Reactions of Ethanal

Question 1

what are the 3 reactions of ethanal

Answer 1

reaction with acidified potassium permanganate
tollens reagent
fehlings reagent

Question 2

what is added to the test tube at the start of reaction with APP

Answer 2

dilute potassium permanganate
equal amount of sulphuric acid

Question 3

why is sulphuric acid added to the test tube in the reaction with APP

Answer 3

acidifying is needed to ensure complete reduction of Mn +7 to Mn+2 without forming the Mn+4 intermediate

Question 4

what does Mn+4 look like

Answer 4

brown ppt

Question 5

what is added to the acidified potassium permanganate

Answer 5

5 drops of ethanal + put in water bath for 5 min

Question 6

what is the control for the reaction with APP

Answer 6

propanone

Question 7

why is propanone used as a control in the reaction with APP

Answer 7

it is a ketone so it does not oxidise so there is no colour change

Question 8

what colour change is noted in the test tube in the reaction with APP

Answer 8

purple to colourless

Question 9

why is there a colour change from purple to colourless in the reaction with APP

Answer 9

ethanal is oxidised to ethanoic acid by the acidified potassium permanganate oxidising agent

Mn+7 (purple) reduced to Mn + 2(colouress)

Question 10

explain the colour change with reference to the ions in the reaction with APP

Answer 10

Mn+7 (purple) reduced to Mn + 2(colouress)

Question 11

why does the propanone (control) stay colourless in the reaction with app

Answer 11

ketones are not easily oxidised by potassium permanganate

Question 12

what is ethanal oxidised into in the reaction with APP

Answer 12

ethanoic acid

Question 13

write the balanced equation for the reaction with APP

Answer 13

MnO4- + 8H+ + 5e- ——> Mn+2 + 4H20

Question 14

why does tollens reagent (TR) have to be freshly prepared and never stored?

Answer 14

it forms explosive products

Question 15

how do you make tollens reagent

Answer 15

add 1cm3 o silver nitrate to a test tube + add one drop of sodium hydroxide solution, brown ppt forms
add dilute ammonia solution drop by drop until brown ppt dissolves

Question 16

what is the brown ppt that forms when making tollen reagent

Answer 16

silver oxide

Question 17

how do we ensure we thoroughly clean the test tube in the reaction with tollens reagent

Answer 17

rinse with detergent, water, and propanone

Question 18

why do we rinse the test tube with propanone

Answer 18

so it dries faster

Question 19

what is the reaction with tollens reagent also known as

Answer 19

the silver mirror test

Question 20

how do we form the silver mirror

Answer 20

pour tollens reagent into clean test tube
add 3 drops of ethanal
place into hot water bath or 5 min
note silver mirror forms on inside of the test tube

Question 21

control for reaction with tollens reagent

Answer 21

propanone

Question 22

why is no change observed when experiment is repeated with propanone (TR)

Answer 22

ketones are not easily oxidised by tollens reagent

Question 23

is tollens reagent a strong or weak oxidising agent

Answer 23

weak

Question 24

what oxidising agent will oxidise ketones

Answer 24

strong oxidising agents such as nitric acid

Question 25

what is happening in side the test tube in the silver mirror test

Answer 25

ethanal is oxidised to ethanoic acid ag+ is reduced to a silver ppt which coats the inside of the test tube

Question 26

write a balanced equation which shows what happens in the silver mirror test

Answer 26

Ag+ + e- ----> Ag(downward arrow) ethanal----> ethanoic acid

Question 27

what is fehlings reagent?

Answer 27

equal amounts of Fehlings A and B

Question 28

what colour is Fehlings reagent

Answer 28

royal blue

Question 29

what is Fehling's A

Answer 29

solution of copper sulphate in water

Question 30

what colour is fehlings A

Answer 30

blue

Question 31

what is fehlings B

Answer 31

potassium sodium tartrate and sodium hydroxide in water

Question 32

what colour is fehlings B

Answer 32

colourless

Question 33

what is fehlings reagent used for

Answer 33

it is an oxidising agent used to differentiate between aldehydes and ketones

Question 34

how does fehlings reagent help us to distinguish between aldehydes and ketones

Answer 34

aldehydes are oxidised by Fehling's reagent but ketones are not

Question 35

procedure for reaction with fehlings reagent

Answer 35

add 5 drops of ethanal into fehlings reagent and put in a hot water bath for a few min

Question 36

what colour ppt is formed from the reaction of ethanal with fehlings reagent

Answer 36

brick red ppt

Question 37

what is the colour change observed in the reaction with fehlings reagent

Answer 37

royal blue to brick red

Question 38

why is a brick red ppt formed when ethanal reacts with fehlings reagent

Answer 38

ethanal is oxidised to ethanoic acid Cu+2 ions (blue) have been reduced to Cu+ ions (red)

Question 39

why is there no colour change when propanone reacts with fehlings reagent (control)

Answer 39

ketones are not oxidised by fehlings reagent

Question 40

writes the balanced equation of the reaction with fehlings reagent

Answer 40

Cu+2 + e- -----> Cu+