PAG 07: Qualitative analysis of organic functional groups

Question 1

What reagent is used to test for the presence of an alkene?

Answer 1

Bromine water

Question 2

What is observed when bromine water is added to an alkene?

Answer 2

The solution will change from orange to colourless

Question 3

What is observed when bromine water is added to a saturated hydrocarbon?

Answer 3

The solution remains orange

Question 4

Briefly describe how to test for haloalkanes

Answer 4

1. Heat the sample with a solution of sodium hydroxide under reflux to release any halide ions
2. Add some dilute nitric acid
3. Add silver nitrate solution and observe the colour of any precipitate formed

Question 5

What colour precipitate forms when silver nitrate solution reacts with chloride ions?

Answer 5

White

Question 6

What colour precipitate forms when silver nitrate solution reacts with bromide ions?

Answer 6

Cream

Question 7

What colour precipitate forms when silver nitrate solution reacts with iodide ions?

Answer 7

Yellow

Question 8

Describe the solubilities of the silver halide precipitates in dilute and concentrated ammonia

Answer 8

AgCl- soluble in dilute and concentrated ammonia
AgBr- soluble in concentrated ammonia
AgI- insoluble in ammonia

Question 9

Briefly describe how to test for carboxylic acids

Answer 9

Add some sodium carbonate solution to the sample. If a carboxylic acid is present, there will be effervescence and CO2 will be produced.
To test the gas, bubble it through limewater. CO2 will cause the limewater to turn cloudy.

Question 10

What reagent can be used to test for presence of an alcohol?

Answer 10

Acidified potassium dichromate solution

Question 11

What is observed when a primary or secondary alcohol is heated under reflux with acidified potassium dichromate solution?

Answer 11

There is a colour change from orange to green

Question 12

What is observed when a tertiary alcohol is heated under reflux with acidified potassium dichromate solution?

Answer 12

No colour change- solution remains orange

Question 13

What 2 reagents can be used to distinguish between an aldehyde and a ketone?

Answer 13

1. Fehling’s solution
2. Tollen’s solution

Question 14

What is observed when Fehling’s reagent is heated with:
a) an aldehyde?
b) a ketone?

Answer 14

a) brick red precipitate forms
b) no precipitate forms, solution remains a deep blue colour

Question 15

What is observed when Tollen’s solution is heated with:
a) an aldehyde?
b) a ketone?

Answer 15

a) silver mirror forms on surface of test tube
b) no change observed

Question 16

What is Brady’s reagent used to identify? Describe the positive result for this test

Answer 16

Used to identify carbonyls (aldehydes or ketones)
A yellow-orange precipitate forms in presence of a carbonyl

Question 17

Briefly describe test for phenol and positive test result for this

Answer 17

Add bromine water
If phenol is present, solution will change from orange to colourless and a white precipitate will form

Question 18

When would a water bath or electric heater b used instead of a bunsen burner to heat a sample?

Answer 18

• When temperature needs to be more controlled
• When the chemicals being heated are highly flammable