Simple and Fractional Distillation

The purpose of this experiment was to separate two liquids using Simple and Fractional Distillations. Distillation separates the components of a liquid mixture by their differences in volatility: the component with the higher vapor pressure (lower boiling point) evaporates more readily, so the vapor, and the distillate condensed from it, is enriched in that component. Those volatility differences arise from differences in the chemicals’ intermolecular forces.

Reagents in the Experiment:

Cyclohexane Toluene

Analysis:

This experiment attempted to separate two liquids, cyclohexane and toluene. These chemicals have different intermolecular forces and therefore have dissimilar boiling points. Distillation is an excellent way to isolate two compounds if their boiling points are sufficiently far apart.

In the simple distillation, heat was applied to a flask containing the two solvents. The flask was connected to the distillation apparatus. The solvents’ vapor slowly escaped the flask, contacted the thermometer, and wisped into the condensing chamber. The condensing chamber is continuously cooled by water, thereby forcing the solvent vapor to liquefy. The distilled solvents were collected. It was expected that the solvents’ evaporate with different rates, particularly that, cyclohexane with a lower boiling point, would have a greater concentration in the distillate beaker than toluene. A graph of temperature on volume is shown below for the simple distillation. The results show that the distillate contains a volume of toluene and cyclohexane; separation was not successful.

Fractional distillation differs from simple distillation because it includes a ‘fractioning column’. The column towers above the heated flask and was stuffed with a metal sponge, allowing for numerous liquid/vapor equilibrium phases of the distillate across the sponge’s surface. As the fractional distillation graph above indicates, this method was highly successful in separating the two liquid chemicals. Unlike the scattered data in the simple distillation plot, there was a defined temperature plateau for cyclohexane (~72 °C observed) and another for toluene (~90 °C observed). Both readings are below the literature boiling points (80.7 °C and 110.6 °C), likely because of thermometer placement or heat loss, but the two distinct plateaus show the separation worked. Fractional distillation was better suited for this separation than simple distillation.

The Science Behind the Separation

Volatility and Vapor Pressure

Every liquid has a vapor pressure, the pressure exerted by its vapor in equilibrium with the liquid at a given temperature. A liquid with weaker intermolecular forces has a higher vapor pressure and a lower boiling point; it is described as more volatile. When a mixture of two miscible liquids is heated, both components contribute to the vapor, but not equally.

Raoult’s Law and Ideal Mixtures

For an ideal mixture, Raoult’s law states that the partial vapor pressure of each component equals its vapor pressure as a pure liquid multiplied by its mole fraction in the liquid (P_A = χ_AP°_A). In other words, a component’s vapor pressure is lowered in proportion to how diluted it is in the solution. Because the more volatile component has the larger pure-liquid vapor pressure, it makes up a larger share of the vapor than it does of the liquid. Condensing that vapor therefore gives a distillate enriched in the lower-boiling component. Real mixtures deviate from Raoult’s law to varying degrees, especially when the attractions between unlike molecules differ from those between like molecules. Strongly non-ideal mixtures can form azeotropes, such as about 95.6% ethanol and 4.4% water, which boil at a constant composition and cannot be separated further by ordinary distillation. Cyclohexane and toluene behave closely enough to ideal for distillation to work.

Theoretical Plates

One complete vaporization–condensation cycle, in which liquid and vapor reach equilibrium, is called a theoretical plate. A simple distillation provides roughly one theoretical plate, so it only partially enriches the distillate. That is enough when the boiling points are far apart, but simple distillation cannot significantly purify components whose boiling points are close; a commonly cited teaching guideline is that a fractionating column is needed when they differ by less than about 100 °C. Cyclohexane (80.7 °C) and toluene (110.6 °C) differ by only about 30 °C, which is why the simple distillation failed.

Why a Fractionating Column Works Better

A fractionating column, packed with a high-surface-area material (like the metal sponge used in this experiment) or built with indentations as in a Vigreux column, lets many successive distillations happen at once. Rising vapor condenses on the packing, the condensate is re-vaporized by hotter vapor coming up from below, and each cycle enriches the vapor further in the more volatile component. Each equilibration adds a theoretical plate, so a column with several plates can separate liquids with close boiling points that simple distillation cannot.

In practice, separation also depends on running the column slowly enough for equilibrium to be approached at each stage. Both liquids are flammable, so heat them with a heating mantle in a fume hood, never over an open flame. For a related experiment, see Distillation – Lab Report.

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