6.1.3 chromatography and qualitative analysis

Question 1

what is the test for alkenes and what is the positive result

Answer 1

add equal parts of the substance you are analysing and bromine water. if bromine water turns from orange to colourless a c=c is present

Question 2

what is the test for haloalkane and what is the positive result

Answer 2

add 5 drops of unknown to a test tube. add 1cm3 of ethanol and 1cm3 aqueous silver nitrate. warm test tube in water bath.
if present a coloured precipitate forms
- chloroalkane forms white precipitate
- bromoalkane forms a cream precipitate
- iodoalkane forms a yellow precipitate

Question 3

what is the test for phenols and what is a positive result

Answer 3

1. add a small spatula of solid NaOH. the solid will dissolve is acid (phenol) is present. or use indicator to show acidity
2. add a solid carbonate if nothing happens phenol is present as only a strong acid will react with it

Question 4

what is the test for carboxylic acids and what is a positive result

Answer 4

1. add sodium carbonate and if gas is formed then bubble it through limewater. carboxylic acid is present the limewater turns cloudy

Question 5

what is chemical is bradys reagent

Answer 5

2,4-DNP (2,4-dinitrophenylhydrazine)

Question 6

test for aldehydes and ketones and what is the positive result

Answer 6

dissolve 2,4-DNP in sulfuric acid , water and methanol. add this to the unknown and shake. if an orange precipitate forms aldehydes and ketones are present.

Question 7

what is the test for aldehydes and what is the positive result

Answer 7

we use tollens reagent.
add silver nitrate solution and a few drops of NaOH. a brown precipitate should form. add dilute ammonia until the precipitate dissolves.
add unknown and place in a water bath. if solution forms a silver mirror an aldehyde is present

Question 8

test for primary and secondary alcohol

Answer 8

add acidified dichromate and heat in a water bath. colour change from orange to green if alcohol is primary or secondary

Question 9

what is thin layer chromatography

Answer 9

use a chromatography plate covered in a thin layer of silica gel. draw a pencil line near bottom and a spot of unknown. place in a beaker of solvent. product separate out. draw a pencil line where solvent ends up.

Question 10

what is the formula for rf values

Answer 10

distance travelled by spot / distance travelled by solvent

Question 11

what is gas chromatography

Answer 11

sample analysed is injected into a stream of gas which carries it through a coiled tube coated with liquid (oil) or solid.
it constantly dissolves and evaporates as it goes through. as it leaves the tube a peak is recorded

Question 12

what is retention time

Answer 12

time taken for substance to go through the tube and reach the detector

Question 13

what does the area under a peak show

Answer 13

relative amount of substance in mixture

Question 14

how do we determine concentration of a substance

Answer 14

create standard solutions of substance at different concentrations. run each solution through the gas chromatography machine and calculate area under each peak. plot a area concentration graph and use this to determine the concentration of the peak in the first gas chromatogram

Question 15

how does solubility effect retention time

Answer 15

it determines how long each component spends dissolved in the oil or solid and how long the take moving along the tube in the gas. a highly soluble substance will have a longer retention time

Question 16

how does boiling point effect retention time

Answer 16

a substance with a high boiling point will spend more time condensed as a liquid than as a gas so will take longer to travel through the tube.

Question 17

how does temperature effect retention time

Answer 17

a higher temperature means that substances will spend more time evaporated. it shortens all retention times.