Distillation

Distillation

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...

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...iling points will have a constant boiling point.

6. When water is distilled, it does not vaporize all at once when the boiling point is reached. When some water molecules evaporate, the kinetic energy of the remaining liquid goes down and the temperature drops slightly. As a result, the rest of the water needs to be heated again before more molecules of water evaporate. A constant source of heat is needed.

8. It is dangerous to carry out a distillation in a closed apparatus because vapor takes up more space than a liquid. So in a closed apparatus, the vapor pressure would build up, and the apparatus would explode.

9. Slower distillation results in better separation of liquids, because time is needed for the liquids to meet the vapors in the distillation/condensation process. If this is done too fast, then the vapor doesn’t go through as many distillation/condensation cycles, and the final sample will not be separated into its too components as much.

10. A packed fractionating column is more efficient than an unpacked one because a packed column provides more surface area for the vapor to condense on. The more it condenses, the more efficient the separation of the liquids.