chemistry - 3f - practical skills and techniques

Question 1

What does colorimetry use?

Answer 1

The relationship between colour intensity of a solution and the concentration of the coloured species present.

Question 2

What is a colorimeter or a spectrophotometer used for?

Answer 2

To measure the
absorbance of light of a series of standard solutions, and this data is
used to plot a calibration graph.

Question 3

How is the concentration of the solution being tested determined from?

Answer 3

Its absorbance and by referring to the calibration curve.

Question 4

Where must the concentration of coloured species in the solution being tested lie?

Answer 4

In the straight line section of the calibration graph.

Question 5

What is distillation used for?

Answer 5

Identification and purification of organic
compounds.

Question 6

What is the boiling point of a compound, determined by distillation, one of?

Answer 6

The physical properties that can be used to confirm its identity.

Question 7

How can distillation be used to purify a compound?

Answer 7

By separating it from less volatile substances in the mixture.

Question 8

What does heating under reflux allow?

Answer 8

Heat energy to be applied to
a chemical reaction mixture over an extended period of time without volatile substances escaping.

Question 9

When carrying out heading under reflux, where is the reaction mixture placed?

Answer 9

In a round-bottomed flask with anti-bumping granules and the flask is fitted with a condenser. The flask is then heated using an appropriate
source of heat.

Question 10

What does vacuum filtration involve?

Answer 10

Carrying out a filtration under reduced pressure and provides a faster means of separating from a filtrate.

A Büchner, Hirsch or sintered glass funnel
can be used during vacuum filtration.

Question 11

What can be used during vacuum filtration?

Answer 11

A Büchner, Hirsch or sintered glass funnel.

Question 13

How is the solvent for recrystallisation chosen?

Answer 13

So that the compound
being purified is completely soluble at high temperatures and only
sparingly soluble at lower temperatures.

Question 14

What does solvent extraction involve?

Answer 14

Isolating a solute from a liquid mixture or solution by extraction using an immiscible solvent in which
the solute is soluble.

Question 15

What happens in the separating funnel when carrying out a solvent extraction?

Answer 15

The two immiscible solvents
form two layers in the separating funnel. The solute dissolves in both
solvents and an equilibrium establishes between the two layers.

Question 16

Solvent extraction: What is the ratio of solute dissolved in each layer determined by?

Answer 16

The equilibrium constant K.

Question 17

Solvent extraction: What happens to the lower and upper layers?

Answer 17

The lower layer is run off into a container and the upper layer is poured into a second container. This process is repeated to
maximise the quantity of solute extracted.

Question 18

Solvent extraction: When is the quantity of solute extracted greater?

Answer 18

If a number of extractions
using smaller volumes of solvent are carried out rather than a single
extraction using a large volume of solvent.

Question 19

Solvent extraction: What should the solvent used be?

Answer 19

• immiscible with the liquid mixture or solution (usually water)
• one in which the solute is more soluble in than the liquid mixture or solution (usually water)
• volatile to allow the solute to be obtained by evaporation of the
  solvent
• unreactive with the solute