CHEM*2700: Experiment #2

Fractional distillation of an acetone/toluene mixture

Important:

Ceramic heating mantle

This heating mantle is our heat source for the upcoming distillation. Notice that there is no ‘ON/OFF’ switch, nor is there an additional way to control ‘heat’. Once this is plugged into a wall outlet, it’s ‘ON’, and will quickly heat up to its maximum operating temperature. We’ll be using a similar model shown, which allows for operation at up to 550 ˚C easily. So don’t plug it directly into the wall outlet.

Variac (Variable A.C.)

Instead, we’ll plug our heating mantle into a Variac (variable autotransformer), which will then be plugged into the wall outlet. This allows us to to smoothly select a variable voltage, anywhere from 0 v to 120 V, at which the heating mantle can operate. In short, by modulating the voltage we can control the heat, at which the heating mantle operates.

Distillation apparatus:

• Start assembly from the bottom. The picture in page 38 is unclear, but the stir plate rests directly on the lab bench, and from there the rest of the apparatus is supported by it.
• Ideally, we will only need to use two clamps (as opposed to three, shown on page 38). One will be used to hold the round bottom flask in place and another to hold the collecting flask. Here’s a sample set up, with the stir plate and heating mantle excluded.

Sample distillation set-up

Note:

• A clamp is placed on the neck of the round bottom flask and another for the receiving flask.
• The incoming water hose is placed on the lower inlet to prevent an air bubble from forming.
• The thermometer bulb is positioned by the entrance port of the still-head for accurate vapour temperature readings.
• Don’t forget to add a boiling chip into your round bottom flask (this will prevent the boiling solution from getting too ‘violent’).

Temperature vs. Time

The different segments of an ideal separation of a two-component fractional distillation

Ideally, we’ll have a graph that looks like this.

(1) Here we’ll see the initial heating period.

(2) Here the vapours are making their way to the still-head as we increasingly heat the solution.

(3) As we appraoch the boiling point of acetone (56 ˚C), we should expect a plateau, since temperature stays constant during a phase change (i.e. acetone boiling from liquid to gas, and condensing from gas to liquid again.)

(4) Once the temperature begins to rise again, we should switch to a new collecting flask. This indicates that a large proportion of the acetone has been removed.

(5) As we approach the boiling point of tolune (110 ˚C) we should expect another plateau to indicate a phase change is occurring. At this point, we should switch to a new collection flask.