Boiling Point Composition Curve Interpretation

The best way to understand what is taking place in a distillation is by looking at a plot called a boiling point composition curve.

A boiling point composition curve of a two-component mixture has temperature on the y-axis and mole percent of one of the components on the x-axis. The lowest curve on a boiling point composition diagram is called the liquid composition line. This line tells you the mole percent of the compounds when they are in the liquid phase at a given temperature. The highest curve is called the vapor composition line. This line tells you the mole percent of the compounds when they are in the vapor phase at a given temperature.

Realize the liquid and vapor compositions are different at different temperatures because some compounds are converted into gases easier than others; they have lower boiling points. The vapor will always be enriched in the lower boiling compound.

The boiling point composition curve of a mixture of cyclohexane and toluene is shown below.

To use a boiling point composition curve, you first need to determine the mole percent of one of your compounds in the two-component mixture.

In the example above, you would determine the mole percent cyclohexane in your mixture. Mole percent is calculated by taking the moles of one substance divided by the total moles in the solution (the moles of both substances) x 100%. This number tells you where to start on the x-axis of the boiling point composition curve.

When you perform a simple distillation, you are heating the mixture, so the temperature increases. The temperature will increase until it hits the liquid composition line. This is the temperature at which the mixture will begin to boil.

Realize that these plots are made assuming an atmospheric pressure of 760 mm Hg. When a substance begins to boil, the temperature remains constant; the liquid is turning into a gas. To visualize this on the boiling point composition curve, you trace a horizontal line across from the liquid composition curve to the vapor composition curve. If you trace down from where your horizontal line intersects the vapor composition line, you can determine the mole percent of the compound in the vapor phase at that temperature.

Once again using the cyclohexane/toluene example, if you start with a 50 mole percent solution of cyclohexane in toluene, you can see from the boiling point composition curve that the liquid will boil at around 91 ^oC and the composition of the vapor will be approximately 80% cyclohexane. The picture below represents one theoretical plate – one vaporization-condensation cycle.