Paper 3: Required practical 10 - 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 (using reflux, distillation)
2. Filtration (usually vacuum filtration)
3. Purification (recrystallisation)

Question 2

How do you use lab equipment to heat under reflux?

Answer 2

• Substance is boiled in a pear-shaped or round-bottomed flask
• As it evaporates, it is cooled by the water in the liebig condenser and so condenses back into a liquid and drips 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 dry boiling
• Prevents volatile reactants/products escaping
• Ensures even heating

Question 4

How do you filter under pressure?

Answer 4

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 5

How do you purify a solid product?

Answer 5

• Add a minimal amount of hot solvent
• Filter to remove insoluble impurities
• Place in an ice bath
• Filter under reduced pressure
• Wash and dry to remove soluble impurities

Question 6

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

Answer 6

• Place a small sample of the solid in a capillary tube
• Melt using the melting apparatus available, measuring the temp with a thermometer
• A pure substance will usually melt at a single temperature but an impure substance wil melt over a range of temperatures (lower than the pure substance)
• Record the starting and ending points of the melting, ehen the first crystal can be seen to melt and when the last crystal becomes liquid respectively
• You can compare the melting point to known values to identify the substance

Question 7

How do you purify a liquid organic product?

Answer 7

• Use a separating dunnel to isolate the organic layer from the aqueous layer
• Add anhydrous MgSO4 to the organic product to get rid of all water
• You can then purify the liquid by distillation, which separates the substances by boiling point