Fractional Distillation

Topics: Distillation, Evaporation, Water Pages: 6 (1567 words) Published: June 24, 2013
The experiment ‘Distillation’ was performed with the purpose of purifying liquids and determining boiling points in order to identify an unknown substance.

Distillation is one of the oldest and still most common methods for both the purification and the identification of organic liquids. It is a physical process used to separate chemicals from a mixture by the difference in how easily they vaporize. As the mixture is heated, the temperature rises until it reaches the temperature of the lowest boiling substance in the mixture, while the other components of the mixture remain in their original phase in the mixture. The resultant hot vapor passes into a condenser and is converted to the liquid, which is then collected in a receiver flask. The other components of the mixture remain in the their original phase until the most volatile substance has all boiled off. Only then does the temperature of the gas phase rise again until it reaches the boiling point of a second component in the mixture, and so on. The boiling point of a substance (determination by distillation) is a useful physical property for the characterization of pure compounds. At any given temperature a liquid is in equilibrium with its vapor. This equilibrium is described by the vapor pressure of the liquid. The vapor pressure is the pressure that the molecules at the surface of the liquid exert against the external pressure, which is usually the atmospheric pressure. The vapor pressure is a very sensitive function of temperature. It does not increase linearly, but in fact increases exponentially with temperature. When the vapor pressure equals the applied pressure, the liquid boils. Thus, the boiling point of a liquid is the temperature at which the vapor pressure equals the applied pressure. The normal boiling point of a liquid is the temperature at which the vapor pressure of a liquid equals atmospheric pressure (1 atm), so the boiling point of a liquid is a measure of its volatility. These concepts can be used to identify unknown substances.

* Long-necked flask
* Short-necked flask
* Cyclohexane
* Toluene
* Boiling chip
* Viton connector
* Thermometer
* Rubber connector
* Distilling head
* Receiving vial
* Beaker
* Ice bath
* Sand bath
* Heating plate
* Bosshead clamp holder
* Flask clamp
* Ring stand
* Metal spatula
* Copper sponge
* Pasteur pipette
* Cotton
* Syringe
* Distilling column
* Unknown #31


A simple distillation of a cyclohexane-toluene mixture was carried out by adding 2.0 mL of dry cyclohexane, and 2.0mL of dry toluene into a 5-mL long-necked, round-bottomed flask along with a boiling chip. The long-necked flask was connected to a distilling head with a rubber connector. Another rubber connecter was used on the other end to connect a thermometer so that the bulb was completely below the side arm of the Claisen head to ensure that the mercury reached the same temperature as the vapor that distilled. A Bosshead clamp holder held this part of the set-up over a sand bath that was heated by a heating plate. A receiving vial was set up so that the end of the distilling head dipped well down into the vial. The vial sat in a 50 mL beaker filled with ice that was held by a flask clamp on a separate ring stand. The neck of the distilling head was wrapped with cotton saturated with cold water. Once the set-up was completed, the heating plate was turned on and the long-necked flask was covered with sand. Once the solution started boiling, the temperature was recorded and the sand was scraped away from the flask so that the rate of the rising vapor was not too high. This was to ensure that the rate of distillation was no faster than 2 drops per minute. Then for every two drops of solution that dropped into the receiving vial, the temperature was recorded. This process continued until there was about 0.4 mL...
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