practical techniques

Question 1

What is heat under reflux used for?
+ diagram

Answer 1

• Ensures even heating
• Prevents volatile reactants/ products escaping
• Allows heating for long periods of time

Question 2

Simple distillation diagram

Answer 2

…

Question 3

Why are anti-bumping granules used?

Answer 3

Reduces the size of the bubbles to allow smooth boiling.

Question 4

Why is steam disillation used?

Answer 4

To seperate an insoluble liquid from an aqueous solution.

Question 5

Why is fractional distillation used?

Answer 5

Fractionating column filled with glass beads, allows gases to pass through but condenses vapours and returns them to the reaction/ heating vessel.

Question 6

What is solvent extraction used for?
+ digram
+ ideally….

Answer 6

Using a solvent to remove a desired component from a reaction mixture.
Ideally, there should be a large difference in solubility of the solvent used for extraction and the initial solvent.

Question 7

What is vacuum filtration used?
+ 2 types (and diagrams)

Answer 7

To remove soluble impurities.
- Buchner funnel and Hirsch funnel

Question 8

What is drying used for?

Answer 8

To remove water using drying agents (e.g. calcium chloride, magnesium sulfate and sodium sulfate)

Question 9

What is hot filtration used for in recrystallisation?

Answer 9

To remove undissolved insoluble impurities.

Question 10

What is cold filtration used for in recrystallisation?

Answer 10

Allows soluble impurites to be removed. Cold minimises the loss of product by dissolving.

Question 11

How to test for purity?

Answer 11

Measure the MP of the organic compound and compare it with the know MP value of the pure compound.

Question 12

Describe the recrystallisation process to obtain a pure, dry sample of paracetamol.

Answer 12

-dissolve crystals in the minimum volume of hot water / solvent
-filter hot and allow to cool
-filter and wash with a small amount of cold solvent
-dry crystals in a warm oven

Question 13

Determination of enthalpy change
using Hess’s Law.

Answer 13

-use a measuring cylinder to measure 50cm3
of hydrochloric acid and pour it into a polystyrene cup
-measure the temperature of the hydrochloric acid every
30 s for 2½ minutes
-weigh the test tube containing sodium carbonate
-add the sodium carbonate at exactly 3 minutes
-stir and measure the temperature of the mixture every
30 s for another 5 minutes
-weigh the empty test tube.
-plot a graph of temperature against time
-draw line of best fit straight lines and extrapolate the lines to the time of mixing and determine the maximum temperature change at that time