Chemistry 29a M. Walker

In this experiment we will be comparing simple distillation with fractional distillation. We will perform experiments 2 & 3, similar to the first part of each. However we will be separating methylcyclohexane (B.P. 98^oC) from hexane (B.P. 68^oC), in place of the toluene and cyclohexane used in the book. [We will not be doing any parts involving separation of ethanol from water]. The simple distillation will be done by individual students, but the fractional distillation will be done in pairs. In both experiments, the distillations must be carried out slowly.

Simple distillation involves turning a liquid into a vapor, then condensing that vapor back to a liquid (in a "condenser") which is collected in a separate container. In this way, impurities which boil at a much higher temperature can be left behind. However, liquids still have a significant vapor pressure below their boiling points (this is why liquids evaporate). Thus even at 68^oC, methylcyclohexane will have a significant amount of vapor present, and this vapor will be carried over with the hexane vapor into the condenser, giving a distillate which is not pure hexane. Therefore simple distillation is not very effective for separating a mixture of liquids which are close in their boiling points, although repeating the distillation many times over will produce a separation (except with inseparable azeotropes such as ethanol-water).

Fractional distillation is performed as with simple distillation, but with the addition of a distillation column containing a solid packing of high surface area (we will use steel wool). This packing allows the liquid to repeatedly boil and condense many times over inside the column, before the substance reaches the condenser. This is equivalent to performing several simple distillations at once, and it results in a much better separation than is observed with simple distillation (again, though, it fails for certain azeotropes such as ethanol-water).

For this experiment (both parts) we will be collecting distillate in tared (i.e. preweighed) sample vials. For both parts we will collect drops 1-5 (from each distillation) in one vial, then we will collect the remaining drops in a second vial, and we will weigh then run a GC of all samples. You can see how close you are to getting pure hexane in drops 1-5, in both experiments, so you can compare the effectiveness of the methods of distillation.

Assigned Problems:

1. Using your data for mass and composition (GC), calculate how much hexane remains undistilled in your "still bottoms" (distillation residues in the flask).

2. When is a single simple distillation an effective method for separation? Give examples.