RP10: Preparation of organic solids and liquids

Question 1

What are the main steps in producing a pure organic solid?

Answer 1

1. Synthesis of the compound (usually using reflux, distillation)
2. Filtration (usually vacuum filtration)
3. Purification (recrystallisation)

Question 2

How do you use laboratory equipment to heat under reflux?

Answer 2

Quick-fit apparatus is used to heat a substance under reflux

The substance is boiled in a pear-shaped flask.

As it evaporates, it is cooled by the water in the liebig condesner and so condenses back down into the flask to be heated again.

Question 3

Why is heating under reflux used?

Answer 3

• Allows heating for a long period of time
• Prevents the flask from boiling dry
• Prevents volatile reactants/products escaping
• Ensures even heating

Question 4

Why are anti-bumping granules used when heating under reflux and distillation?

Answer 4

To allow smooth boiling - they prevent splashing up the sides of the flask due to bubbles caused by vapours

Question 5

How do you use laboratory equipment to filter under reduced pressure?

Answer 5

Using a Buchner funnel and Buchner flask, connected by rubber tubing to the vacuum source.

• The funnel contains a layer of filter paper
• Pour the substance onto the filter paper and the liquid will be sucked through via vacuum filtration into the flask
• The solid will remain on the paper

Question 6

How do you purify a solid product?

Answer 6

By recrystallisation

1. Add minimum amount of warm solvent to the impure sample until it has dissolved
2. Allow to cool, crystals should form
3. When no more form, you can filter under reduced pressure to obtain a dry crystalline solid

Question 7

How do you determine the melting point of a substance and why can this information be useful?

Answer 7

1. Place a small sample of the solid in a capillary tube
2. Melt using the melting apparatus available, measuring the temperature with a thermometer
3. A pure substance will usually melt at a single temperature but an impure substance will melt over a range of temperatures
4. Record the starting and ending points of the melting, when the first crystal can be seen to melt and when the last crystal becomes liquid respectively.
5. You ccan

Question 8

How do you determine the melting point of a substance and why can this information be useful?

Answer 8

1. Place a small sample of the solid in a capillary tube
2. Melt using the melting apparatus available, measuring the temperature with a thermometer
3. A pure substance will usually melt at a single temperature but an impure substance will melt over a range of temperatures
4. Record the starting and ending points of the melting, when the first crystal can be seen to melt and when the last crystal becomes liquid respectively.
5. You can then compare the melting point to known values to identify the substance.

Question 9

How do you purify a liquid organic product?

Answer 9

You can use a separating funnel to isolate the organic layer from the aqueous layer.

Add anhydrous MgSO4 to the organic product to remove all water.

You can then purify the liquid by distillation which separates the substances by boiling point.

Question 10

Percentage Uncertaintity

Answer 10

(Actual/Theoretical) x 100