Practical Techniques

Question 1

Why is refluxing done?

Answer 1

It allows flammable substances in slow reactions to continue reacting without losing any of the products.

Question 2

What would happen to substances that need to be refluxed if they were heated in a beaker instead?

Answer 2

They would evaporate or catch fire.

Question 3

Describe the structure of refluxing equipment.

Answer 3

• Round-bottomed flask with anti-bumping granules in the reaction mixture
• Heat is applied from the bottom
• Liebig condenser attached to the top -> Water flows in at the bottom and leaves at the top

Question 4

When refluxing, what type of heating is usually used?

Answer 4

Electrical (e.g. hot plates), so as to avoid naked flames that might ignite the compounds.

Question 5

Describe the structure of distillation equipment.

Answer 5

• Round-bottomed flask
• Thermometer at the top near the junction with a Liebig condenser
• Heat is applied to the bottom of the flask
• Conical flask at the end of the Liebig condenser to collect the products

Question 6

When might distillation be used?

Answer 6

• Separating substances with different boiling points

* Separating off a product of lower boiling point as it forms (e.g. an aldehyde)

Question 7

What piece of equipment is used to separate aqueous and organic mixtures?

Answer 7

Separating funnel

Question 8

Describe how you can remove traces of water from a mixture.

Answer 8

• Add an anhydrous salt, such as MgSO₄ or CaCl₂ -> This binds to any of the water
• At first, the organic layer will be lumpy.
• Add more until the mixture looks like a snow globe when swirled.
• Filter to remove the drying agent.

Question 9

Give two examples of drying agent.

Answer 9

• MgSO₄

* CaCl₂

Question 10

How can you remove a drying agent from an organic mixture?

Answer 10

Filtration

Question 11

What is a good physical way of determining the purity of a liquid organic product?

Answer 11

Measuring the boiling point using distillation apparatus. See the range at which it boils too.

Question 13

Why might simple distillation not always be suitable for separating organic products? What is the solution?

Answer 13

Some organic liquids:
• Have high boiling points
• Decompose when they’re heated

Instead, steam distillation can be used.

Question 14

What is steam distillation and why is it used?

Answer 14

• It is normal distillation, except steam is pumped into the immiscible product
• This lowers the boiling point of the organic product so that it can be distilled out of the mixture and before it decomposes

Question 15

Will steam distillation work with all products?

Answer 15

No, the product must be immiscible with water.

Question 16

Describe the setup for steam distillation.

Answer 16

Normal distillation setup, except:
• Conical flask with water being heated
• This is connected to a tube that feeds steam into the organic product

(See diagram pg 224 of revision guide)

Question 17

In steam distillation, what happens if the product you are trying to collect is less volatile (higher boiling point) than the other components?

Answer 17

• The organic product and the steam will evaporate out of the impure mixture together.
• These can then be separated using separating funnel or solvent extraction.

Question 18

What technique is used to separate an organic product from an impurity when both are soluble in water?

Answer 18

Solvent extraction

Question 19

Describe solvent extraction.

Answer 19

There is an organic product and impurity, both of which are soluble in water:

1) Add the impure compound to a separating funnel and add some water. Shake well.
2) Add an organic solvent in which the product is more soluble than in water. Shake well. This will dissolve the product into the organic solvent.
3) You can also add a salt to the mixture, which causes the organic product to move into the organic layer.
4) Open the tap and run each layer off into a separate container.

Question 20

In solvent extraction, why might a salt be added? How does this work?

Answer 20

• The salt causes the organic product (which is also soluble in water) to move more into the organic layer.
• This is because the salt makes the water even more polar, so the organic product is less soluble in it.

Question 21

What is washing?

Answer 21

Adding another liquid to an impure liquid and shaking.

Question 22

Does washing have to be with water?

Answer 22

No

Question 23

What is washing used for?

Answer 23

Removing impurities that cannot be removed by solvent extraction (i.e. impurities that are also in the organic layer when you want the organic product).

Question 24

How can you remove an organic acid impurity from the organic layer in a separating funnel?

Answer 24

“Washing”:
• Add aqueous sodium hydrogencarbonate
• This reacts to give CO₂ and a salt of the acid, which dissolves in the aqueous layer
• The organic product remains in the organic layer and can be separated from the aqueous layer using a separating funnel

Question 25

What are the two types of filtration?

Answer 25

• Gravity filtration

* Filtration under reduced pressure

Question 26

What filtration techniques are used when you want to:
• Keep the liquid and discard the solid
• Keep the solid and discard the liquid

Answer 26

• Keep the liquid and discard the solid -> Gravity filtration
• Keep the solid and discard the liquid -> Filtration under reduced pressure

Question 27

Describe gravity filtration.

Answer 27

1) Place a piece of fluted filter paper in a funnel that feeds into a conical flask
2) Gently pour the mixture to be separated into the filter paper. The solution will pass through, while the solid is trapped.
3) Rinse the solid left in the filter paper with a pure sample of the solvent present in the solution.

Question 28

In gravity filtration and filtration under reduced pressure, what might the solid in the filter paper be rinsed with?

Answer 28

A pure sample of the solvent present in the solution.

Question 29

Describe how you can remove water soluble impurities from an insoluble product.

Answer 29

• Pour the product into a separating funnel
• Add water
• Shake the funnel and allow it to settle
• The organic layer and aqueous layer separate out into distinct layers
• Open the tap and run each layer off into a container

Question 30

Name the process by which a solid organic product can be purified.

Answer 30

Recrystallisation

Question 31

Describe the process of recrystallisation to purify an organic solid.

Answer 31

1) Add very hot solvent is added to the impure solid until it just dissolves. This should give a saturated solution of the impure product.
2) Filter by gravity filtration while the solution is still hot to remove any insoluble impurities.
3) Leave the solution to cool down slowly. Crystals of the product form as it cools. The impurities stay in the solution as they’re present in much smaller amounts than the product, so they take much longer to crystallise out.
4) Remove the crystals by filtration under reduced pressure and washed with ice-cold solvent.
5) Dry the crystals.

Question 32

In recrystallisation, why are soluble impurities removed from the product?

Answer 32

The impurities are present in smaller amounts than the product, so they take much longer to crystallise.

Question 33

When choosing a solvent for recrystallisation, what is it important to ensure?

Answer 33

The solid must be very soluble in the hot solvent, but nearly insoluble when the solid is cold.

Question 34

What is a good physical way of determining the purity of a solid organic product? Describe the process.

Answer 34

Use melting point apparatus:

1) Pack a small sample of the solid into a glass capillary tube and place it inside the heating element.
2) Increase the temperature until the sample turns from solid to liquid.
3) Measure the temperature range between when the solid starts t melt and when it has completely melted.
4) Look up the melting point of a substance in data books and compare it to your measurements.
5) Impurities from the sample will lower the melting point and broaden the melting range.

Question 35

Describe the setup for filtration under reduced pressure.

Answer 35

• Side-arm flask connected to a vacuum
• Bung in the top, with a Büchner funnel through it
• Filter paper on the base of the Büchner funnel
• Filtration mixture on top of the filter paper

Question 36

How do impurities affect the boiling point of organic liquids and solids?

Answer 36

• Liquids -> Increase it

* Solids -> Decrease it

Question 37

Describe the structure of melting point apparatus.

Answer 37

• Heating element
• Thermometer inserted into it
• Sample in capillary tube inserted into heating element

Question 38

Describe filtration under reduced paper.

Answer 38

1) Place a piece of filter paper, slightly smaller than the diameter of the funnel, on the bottom of the Büchner funnel so that it lies flat and covers all the holes.
2) Wet the paper with a little solvent, so that it sticks to the bottom of the funnel, and doesn’t slip around when you pour your mixture.
3) Turn the vacuum on, and then pour your mixture into the funnel. As the flask is under reduced pressure, the liquid is being sucked through the funnel into the flask, leaving the solid behind.
4) Rinse the solid with a little of the solvent that your mixture was in. This will wash off any of the original liquid from the mixture that stayed on your crystals, leaving you with a more pure solid.
5) Disconnect the vacuum line from the side-arm flask and then turn off the vacuum.
6) The solid will be wet from the solvent, so leave it to dry completely.