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exp2w08

# exp2w08 - Experiment 2 separation of water and...

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Experiment 2: separation of water and ethanol (Simple and Fractional Distillation) Mohrig, Chapter 11 Distillation is undoubtedly the most important purification technique for organic liquids (bp < 300 °C). It involves heating a liquid to its boiling point at atmospheric or reduced pressure to convert it to a vapor, and then condensing the vapor back to the liquid by cooling. The boiling point of a liquid is the temperature at which the vapor pressure (es- caping tendency) of the liquid equals the atmospheric or applied pressure; that is, when liquid and vapor are in equilibrium. Vapor pressure is related to the strength of the weak forces as well as the temperature but not to the exposed surface area or the volume of the space above the liquid (assuming there is not so much space that all of the liquid evaporates before equilibrium can be established). These other factors may influence the rate at which vapor pressure equilibrium is achieved, but the ultimate result will be the same for a given liquid at a given temperature. The amount of vapor pressure generated is thus tied to the energy needed to overcome the forces that hold the liquid together. Va- por pressure is not a linear function of temperature, but instead has a more complicated logarithmic relationship. A homogenous mixture (i.e., a solution) of two liquids boils when the vapor pressure of the mixture is equal to the applied pressure, that is, when the sum of the partial pressures of the components (P A , P B , P C ...) equals the applied pressure, P total . Thus, at the boiling point [P total = P A + P B + P C ]. In this experiment, simple distillation [Part 1] will be compared to fractional distillation [Part 2]. Like melting points, a sharp distillation point is indicative of a pure organic sub- stance. The procedure only works if you distill very slowly. It is best not to do this experiment in the hood, because the draft cools the column. The column may be insulated by wrapping a paper towel around it or wrapping the column loosely with aluminum foil or glass wool. The apparatus set-ups for these proce- dures in semi-micro scale are shown. Assemble the apparatus shown for simple distillation, and add 15 mL of a 20% alcohol- water solution to a 25 mL or larger round bottom flask. No column is used in the simple distillation. Before beginning heating on the hot-plate, add one or two tiny boiling stones.

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exp2w08 - Experiment 2 separation of water and...

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