Simple Distillation
At the end of this lab, students should know...
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.
Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA
- Preparation of the Laboratory
Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.
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Before you begin, put on a lab coat, splash-proof safety glasses, and nitrile gloves.
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Set out the following glassware and equipment that the students will need to build the distillation apparatus (we suggest that students work in pairs):
2Lab stands 1Stir plate 1Heating mantle 1Temperature controller 1Round-bottom flask 1Cork lab ring 1Distilling head 1Condenser 1Connecting tube 110-mL graduated cylinder 21-mL volumetric pipettes 1Pipette bulb or controller 1Thermometer adapter 1Small beaker of sand 1Thermometer 1Stir bar 4Joint clips 2Pieces of rubber tubing 1Tube of vacuum grease 2Stand clamps -
Set up an example distillation apparatus for the students to refer to. First, place two stands next to each other in the hood and set a stir plate on the base of one of the stands.
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Place a heating mantle on the stir plate and plug the mantle into a temperature controller. Clamp the round-bottom flask to the stand over the heating mantle.
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Next, carefully insert the thermometer into the adapter by gently pushing and twisting it through the top. Insert the adapter into the distilling head and adjust the thermometer until the bulb is below the bend. Then, insert the distilling head into the round-bottom flask.
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Now, clamp the condenser to the second stand, with it oriented diagonally toward the side joint of the distilling head.
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Connect a piece of rubber tubing to the tap water faucet in the hood and to the inlet port of the condenser, which is the port further from the distilling head.
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Attach the other tubing to the outlet port, which is the port closer to the distilling head, and place the free end in the drain.
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Connect the condenser to the distilling head, and then attach the connecting tube to the other end of the condenser.
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Finally, place a 10-mL graduated cylinder below the connecting tube and secure each joint with a plastic clip.
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Prepare the two solvents the students will use in the experiment. Each group will need 2 mL of toluene and 2 mL of cyclohexane, so ensure that you have enough for all groups.
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Pour an appropriate amount of each solvent into a labeled glass bottle, so that the students can measure their solvents without handling any large stock bottles.
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Set these solvents in the instructor hood for students to access during the experiment.
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Q1: 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.
Q2: 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.
Q3: 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.
Q4: 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.
Q5: 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.
Q6: 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.
Q7: 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.