Practical Techniques

Question 1

What is reflux used for?

Answer 1

Used to heat reactions involving volatile flammable organic compounds. Organic reactions proceed slowly and low boiling point, so heating in an open vesicle causes evaporation before reaction can take place.

Question 2

Draw a reflux experiment/ CONDENSER (describe it)

Answer 2

Pear bottom flask- heat (usually use electrical heat to prevent ignition of flammable vapours)
Anti bumping granules (prevent violent boiling)

CONDENSER- water in at bottom/ left and water out at top/ right

Question 3

List the 4 steps in purification

Answer 3

Re distillation
Extraction
Drying
Filtration

Question 4

What is used for purification

Answer 4

Separating funnel (organic layer on top, aqueous layer on bottom)

Question 5

Describe redistilation

Answer 5

Repeated distillation to seperate pure volatile liquid from impurities with different BP.

Question 6

Describe extraction

Answer 6

Water added to organic liquid and solvent, 2 layers form

Organic liquid dissolves in organic layer

Water soluble impurities dissolve in aqueous layer (drained off using tap)

Question 7

Explain drying

Answer 7

Traces of water can be removed using drying agents eg. Anhydrous magnesium sulfate

Question 8

Explain filtration

Answer 8

After drying, solid dry agent removed by filtering mixture leaving behind purified organic product

Question 9

What is the role of distillation

Answer 9

Separate based on boiling point

Question 10

Draw/ describe distillation apparatus

Answer 10

Pear bottom flask- heat reactions
Anti bumping granules
Thermometer
Condenser
Water in bottom right
Water our top left
Distillate collected

Question 11

First three steps in distillation

Answer 11

Mixture heated gently

Evaporate in increasing BP

Thermometer monitors BP (help identify substance)

Question 12

Steps 4-6 in distillation

Answer 12

If BP of desired product known collection can start at this temp

Sometimes used when product has lower BP than reactants

As reaction progresses it evaporates and collected separately via condensation preventing further reaction

Question 13

List the two steps of filtering solids under reduced pressure

Answer 13

Transfer reaction mixture to Büchner funnel with filter paper connected to vacuum liner.

Apply suction to draw liquid through filter, leaving dry solid.

Question 14

List the 4 steps in recrystallisation of organic solids

Answer 14

Dissolve impure solid in hot solvent creating a saturated solution

Allow solution to cool quickly encouraging formation of pure crystals, impurities remain

Filter mixture under reduced pressure to collect crystals and wash in cold water to remove impure residue

Dry purified crystals

Question 15

What should the solvent do? (In recrystallisation)

Answer 15

Fully dissolve solid at high temp
Promote crystallisation of solid as it cools

Question 16

What does the correct solvent ensure?

Answer 16

Maximum amount of product of recrystallisation

Question 17

Solubility is low when hot. Why?>

Answer 17

Does not fully dissolve even boiling
Prevents recrystallisation as crystals cannot form from solution

Question 18

Solubility too high when cold. Why?>

Answer 18

Product remains dissolved in cold solution
Crystals fail to form leading to product loss rather than impurity removal.

Question 19

How does MP indicate purity?

Answer 19

Pure substances melt at precise temperatures. Impure over a range of temperatures.

Question 20

What does a MP between 1-2 degrees indicate

Answer 20

High purity

In contrast a wider range indicates impurities

Question 21

How to accurately determine MP of organic solid (1-4)

Answer 21

Place small amount in capillary tube at bottom

Place in oil bath and heat while stirring bath and monitoring temperature

Record melting range of solid, noting the temperature which solid started melting and completely Melted

Repeat to confirm accuracy