prevent excess water from entering the beaker. After the representative sample was burned, about 4 small droplets remained on the watch glass.

During the fractional distillation, the sample started boiling around 74 C. The temperature rose until 80 C. It remained at this temperature for awhile as much of the sample was evaporating and condensing on the copper wool. At 90 C, the heating was terminated to prevent excess water from evaporating. After the representative sample