Fractional distillation is an important method of separating complex mixtures. It relies on the principle that different substances have different boiling points. You are to simulate the separation of a mixture using the apparatus shown. You will follow the temperature changes as the liquids are distilled off, and investigate the composition of the liquids collected. You will then learn about the application of fractional distillation to separating the complex mixtures of substances found in crude oil.
Separating a mixture using distillation
- Open Yenka file Model 1.
- You have been provided with the essential items required to build a distillation apparatus. The condenser has a hidden connection to a water supply that keeps the cooling jacket cold.
- Pour the mixture to be separated into the distillation vessel (you can do this without taking the connector out!). Select the distillation vessel and write down the chemicals present.
AnswerSodium chloride, ethanol, water.
- Click on the graph and then erase the data. Now set the temperature to 97 °C. Which of the graph lines shows an increase in temperature, and what does it change to? Suggest why the condenser shows no change in temperature.
AnswerThe one in the distillation vessel changes to 90 °C.
The one in the condenser has a steady flow of water to cool it.
- The liquid in the distillation vessel is boiling because it contains ethanol (boiling point 78 °C). What is collecting in the receiver? Select the receiver and open its reaction details to see exactly what is present.
- Allow the distillation to continue for several minutes. Watch the graph. As soon as there is a change, pause the simulation. What happened? Select the receiver and write down exactly what it contains.
AnswerTemperature of distillation vessel starts to rise.
E.g. receiver contains 7 cm3 ethanol.
- Move the receiver to one side, and replace with another. Restart the simulation. What happens and what is the temperature inside the distillation vessel?
AnswerNo more liquid collected, and mixture in distillation vessel stops boiling. Temperature steadies at 96 °C.
- Select the distillation vessel and write down the substances it contains. What have you achieved?
AnswerSodium chloride and water - all ethanol removed.
- Turn the heater up to 181 °C, select the new receiver, and describe what happens.
AnswerLiquid in distillation vessel boils again, temperature rises to, e.g., 104 °C, and water starts being collected.
- Why does water have a higher boiling point than ethanol?
AnswerStronger intermolecular forces between water molecules than between ethanol molecules.
- Allow the distillation to continue for a minute or so. What happens?
AnswerSolid sodium chloride remains in distillation vessel, temperature rises to 180 °C, and, e.g., 3 cm3 of water is collected.
- Select the distillation vessel and open the atom viewer. Describe what you see.
AnswerSodium ions and chloride ions forming a solid lattice.
- Sodium chloride cannot be either melted or boiled using this apparatus. Explain, in terms of the bonding present, why sodium chloride has such a high melting point.
AnswerIonic bonding, therefore very strong electrostatic attractions between oppositely charged ions.
By gradually heating up the sample mixture you have collected relatively pure samples of water and ethanol. Sodium chloride has a very high boiling point and remains as the residue.
- The laboratory method of distillation is a batch process and involves heating a sample up slowly, and collecting the fractions at the same location, but at different times. Open the link to a website demonstrating industrial fractional distillation. This shows how the various components in crude oil are separated. Give two differences between the laboratory method and the industrial method.
AnswerE.g., industrial method is continuous; fractions condensed and removed at different heights up the column.
- Explain why it is so important to separate crude oil into groups of compounds with similar boiling points (fractions).
AnswerCannot make use of such a complex mixture of chemicals, need to separate into groups of chemicals that have similar properties and will therefore be equally suited to a particular use.
- Explain why the compounds in the diesel fraction have higher boiling points than those in the naphtha fraction.
AnswerLonger chain lengths, higher molecular mass, bigger molecules, larger intermolecular forces (Van der Waals' forces).
Industrial fractional distillation is a large-scale continuous process which separates the complex mixture of hydrocarbons in crude oil into groups of chemicals with similar boiling points.
- Simulation involves connecting up tubes and placing temperature probes at various points so pupils need to have experience of these techniques.
- Pupils need access to the Internet in order to research answers for second task.
- This activity builds on Separation by Distillation.