Q2. * Properties Of Ethanal Flashcards
What three chemicals will you study the reactions of with ethanal?
Acidified potassium manganate (VII) solution
Fehling’s reagent
Ammoniacal silver nitrate
What are these tests repeated with?
Propanone (acetone)
Why are these tests repeated with propanone (acetone)?
To show that ketones are not easily oxidised
What is the main use of Fehling’s solution?
It can be used to distinguish between aldehydes and ketones
List the steps to the reaction of ethanal with acidified potassium permanganate.
(3)
Add about one-tenth of very dilute potassium permanganate solution to a test-tube
Add about five drops of ethanal to the test-tube
Place the tube in a beaker of hot water for a few minutes
Note the colour change from purple to colourless
How much acidified dilute potassium permanganate is needed?
About one-tenth of a test tube
How much ethanal should be added to the potassium permanganate?
About five drops
What should be done after the ethanal is added to the test tube of potassium permanganate?
It should be placed in hot water for five minutes
What colour change takes place when ethanal is heated with potassium permanganate?
purple to colourless
Why does the potassium permanganate and ethanal solution change from purple to colourless?
(2)
This is because the ethanal has been oxidised to ethanoic acid
The Mn(VII) ions have been reduced to Mn2+ ions
What happens when the potassium permanganate reaction is carried out with propanone?
There is no colour change
Why is there no colour change when the potassium permanganate reaction is carried out with propanone?
This is because the ketones are not oxidised by potassium permanganate
Describe the steps to the Fehling’s reagent reaction with ethanal.
(4)
Make up some Fehling’s reagent by mixing about one-tenth of a test-tube of Fehling’s A with Fehling’s B (should be royal blue)
Add about five drops of ethanal to the test tube
Stand the test-tube in a beaker of hot water for a few minutes
Note the brick-red colour
How do you make up some Fehling’s reagent?
Mix about one-tenth of a test-tube of Fehling’s A with the same amount of Fehling’s B (should be royal blue)
What colour is Fehling’s reagent?
A royal blue colour
What colour change takes place when ethanal is heated with Fehling’s reagent?
Royal - blue to brick red
Explain what has happened when the ethanal and Fehling’s reagent solution turns brick red.
(2)
Ethanal has been oxidised by Fehling’s reagent
The Cu2+ ions (blue) have been reduced to Cu+ ions (red) by the ethanal
What is the silver mirror test also called?
Ammoniacal silver nitrate test
Describe the steps to forming Tollens’ reagent.
3
Add one drop of dilute sodium hydroxide solution to 1 cm cubed of a silver nitrate solution
A brown precipitate of silver oxide is formed
Add dilute ammonia solution, drop by drop, until the brown precipitate dissolves
How much dilute sodium hydroxide solution is needed to form Tollens’ reagent?
One drop
What should the dilute sodium hydroxide be added to to form Tollens’ reagent?
1 cm cubed of a silver nitrate solution
How much silver nitrate solution do you need to make Tollens’ reagent?
1 cm cubed
What is formed when one drop of dilute sodium hydroxide solution is added to 1 cm cubed of a silver nitrate solution ?
A brown precipitate of silver oxide
What should be added once the silver oxide is formed when making Tollens’ reagent?
Dilute ammonia solution
Describe the steps to carrying out the silver mirror test.
5
Wash out a test-tube with detergent, then water then propanone to remove any droplets of water and allow to dry
Pour the Tollens’ reagent into the clean test-tube
Add about three drops of ethanal
Stand the test-tube in a beaker of boiling eater for a few minutes
A silver mirror is formed
What needs to be done to the test tube before the Tollens’ reagent is added?
(4)
Washed with detergent
Washed with ordinary water
Washed with propanone to remove any droplets of water
Allow to dry
How much ethanal is needed in the silver mirror test?
Three drops
What is formed when ethanal is heated with Tollens’ reagent?
A silver mirror
What happens when propanone is used in the Fehling’s reagent test?
No red precipitate is observed
What happens when propanone is used in the silver mirror test?
No change is observed
Why is no silver mirror formed with propanone?
This is because ketones are not oxidised by Tollens’ reagent as it is not a strong enough oxidising agent
