EXPERIMENT 7 - Distillation – Separation of a Mixture
a) To purify a compound by separating it from a non-volatile or less-volatile material. b) To separate a mixture of two miscible liquids (liquids that mix in all proportions) with different boiling points. Equipment / Materials: large test tubes(3) clamp (1 or 2) ringstand boiling chips test tube rack (1) heating mantle glass adaptor thermometer adaptor 10-mL graduated cylinder condenser (1 or 2) grease 50- mL round bottom flask 50- mL round bottom flask thermometer rubber tubing (2) unknowns ( A and B)
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 their original phase until the most volatile substance has all boiled off. Only then does the temperature of the gas phase rises again until it reaches the boiling point of a second component in the mixture, and so on. The boiling point of a substance—determined 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. The vapor pressure of a substance roughly doubles for every increase in 10 °C, Figure 1. When the vapor pressure of the liquid 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). The boiling point of a liquid is a measure of its volatility. 1
CHEM 2423 Distillation of a Mixture
Fig 1. Vapor pressure dependence on temperature for water The successful application of distillation techniques depends on several factors. These include the difference in vapor pressure (related to the difference in the boiling points) of the components present, the size of the sample, and the distillation apparatus. Distillation relies on the fact that the vapor above a liquid mixture is richer in the more volatile component in the liquid, the composition being controlled by Raoult’s law: In an ideal solution the partial pressure (PA) of component A at a given temperature is equal to the vapor pressure PoA of pure A multiplied by the mole fraction of A (XA) in solution. Consider an ideal solution of A and B: XA = nA/ (nA + nB) , XB = nB/ (nA + nB) and XA + XB = 1 nA and nB represent the number of moles of components A and B. PA = XAPoA PB = XBPoB and PT (total vapor pressure) = PA + PB
This relationship, derived from Raoult’s law, is capable of describing the boiling point behavior of compound A in a mixture of compounds under a variety of different circumstances. The boiling point of the solution is reached when PT is equal to the pressure applied to the surface of the solution. There two major types of distillation are to be considered: Simple Distillation - used frequently in the organic chemistry teaching labs, Figure 3. It is often considered when: a) the liquid is relatively pure to begin with...
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