4f- Practical skills and techniques

Question 1

What relationship 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 used to measure?

Answer 2

The absorbance of light of a series of standard solutions, and this data is used to plot a calibration graph

Question 3

In colorimetry, how is the concentration of the solution being tested determined?

Answer 3

From its absorbance value and by referring to the calibration curve

Question 4

What is distillation used for?

Answer 4

Identification and purification of organic compounds

Question 5

What physical property of a compound that can be used to confirm its identity, can be determined through distillation?

Answer 5

Boiling point

Question 6

How can distillation be used to purify a substance?

Answer 6

By separating it from less volatile substances in the mixture

Question 7

What does heating under reflux allow for?

Answer 7

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

Question 8

What must be ensured when carrying out heating under reflux?

Answer 8

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 using an appropriate source of heat

Question 9

What does vacuum filtration involve?

Answer 9

Carrying out a filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate. A Buchner glass funnel can be used during vacuum filtration

Question 10

What are the steps in the technique of recrystallisation to purify an impure solid?

Answer 10

• 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 11

How is the solvent for recrystallisation chosen?

Answer 11

S o that the compound being purified is completely soluble at high temperatures and only sparingly soluble at lower temperatures

Question 12

What does solvent extraction involve?

Answer 12

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

Question 13

What should the solvent used in solvent extraction be?

Answer 13

• immiscible with the liquid mixture or solution

- one in which the solute is more soluble in than in the liquid mixture or solution

Question 14

What should the solvent used in solvent extraction be?

Answer 14

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

Question 15

What is the melting point of a substance?

Answer 15

The temperature range over which the solid first starts to melt, to when all of the solid has melted

Question 16

How can the identity of a pure compound be confirmed?

Answer 16

Through melting point analysis and a comparison of the experimentally determined melting point with a literature or known melting point value

Question 17

What can the determination of the melting point give an indication of?

Answer 17

The purity of a substance, the presence of impurities in the compound lowers the melting point and broadens its melting temperature range due to the disruption in intermolecular bonding in the crystal lattice

Question 18

What does the determination of a mixed melting point involve?

Answer 18

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 19

What is the technique chromatography used for?

Answer 19

To separate the components present within a substance

Question 20

What does chromatography separate substances based on?

Answer 20

The differences in their polarity and molecular size

Question 21

How does thin-layer chromatography (TLC) work?

Answer 21

TLC uses a fine film of silica or aluminium oxide spread over glass, aluminium foil or plastic. A small sample of the mixture being tested is spotted onto the base (pencil) line of the chromatogram. 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. A developing agent or ultraviolet light is normally required to visualise the spots on the chromatogram

Question 22

What is the calculation used to determine the Rf value of a compound during TLC?

Answer 22

Rf= distance travelled by the sample / distance travelled by the solvent

Question 23

What conditions can affect the Rf value of a compound?

Answer 23

• temperature
• solvent used
• saturation levels

Question 24

How can the identity of a compound be confirmed in TLC?

Answer 24

• comparing the experimentally determined Rf 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 25

What is TLC used to assess?

Answer 25

The purity of a substance

Question 26

What will a pure substance look like on a TLC plate?

Answer 26

It would appear as a single spot, the presence of more than one spot shows that impurities are present (some impurities may not be visible by TLC analysis)