October 1, 2001
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.
Raoults 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. Daltons 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.
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.
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 was burned, no water droplets remained on the watch glass.
From this experiment, it is obvious that the more efficient way to collect ethanol from an ethanol/water mixture is through fractional distillation. The packed copper wool provided much surface area for the mixture to condense and evaporate on. Each evaporation caused the vapor to be more highly concentrated in ethanol.
A 50% ethanol/water mixture will burn. Water will not burn. So the droplets that remained after the simple distillation were greater than 50% water. The lack of droplets after fractional distillation does not mean that I collected a pure sample of ethanol. This can not be done since the azeotrope of ethanol is 95% ethanol/5% water. The sample that I collected was simply greater than 50% ethanol, because anything greater than this would burn.
1.The boiling point of pure ethanol is 78.3 C, and the boiling point of pure water is 100 C. The boiling point that I observed was actually the not the boiling point of the liquid but the temperature of the vapor at the tip of the thermometer. However, it is known that the boiling point of a 10-20% ethanol/water mixture is around 85 C, somewhere between the boiling points of the two pure liquids.
4.The thermometer stayed around 80 C when most of the sample was collected. However, there was some volume collected below and above 80 C. This is a sign of impurity. A constant boiling point does not necessarily mean one has a pure substance. A sample of 2 liquids that each have the same boiling 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 doesnt 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.