Expt. 2

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Mixtures with a composition to the left of C MIN , in Figure 4A (or C MAX in Figure 4B), can beseparated into pure ethanol (or formic acid in 4B) and the constant boiling mixture, but purewater cannot be obtained from such a mixture by distillation. Conversely, mixtures with acomposition to the right of C MIN , in Figure 4A (or C MAX in 4B), can be separated into pure waterand the constant boiling mixture, but can never furnish pure ethanol/formic acid by distillation.Fractional DistillationIt was noted from Figure 3 that even the first drop of distillate (fraction collected) obtained wasnot pure, but rather a mixture, containing mainly the lower boiling point compound (carbontetrachloride). If these first fractions were to be combined and redistilled, the first vapour to becondensed would be even richer in carbon tetrachloride. Repetition of this process (vaporization,condensation, and revaporization) could eventually lead to isolation of pure carbon tetrachloride.Similar redistillation of the higher-boiling fractions could lead to isolation of pure toluene in thefinal fractions. Clearly this repeated redistillation is a laborious process.Figure 5: : Close-up of the Fractionating Section of a Hickman StillThe fractionating column is a device for increasing the efficiency of this redistillation process. Itconsists of a vertical column packed with some inert material, such as glass beads or glasshelices, or provided with some other device (indentations are used in our apparatus – see Figure5) for increasing the surface upon which the vapour may condense. As the hot vapours risethrough the column, they condense and flow back down the column. The condensate, as it hitsthe lower, hotter portions of the column, is revaporized, and the more volatile componentsproceed up the column once again. This process is repeated numerous times in the fractionatingsection of the Hickman still, and the distillate will consist of the lowest-boiling component in anearly pure form. Figure 6 illustrates the process graphically.
Figure 6: Liquid-Vapour Diagram, illustrating the Principles of FractionalDistillationAn A-B mixture (Figure 6) with composition C 1 boils at temperature T C1 and the vapours enterthe column at that temperature. If they condense in the column, the condensate will have thecomposition C 2 . This vaporizes near the bottom of the column at temperature T C2 , producingvapours with composition C 3 . These may condense further up the column at T C3 ; vaporizationnow gives vapour with composition C 4 , etc. If the column contains enough surface area for manysuccessive vaporizations and condensations (an ideal distillation column), the first distillate thatcomes over will be nearly pure 100% A with a boiling point close to that of pure A. This willcontinue until all of A is removed, after which B will start distilling, and the temperature will riserather abruptly to the boiling point of B. In practice, distilling columns are not 100% efficient,but columns have been designed which can separate liquids that boil as little as 2 o C apart.Thermometer CorrectionGenerally speaking, thermometers may be of two types: total immersion or partial immersion. Ifthe thermometer is not immersed totally or to the immersion line on the thermometer, thetemperature reading will be slightly lower than the true boiling point of your distillate. The stemcorrection may be calculated according to the formula:T c = T o + F x L(T o - T m )where T c = corrected temp.,T o = observed temp.,T m = mean temp. of exposed stem,L = length of exposed mercury column in degrees,F = correction factor (for mercury in pyrex, a value for F of 0.00016 is used).Although you are not expected to perform this correction, keep in mind that your observedtemperature will be lower than the true boiling point, especially for higher temperatures.The Melting Point ApparatusSeveral types of apparatus used to determine melting points are shown below. Types A to C areolder versions and use liquids as the heat transfer agents. In this laboratory you will useapparatus D which comes in two versions: one for use with a capillary tube and the other for use
Page 1 and 2: Experiment 2. Separation of Liquids
Page 6 and 7: with a microscope cover slide. You
Page 8 and 9: your table for up to 10 fractions.

Expt. 2

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