Researching Chemistry 3 - Techniques Used In the Preparation and Purification of Compounds

Question 1

What are the five techniques that can be used to isolate and purify compounds?

Answer 1

distillation, solvent extraction, vacuum filtration, recrystalisation, refluxing

Question 2

Refluxing is a technique used to…

Answer 2

heat substances for a prolonged period of time without volatile substances escaping

Question 3

(Refluxing)
The reaction mixture is placed in a… with a few… and a ___ is fitted to the neck of the flask.
What should be used to heat flammable substances?

Answer 3

a round-bottomed flask, anti bumping granules, condenser.
a heating mantle

Question 4

(Refluxing)
The condenser should be connected to a… with water coming…
What does the condenser prevent?

Answer 4

cold water tap, with water coming in at the bottom and out at the top.
volatile substances escaping

Question 5

Recrystalisation is a technique used to…

Answer 5

purify an impure solid

Question 6

Recrystalisation depends on the solid being…

Answer 6

soluble in the solvent at high temperatures and insoluble at low temperatures

Question 7

What are the four steps to recrystalisation?

Answer 7

1. Dissolve the solid in a minimum volume of hot solvent.
2. Filter to remove insoluble impurites
3. Slowly cool the mixture to allow the solid to crystalise
4. Filter to remove soluble impurities

Question 8

The solvent for recrystalisation must be carefully selected so that…

Answer 8

the substance to be purified is soluble at high temps and insoluble at low temps

Question 9

What is vacuum filtration?
What is its benefit when compared to regular filtration?

Answer 9

filtration carried out at a low pressure.
faster

Question 10

What are the three pieces of equipment which can be used in vacuum filtration?

Answer 10

a Buchner, Hirsch or sintered glass funnel

Question 11

What is distillation?
What is it used for?

Answer 11

a process whereby a mixture is heated, and the vapours collected through condensation.
separating the components of a mixture by their boiling points

Question 12

Distillation is used for the ___ and ___ of compounds.

Answer 12

preparation and purification

Question 13

The __ point of a compound is one of the physical properties that can confirm its ___ and can be determined through ___.

Answer 13

boiling, identity, distillation

Question 14

Distillation is used to ___ a compound by separating it from…

Answer 14

isolate, substances in the mixture with different volatilities

Question 15

Volatility ___ as boiling point increases.

Answer 15

decreases

Question 16

Does refluxing apparatus have a lid?

Answer 16

no

Question 17

What does solvent extraction involve?

Answer 17

separating a solute from a solvent using a second solvent which is immiscible in the first, and which the solute has a higher solubility in

Question 18

In solvent extraction, the two immiscible liquids form… within the…

Answer 18

two layers within the separating funnel

Question 19

In solvent extraction, the solute dissolves in the two liquids according to…
What does this establish?

Answer 19

the equilibrium constant, K - (ratio of solubility).
an equilibrium between the two layers

Question 20

Describe solvent extraction.

Answer 20

The solvent containing the solute to be extracted and the secondary solvent are placed in a separating funnel and shaken. You should open the valve to release any pressurised gas. The solute then dissolves in the two layers according to the equilibrium constant, K, and then the bottom layer is carefully removed. More of the second solvent is added to the solvent containing the solute and this process is repeated

Question 21

In solvent extraction, the amount of solvent extracted is far greater when…

Answer 21

multiple extractions using smaller volumes of solvent are used, when compared to one extraction using a large volume of solvent

Question 22

List four properties that the solvent used to extract the solute should have in solvent extraction.

Answer 22

1. Immiscible with the other solvent
2. unreactive with both the solute and other solvent
3. the solute should have a higher solubility in it than the other solvent
4. High volatility/low boiling point to allow extraction of the solute by evaporation at the end