Practical Skills and Techniques

Question 1

What is distillation used for

Answer 1

Distillation is used for identification and purification of organic compounds

Question 2

What can the boiling point of a compound confirm

Answer 2

The boiling point of a compound determined by distillation is one of the physical properties that can be used to confirm its identity

Question 3

How does distillation purify the compound

Answer 3

Distillation can be used to purify a compound by separating it from less volatile substances in the mixture

Question 4

Refluxing

Answer 4

Heating under reflux allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping

Question 5

What is the procedure of carrying out heating under reflux

Answer 5

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

Question 6

Vacuum filtration

Answer 6

Vacuum filtration involves carrying out a filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate

Question 7

Which flasks can be used during vacuum filtration

Answer 7

Buchner
Hirsch
Sintered glass funnel

Question 8

What is used to draw the filtrate through the filter paper

Answer 8

Uses a vacuum pump to draw the filtrate through the filter paper at much greater speed

Question 9

What is used with a vacuum flask

Answer 9

A Buchner or Hirsch funnel is used with a vacuum flask which are designed to cope with increased pressures involved

Question 10

What is the melting point of a substance

Answer 10

The melting point of a substance is the temperature range over which the solid first starts to melt to when all of the solid has melted

Question 11

What can the identity of a pure compound be confirmed

Answer 11

The identity of a pure compound can be confirmed by melting point analysis and a comparison of the experimentally determined melting point with a literature or known melting point value

Question 12

What can determination of the melting point of a compound give an indication of

Answer 12

Determination of the melting point of a compound can give an indication of the purity of a compound. The presence of impurities in the compound lowers the melting point and broadens its melting temperature range due to the disruption in the intermolecular binding in the crystal lattice

Question 13

What does the determination of a mixed melting point involve

Answer 13

Determination of a mixed melting point involves mixing a small quantity of the product with some of the pure compound and determining the melting point. The melting point value and the range of the melting temperature can be used to determine if the product and the pure compound are the same substance.

Question 14

What does colorimetry use the relationship between

Answer 14

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

Question 15

What is a colorimeter or spectrophotometer used to measure

Answer 15

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

Question 16

How is the concentration of the solution being tested determined

Answer 16

The concentration of the solution being tested is determined from its absorbance and by
referring to the calibration curve.

Question 17

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

Answer 17

The concentration of coloured species in the solution being tested must lie in the straight
line section of the calibration graph.

Question 18

What technique is chromatography

Answer 18

Chromatography is a technique used to separate the components present within a mixture.

Question 19

How does chromatography separate substances

Answer 19

Chromatography separates substances by making use of differences in their polarity or molecular size.

Question 20

What type of silica is used and what is it spread over in thin layer chromatography

Answer 20

Thin-layer chromatography (TLC) uses a fine film of silica or aluminium oxide spread over glass, aluminium foil or plastic.

Question 21

What is spotted onto the chromatogram

Answer 21

A small sample of the mixture being tested is spotted onto the base (pencil) line of the chromatogram.

Question 22

What dissolves the compounds in the spot

Answer 22

A solvent dissolves the compounds in the spot and carries the compounds up the chromatogram. How far the compounds are carried depends on how soluble the compounds are in the chosen solvent and how well they adhere to the plate.

Question 23

What is required to visualise spots on the chromatogram

Answer 23

A developing agent or ultraviolet light is normally required to visualise the spots on the chromatogram.

Question 24

Rfvalues can be calculated:

Answer 24

distance travelled by the sample
R, = distance travelled by the solvent

Question 25

What does a compound always have the same of under the same conditions

Answer 25

Under the same conditions (temperature, solvent, and saturation levels) a compound always has the same R, value (within experimental error).

Question 26

The identity of a compound can be confirmed by:

Answer 26

• comparing the experimentally determined R, values with a literature or known value determined under the same conditions - making a direct comparison on a TLC plate between the compound being tested and the pure substance - a co-spot could be used

Question 27

What should appear on a TLC plate

Answer 27

A pure substance, when spotted and developed on a TLC plate, should appear as a single spot. The presence of more than one spot shows that impurities are present.

Question 28

Candidates must be familiar with use of the technique of recrystallisation to purify an impure solid involving:

Answer 28

♦ dissolving an impure solid gently in a minimum volume of a hot solvent ♦ hot filtration of the resulting mixture to remove any insoluble impurities ♦ cooling the filtrate slowly to allow crystals of the pure compound to form, leaving soluble impurities dissolved in the solvent ♦ filtering, washing and drying the pure crystals

Question 29

What is solvent extraction

Answer 29

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