ORGANIC ANALYSIS

Question 1

What test can be used to identify primary, secondary and tertiary alcohols? (2)

Answer 1

using acidified potassium dichromate (VI), you can also use sodium metal

Question 2

what is the test to identify an alcohol using acidified potassium dichromate? (3)

Answer 2

1) add 10 drops of the alcohol to 2cm3 of acidified potassium dichromate solution in a test tube.
2) warm the mixture gently in a hot water bath
3) then watch for a colour change

Question 3

in the test to identify an alcohol what colour will the solution change if it is a primary alcohol? (1)

Answer 3

the orange solution slowly turns green as an aldehyde forms.

Question 4

in the test to identify an alcohol what colour will the solution change if it is a secondary alcohol? (1)

Answer 4

the orange solution slowly turns green as a ketone forms

Question 5

in the test to identify an alcohol what colour will the solution change if it is a tertiary alcohol? (1)

Answer 5

nothing happens (no reaction)

Question 6

what test can be used to distinguish between secondary and primary alcohols? (1)

Answer 6

you must repeat the experiment using acidified potassium dichromate

Question 7

what is the steps in the test to distinguish between a primary and secondary alcohol? (3)

Answer 7

1) add excess alcohol to 2cm3 of acidified potassium dichromate solution in a round bottomed flask
2) set up the flask as part of distillation apparatus
3) gently heat the flask, the alcohol will be oxidised and the product will be distilled off immediately so you can collect it.

Question 8

how can we test if the product produced in the further distillation of the primary and secondary alcohols is a ketone of aldehyde? (2)

Answer 8

using Fehling’s solution and/or benedict’s solution

Question 9

what are the steps of using the Fehling’s reagent and benedict’s solution test? (3)

Answer 9

1) add 2cm3 of the Fehling’s or benedict’s solution to a test tube. (it should be a clear blue solution)
2) add 5 drops of the aldehyde or ketone in the test tube
3) put the test tube in a hot water bath to warm it for a few minutes

Question 10

what colour should the solution be for the Fehling’s/benedict’s solution test if it is an aldehyde? (1)

Answer 10

the blue solution will give a brick red precipitate

Question 11

what colour should the solution be for the Fehling’s/benedict’s solution test if it is an ketone? (1)

Answer 11

nothing happens (no reaction)

Question 12

how do you make up a tollens reagent? (3)

Answer 12

1) put 2cm3 of 0.10mol dm-3 silver nitrate solution in a test tube
2) add a few drops of dilute sodium hydroxide solution, a light brown precipitate should form.
3) add drops of dilute ammonia solution until the brown precipitate dissolves completely.

Question 13

how will the solution react with the tollens reagent if it is an aldehyde? (1)

Answer 13

a silver mirror (a thin coating of silver) forms on the walls of the test tube

Question 14

how will the solution react with the tollens reagent if it is an ketone? (1)

Answer 14

nothing happens (no reaction)

Question 15

why must a solution containing an aldehyde or ketone be heated in a water bath? (1)

Answer 15

aldehydes and ketones are flammable so the test tube must be heated in a water bath rather than over a flame.

Question 16

why do aldehydes get a silver mirror? (1)

Answer 16

because the aldehyde reduces the Ag+ ions to silver atoms

Question 17

what test do we use for alkenes? (1)

Answer 17

use bromine water to test for the presence of double bonds

Question 18

what are the steps we use for the test of bromine water? (3)

Answer 18

1) add 2cm3 of the solution that you want to test to the test tube
2) add 2cm3 of bromine water to the test tube
3) shake the test tube

Question 19

what colour will the solution change if a alkene is present for the bromine test? (1)

Answer 19

the solution will decolourise (go from orange to colourless)

Question 20

what colour will the solution change if an alkene is not present for the bromine test? (1)

Answer 20

nothing happens (no reaction)

Question 21

what test do we do to identify carboxylic acids? (1)

Answer 21

use sodium carbonate, they react with carboxylic acids to form a salt, carbon dioxide and water.

Question 22

what are the steps of the test to identify carboxylic acids? (3)

Answer 22

1) add 2cm3 of the solution that you want to test to the test tube
2) add 1 small spatula of solid sodium carbonate
3) if the solution begins to fizz, bubble the gas that it produces through some limewater in a second test tube

Question 23

what colour will the solution turn if it is an carboxylic acid for the sodium carbonate test? (1)

Answer 23

the solution will fizz, the carbon dioxide that is produced will turn the limewater cloudy

Question 24

what colour will the solution turn if it is not an carboxylic acid for the sodium carbonate test? (1)

Answer 24

nothing happens (no reaction)