What personal protective equipment should you wear before setting up a distillation apparatus?

Before beginning laboratory work, put on a lab coat, splash-proof safety glasses, and nitrile gloves. These items protect you from chemical splashes, vapors, and skin contact with solvents used during distillation experiments. Always ensure all protective equipment is properly fitted and in good condition before starting any lab procedure.

How should the thermometer be positioned in a distillation apparatus?

Insert the thermometer into the thermometer adapter by gently pushing and twisting it through the top. Adjust the thermometer until the bulb sits below the bend in the distilling head. This positioning ensures accurate temperature readings of the vapor being distilled, which is critical for collecting the correct fraction.

What is the correct orientation for connecting the condenser to the distillation setup?

Clamp the condenser to a stand oriented diagonally toward the side joint of the distilling head. Connect rubber tubing from the tap water faucet to the inlet port, which is further from the distilling head, and attach the outlet port to the drain. This configuration ensures efficient cooling of vapors as they pass through the condenser.

How should water flow through the condenser during distillation?

Water enters through the inlet port, which is the port further from the distilling head, and exits through the outlet port closer to the distilling head. This countercurrent flow maximizes heat exchange efficiency, keeping the condenser jacket cool and ensuring effective vapor condensation throughout the distillation process.

What heating equipment is used to control temperature during distillation?

A heating mantle placed on a stir plate provides controlled heat to the round-bottom flask. The heating mantle connects to a temperature controller, which regulates the heating rate and maintains consistent temperatures. This setup allows precise control over the distillation process and prevents overheating of the sample.

How should solvents be prepared and stored for student use during a distillation experiment?

Pour appropriate amounts of each solvent into labeled glass bottles so students can measure their portions without handling large stock bottles. For example, if each group needs 2 mL of toluene and 2 mL of cyclohexane, prepare enough for all groups. Store these prepared solvents in the instructor hood for safe student access during the experiment.

What role do joint clips play in assembling a distillation apparatus?

Plastic joint clips secure each connection point throughout the distillation apparatus, including where the distilling head connects to the flask and where the condenser attaches to the distilling head. These clips prevent glassware from separating during heating and ensure a stable, leak-free setup for safe and effective distillation.

Simple Distillation

Concepts Instructor Prep Student Protocol

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As a liquid is heated, it gains energy until the increased disorder of the gas phase outweighs the intermolecular interactions in the liquid phase. Once enough molecules are in the gas phase, they escape the liquid in the form of bubbles. This effect, called boiling, occurs when the total vapor pressure of the substance is equal to the atmospheric pressure.

Vapor pressure is the pressure of the vapor in equilibrium with its condensed phase, and it varies with temperature. In a mixture of liquids, each component has its own vapor pressure, which we call its partial pressure. The total vapor pressure of the mixture is equal to the sum of the partial pressures. If the liquids are miscible, meaning that they always form a homogeneous solution, the partial pressure of each component is the vapor pressure of the pure compound at that temperature times its mole fraction in the liquid.

The temperature at which the first bubble of vapor starts to form in a liquid mixture is called the bubble point. For a pure liquid, both the bubble point and the temperature at which vapor starts to condense, or the dew point, are the same as the boiling point. However, for a mixture of two miscible liquids, both the bubble point and the dew point will be between the boiling points of the components.

When the mixture first boils, the vapor is rich with the compound with the lower boiling point, or the more volatile compound. This increases the proportion of the compound with the higher boiling point, or the less volatile compound, in the liquid mixture.

Distillation is a separation technique that takes advantage of this phenomenon. In a simple distillation, a homogeneous liquid mixture is boiled. The rising vapor then enters the inner chamber of a water-cooled condenser. The vapor condenses to a liquid, called the distillate, which is then collected in a separate vessel.

As the boiling continues, the compositions of the liquid and the vapor change as the more volatile component is removed. So, if we collect the distillate in small fractions, we'll see that each fraction contains the compounds in a different molar ratio.

As the proportion of the less volatile component in the liquid mixture increases, so do the bubble point and dew point. Plotting the mixture's bubble and dew points versus the mole fractions of the components makes a boiling point diagram. Once we have this diagram, we can use the dew point curve to determine the composition of the vapor at a given temperature.

In this lab, you will set up and perform the simple distillation of a mixture of cyclohexane and toluene and record the temperature of the vapor throughout the experiment. You'll then use the published boiling point diagram for cyclohexane and toluene to determine the composition of the vapor, allowing you to estimate the composition of the liquid mixture throughout the distillation.

At the end of this lab, students should know...

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What is the vapor pressure of a liquid?

The vapor pressure of a pure component is the pressure exerted by the vapor on the liquid in a closed system.

At what point does a liquid boil?

A liquid boils when the vapor pressure of the liquid is equal to the atmospheric pressure.

In a mixture of miscible liquids, at which temperature does the solution boil?

A miscible solution boils at a temperature between the boiling points of the pure components. The vapor pressure of the mixture is equal to the sum of the product of the partial vapor pressures and mole fractions of the components.

How does the composition of the distillate change over time during a distillation of a mixture of liquids?

Initially, the vapor — and by extension, the distillate — is rich with the more volatile component. As that component boils off, the less volatile component enters the vapor phase. At the end of the distillation, the vapor phase is rich with the molecules of the less volatile component.

How is a vapor-liquid equilibrium diagram used to determine the composition of the liquid and vapor in a distillation?

The vapor-liquid diagram is a plot relating the temperature of the vapor and liquid to the mole fraction of each component. At a given temperature, the composition of the liquid and vapor phases can be determined.