exp 2

Distillation

One of the more common methods by which a mixture of two liquids (or, a binary mixture) can be separated in organic chemistry is using the process of distillation. This is based upon the theory that since two different liquids have two different boiling points, the one with the lower boiling point (molecule “A”) will evaporate into the gas phase before the other (molecule “B”).  When it evaporates into the gas phase, it can be condensed into a purified liquid called the distillate in a separate container, as illustrated below.

Figure 2.1 Simple distillation

Distillation Set-Up

In a distillation, the mixture of liquids is heated to a boil in a round-bottom flask.  To prevent “bumping” (when a large amount of liquid evaporates, it forms a large pocket of gas which results in vigorous bubbling which can make the flask move), boiling stones or a stir bar is used in the round-bottomed flask.  The evaporated gas ascends, passes the thermometer (positioned in the center of the 3-way connecter), and then travels to the center of the condenser, where the gas condenses back into a liquid.  Tap water flows through the outer portion of the condenser to help cool off the gases inside of it.  The curved connecter directs the flow of the liquid into the container that is set up to receive the distillate.  The container for Exp. 2 is a graduated cylinder, but can also be another round-bottomed flask.

Distillation Graphed: The Liquid-Gas Phase Boundary

In practice, this process of separation by evaporation is not perfect, especially when the boiling points differ by less than 75 °C, as some of the molecules of liquid “B” will evaporate at a temperature below its boiling point, and some of the molecules of liquid “A” will not evaporate and remain behind in the original mixture of liquids “A” and “B”.  In addition, the greater the attraction between the two molecules in the two liquids, the more likely some of molecule “A” (with the lower boiling point) will remain in the mixture with molecule “B”. 

For example, say that there is originally a liquid mixture of ethanol (boiling point: 78.5 °C) and water (boiling point: 100 °C), which has a 1:1 mol:mol composition (n represents the number of mol): 

Using Figure 2.2, when this mixture (Xethanol = 0.5) and it is heated to a boil (i.e., until the liquid becomes a gas), it has a boiling point of 82.2 °C.  At this temperature, the gas composition is 77% ethanol and 23% water by mol, which, when the gas condenses, will be the composition of the distillate.

 Figure 2.2 Temperature and Composition Graph for Distillation

If the process of distillation is repeated with the purified liquid, this results in further purification.  For example, in the ethanol-water mixture mentioned above, after the first distillation, the distillate is 77% ethanol by mol, which is closer to 100%, but not yet purified.  A subsequent distillation with a mixture at 77% (illustrated by points L2 and G2 in Figure 2.3) results in 85%; a third results in 88% (reference points L3 and G3).

 Figure 2.3 Temperature and Composition Graph Illustrating Sequential Distillations

Fractional Distillation

However, conducting each sequential distillation does take time, and comes with a risk of losing material.  The process of fractional distillation can be used to improve the separation since it allows for many small distillation steps.  Fractional distillation in an organic chemistry lab typically involves the use of a Vigreux column that has many internal “prongs” where the gas can condense and re-evaporate on the way up before reaching the final condensing step. 

Figure 2.4 Process of Fractional Distillation

When graphing the temperature of the still versus the volume of the distillate, a good separation will plateau for awhile around the boiling point of each liquid, while a poor separation will increase in temperature at the same rate for the entire length of the distillation (no fractionation between liquids).  Refer to Figure 2.5 below.

Figure 2.5: Good Fractional Distillation (left) versus a Poor Distillation (right)

Fractional distillation is a common practice used in industry for many things, one being to separate gasoline from crude oil.

Azeotropes

In certain cases, two liquids are inseparable at a certain point using the distillation technique.  This is true for water and ethanol at mol fractions greater than about 0.95 ethanol (95%).  Using the graph in Figure 2.3 above, it can be seen that the liquid phase composition is essentially identical to the gas phase composition at this point (the azeotrope’s boiling point is 78.2 °C).  When the compositions are identical, it called an azeotrope, and is impossible to separate the two liquids.  Ethanol that is 100% must be prepared by another process, which is not one that will be utilized in this experiment.  

Thus, the greatest separation that can take place is to 95% purity for ethanol.  The purity of the distillate will be measured using density at the end of the experiment.  

Distillation Glassware

The glassware used in a distillation is special because each piece has a ground glass section at the ends (“joint”) that is used to connect to the other pieces of glassware.  This ground glass joint effectively seals the glassware components together.  Frequently, however, it seals too well, so joint grease is used to help keep the pieces from locking together. 

The glassware components for the distillation apparatus are many (and costly!).  Be careful when assembling.  Use Keck clips ACROSS the joint to secure joints that are away from the heat source.  Important joints to use clips on are the horizontal ones (i.e., either side of the condenser) since gravity does not help to secure them.

Learning Goals

  • Understand the theory behind distillation and fractional distillation.

  • Learn how to set up a fractional distillation apparatus. 

  • Understand azeotropes.

  • Use density and temperature measurements to determine purity.