Simple Distillation

As a liquid is heated, it gains energy until the increased disorder of the gas phase outweighs the intermolecular interactions in the liquid phase. Once enough molecules are in the gas phase, they escape the liquid in the form of bubbles. This effect, called boiling, occurs when the total vapor pressure of the substance is equal to the atmospheric pressure.

Vapor pressure is the pressure of the vapor in equilibrium with its condensed phase, and it varies with temperature. In a mixture of liquids, each component has its own vapor pressure, which we call its partial pressure. The total vapor pressure of the mixture is equal to the sum of the partial pressures. If the liquids are miscible, meaning that they always form a homogeneous solution, the partial pressure of each component is the vapor pressure of the pure compound at that temperature times its mole fraction in the liquid.

The temperature at which the first bubble of vapor starts to form in a liquid mixture is called the bubble point. For a pure liquid, both the bubble point and the temperature at which vapor starts to condense, or the dew point, are the same as the boiling point. However, for a mixture of two miscible liquids, both the bubble point and the dew point will be between the boiling points of the components.

When the mixture first boils, the vapor is rich with the compound with the lower boiling point, or the more volatile compound. This increases the proportion of the compound with the higher boiling point, or the less volatile compound, in the liquid mixture.

Distillation is a separation technique that takes advantage of this phenomenon. In a simple distillation, a homogeneous liquid mixture is boiled. The rising vapor then enters the inner chamber of a water-cooled condenser. The vapor condenses to a liquid, called the distillate, which is then collected in a separate vessel.

As the boiling continues, the compositions of the liquid and the vapor change as the more volatile component is removed. So, if we collect the distillate in small fractions, we'll see that each fraction contains the compounds in a different molar ratio.

As the proportion of the less volatile component in the liquid mixture increases, so do the bubble point and dew point. Plotting the mixture's bubble and dew points versus the mole fractions of the components makes a boiling point diagram. Once we have this diagram, we can use the dew point curve to determine the composition of the vapor at a given temperature.

In this lab, you will set up and perform the simple distillation of a mixture of cyclohexane and toluene and record the temperature of the vapor throughout the experiment. You'll then use the published boiling point diagram for cyclohexane and toluene to determine the composition of the vapor, allowing you to estimate the composition of the liquid mixture throughout the distillation.