Researching chemistry: Practical skill and techniques

Question 1

how can the Rf value be calculated

Answer 1

by dividing the distance travelled by the substance, by the distance travelled by the solvent

Question 2

what is TLC used for

Answer 2

TLC is used to assess the purity of substances

Question 3

how are you able to identify a pure substance in TLC

Answer 3

A pure substance, when spotted and developed on a TLC plate, should appear as a single spot (some impurities may not be visible by TLC analysis). The
presence of more than one spot shows that impurities are present

Question 4

method of TLC

Answer 4

Thin-layer chromatography (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.

Question 5

what makes the spots travel far

Answer 5

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 6

what is used to visualise the spots on a chromatogram

Answer 6

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

Question 7

how can the identity of a substance be confirmed by TLC

Answer 7

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 8

what is colorimetry

Answer 8

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

Question 9

what is used to measure the absorbance of light

Answer 9

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 10

how is the concentration of the solution determined (colorimetry)

Answer 10

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

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

Question 11

what is distillation used for

Answer 11

Distillation is used for identification and purification of organic
compounds.

Question 12

how is the boiling point of a compound determined

Answer 12

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

Question 13

how can distilation be used to purify a compound

Answer 13

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

Question 14

what is heating under reflux

Answer 14

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 15

method of carrying out heating under reflux

Answer 15

When carrying out heating under reflux, 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 16

what is vacuum filtration

Answer 16

Vacuum filtration involves carrying out a filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate. A Büchner, Hirsch or sintered glass funnel can be used during vacuum filtration.

Question 17

how is the solvent of recrystallisation chosen

Answer 17

The solvent for recrystallisation is chosen so that the compound being purified is completely soluble at high temperatures and only sparingly soluble at lower temperatures.

Question 18

what is solvent extraction

Answer 18

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

Question 19

the technique of recrystalisation

Answer 19

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 20

what happens during a solvent extraction

Answer 20

When carrying out a solvent extraction, 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 21

how is the ratio of solute in each layer determined (solvent extraction)

Answer 21

The ratio of solute dissolved in each layer is determined by the equilibrium constant, K. 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 22

what are the qualities needed in a solvent for solvent extraction

Answer 22

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

Question 23

melting point and mixed melting point

Answer 23

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 24

how can the identity of a pure compound be confirmed

Answer 24

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 25

what can the determination of the melting point of a compound provide

Answer 25

Determination of the melting point of a compound can give an indication of the purity of a compound

Question 26

what is the effect of the presence of impurities on the melting point

Answer 26

The presence of impurities in the compound lowers the melting point and broadens its melting temperature range due to the disruption in inter-molecular bonding in the crystal lattice.

Question 27

how do you determine a mixed melting point

Answer 27

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

Question 28

what can the melting point value and the range of the melting temperature can be used to determine

Answer 28

. 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 29

what is TLC

Answer 29

Chromatography is a technique used to separate the components present within a mixture. Chromatography separates substances by making use of differences in their polarity or molecular size.