6.3.1 Chromatography And Qualitative Analysis

Question 1

what is thin layer chromatography (TLC)?

Answer 1

• stationary phase = solid e.g. aluminium oxide coating a sheet of glass
• mobile phase = liquid e.g. a solvent that moves vertically up the plate

Question 2

what is gas chromatography?

Answer 2

• stationary phase = solid or liquid on an inert solid support e.g. a long chain alkane coating the inside of a coiled tube
• mobile phase = carrier gas e.g. an inert gas like nitrogen

Question 3

what is the separation for TLC?

Answer 3

• the stronger the adsorption (attraction) to the stationary phase the slower they are carried by the mobile phase

Question 4

what is the separation for gas chromatography?

Answer 4

• the greater the solubility in the stationary phase the slower they are carried by the mobile phase

Question 5

how to calculate the Rf value?

Answer 5

distance moved by component / distance moved by solvent front

Question 6

why are Rf values useful?

Answer 6

• Rf values for a particular substance are always the same, if the conditions are the same, so can be used to indemnify the components of the mixture
• a chromatogram of the mixture can be run along side samples of known substances to allow easy identification by comparison

Question 7

what is retention time?

Answer 7

• time taken for a component to pass from the column inlet to the detector
• different compounds have different retention times so a retention time can be compared with those of known compounds to help identify it
• the area under the peak is proportional to the concentration of each component

Question 8

how would you make a calibration curve for a gas chromatogram?

Answer 8

• standard solutions of known concentration are prepared
• gas chromatograms are obtained for each standard solution
• the peak integration is found for each
• a graph of peak integration against concentration is plotted - this is the calibration curve
• compare the peak area of the compound being investigated with the calibration curve under the same conditions to find the concentration

Question 9

what qualitative test would you use to show alkenes?

Answer 9

• add bromine water
• goes from orange to colourless

Question 10

test for haloalkanes

Answer 10

• add aqueous silver nitrate in ethanol
• chlorine forms white precipitate
• bromine forms cream precipitate
• iodine forms yellow precipitate

Question 11

test for phenols

Answer 11

• weak acidity (will react with NaOH)
• no reaction with Na2CO3

Question 12

test for carbonyls

Answer 12

• add 2,4-DNP (Brady’s)
• orange precipitate forms if carbonyl present

Question 13

test for aldehydes

Answer 13

• add Tollen’s reagent (ammoniacal silver nitrate)
• silver mirror forms
• no reaction for ketones

Question 14

test for primary and secondary alcohols and ketones

Answer 14

• heating with acidified potassium dichromate
• Cr(VI) -> Cr(III)
• orange to green
• ketones won’t be oxidised so colour will remain orange

Question 15

test for carboxylic acid

Answer 15

• carboxylic acid will react with Na2CO3 (unlike phenols)