Experiment 3. Procedures.

Your T.A. will assign you and your partner to Part A or Part B.

Experimental Notes and Warnings

1. Before inserting a thermometer or glass tube into a stopper or rubber tube:
i) Be certain the hole is large enough to accommodate the glass.
ii) Lubricate the glass and the rubber with glycerol or stopcock grease.
iii) Protect your hands by holding the stopper and the glass in towels. Hold the stopper by your fingers NOT IN THE PALM OF YOUR HAND.
iv) Grasp the glass close to the end that is to fit into the stopper and twist with an even pressure.

Do not attempt to push or pull glass tubing or thermometers from rubber tubing, corks or stoppers which have become hardened. Cut the rubber or cork from the glass.

2. When starting the water flow through the condenser do not turn the tap on full blast! A gentle flow will suffice.

3. DO NOT allow the distillation to proceed to complete dryness - this is VERY DANGEROUS. Always leave 1-3 mL of liquid in the distillation "pot".

4. Do NOT plug the heating mantle directly into the mains - go through the rheostat on the rack above your bench.

Part A. Separation of a Binary Mixture by Simple Distillation at Atmospheric Pressure

PROCEDURE

Use the apparatus shown in Figure 3.5 (use your 100 mL graduated cylinder to collect the distillate, placing the mouth of the cylinder as far as possible into the receiver-adaptor) to distill a mixture of 25 mL of cyclohexane and 25 mL of p-xylene. Ensure that the top of the thermometer bulb is positioned midway in the opening to the condenser. Use a heating mantle supported on a ring clamp to heat the flask to which has been added a clay boiling chip or "Boileezer".¹ Make sure that the supporting ring clamp is above the level of the desk so that it may be released and the heating mantle removed quickly in the event of any dangerous overheating occurring. Also, be sure that the apparatus is not "closed" to atmosphere; otherwise pressure will build up and possibly cause an explosion.

Connect the mantle to one of the themostats on the bench, and Adjust the setting of the thermostat to control the heat during the distillation so that the distillate drips slowly and steadily into the receiver (about 1 drop per second). Use a graduated cylinder to collect the distillate. Record the temperature every 2 mL as the distillation proceeds until 45 mL of distillate are collected. (See below for how to write this up in your book.)

Please note: This experimental setup is special in that it uses a graduated cylinder to collect the distillate. This is not how distillations are normally carried out - one normally uses a roundbottom flask to collect with, so that loss of vapours is minimized.

The boiling aid which prevents "bumping" due to superheating, is an inert material with small pores which provide sites where bubbles can form, thus inducing even boiling. If, during a distillation, the temperature should drop below the boiling point of the liquid in the flask, liquid will fill the pores and the boiling aid will no longer be effective. In this event, the liquid can be cooled and a fresh chip cautiously added. The new chip should not be added when the liquid is at or near the boiling point, as this may initiate violent boiling.

Part B. Separation of a Binary Mixture by Simple Distillation at Atmospheric Pressure Using a Fractionating Column

PROCEDURE

Assemble the apparatus shown in Figure 3.6; again use your graduated cylinder to collect the distillate. (Only replace the steel wool in the condenser if it is severely rusted. Check with your T.A. if in doubt.)

Place a mixture containing 25 mL of each of cyclohexane and p-xylene in a 100 mL round-bottomed flask (dont forget some boiling chips!), connect it to the fractionating column (Fig. 3.6) and proceed as described above. [NOTE that you will need to start the distillation under fairly high heat, otherwise you will not finish on time.] Again, record the temperature every 2 mL as the distillation proceeds until 45 mL of distillate are collected. Be sure to increase the heating rate near the midpoint of the distillation otherwise the head temperature will drop. (Why?)

Recording of Results (both Parts A & B)

Construct a table in your notebook like that given below, to record the temperature at the distillation "head" as a function of volume distilled. You will record your data in both your and your partner's notebook simultaneously. He/she will do the same.

Volume distilled (mL)	2	4	6	8	10	12	14	16	18
Temperature without column
Temperature with column

As the table is being filled out, plot the boiling point vs. volume distilled for the distillation of the cyclohexane/p-xylene mixture with and without the fractionating column. Both sets of data will be plotted on the same graph, using different symbols. Label the two curves.

Note that no azeotrope is formed in this experiment. Also note that you are plotting temperature vs. volume not temperature vs. composition as is shown in Fig. 3.2. These plots are not expected to be directly related.

In your notebook, comment on the following:

On the basis of your results, which procedure was more efficient at separating the mixture into its components?

From your results what generalization can you formulate about the purity of a liquid as judged by its boiling point?

Questions you should be able to answer

1. Define the term boiling point. What effect would a reduction in the atmospheric pressure have on the boiling point of a liquid?

2. Why would it be dangerous to heat an organic compound in a distilling apparatus that was closed tightly at every joint and having no vent or opening to the atmosphere or to a vacuum pump?

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