6.3.1 Chromatography And Qualitative Analysis Flashcards
1
Q
what is thin layer chromatography (TLC)?
A
- 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
2
Q
what is gas chromatography?
A
- 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
3
Q
what is the separation for TLC?
A
- the stronger the adsorption (attraction) to the stationary phase the slower they are carried by the mobile phase
4
Q
what is the separation for gas chromatography?
A
- the greater the solubility in the stationary phase the slower they are carried by the mobile phase
5
Q
how to calculate the Rf value?
A
distance moved by component / distance moved by solvent front
6
Q
why are Rf values useful?
A
- 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
7
Q
what is retention time?
A
- 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
8
Q
how would you make a calibration curve for a gas chromatogram?
A
- 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
9
Q
what qualitative test would you use to show alkenes?
A
- add bromine water
- goes from orange to colourless
10
Q
test for haloalkanes
A
- add aqueous silver nitrate in ethanol
- chlorine forms white precipitate
- bromine forms cream precipitate
- iodine forms yellow precipitate
11
Q
test for phenols
A
- weak acidity (will react with NaOH)
- no reaction with Na2CO3
12
Q
test for carbonyls
A
- add 2,4-DNP (Brady’s)
- orange precipitate forms if carbonyl present
13
Q
test for aldehydes
A
- add Tollen’s reagent (ammoniacal silver nitrate)
- silver mirror forms
- no reaction for ketones
14
Q
test for primary and secondary alcohols and ketones
A
- heating with acidified potassium dichromate
- Cr(VI) -> Cr(III)
- orange to green
- ketones won’t be oxidised so colour will remain orange
15
Q
test for carboxylic acid
A
- carboxylic acid will react with Na2CO3 (unlike phenols)
