Testing For Organic Functional Groups

Question 1

Test for alkenes

Answer 1

Add bromine water

Solution goes from orange -> colourless

Question 2

Test for an aldehyde

Answer 2

Test with tollens reagent a silver mirror should form

Test with fehlings reagent
Solution goes from blue -> red precipitate

Question 3

Test for carboxylic acid

Answer 3

Add sodium carbonate

Effervescence of carbon dioxide should evolve

Question 4

Test for primary alcohols

Answer 4

Warm solution with acidified potassium dichromate
Colour change from Orange -> green

Under distillation primary alcohols oxidise to aldehydes then carboxylic acids

Test for carboxylic acid: add sodium carbonate
If effervescence of co2 then it’s a primary alcohol

Test for aldehyde: Add Tollens reagent
If silver mirror is formed then it’s a primary alcohol
Add fehlings solution
If colour change from blue -> red precipitate then it’s a primary alcohol

Question 5

Test for secondary alcohols

Answer 5

Warm solution with acidified potassium dichromate
Colour change from Orange -> green

When secondary alcohols are oxidised under reflux a ketone is produced

Test with Fehlings and Tollens a negative results indicates the presence of the ketone and therefore it’s a secondary alcohol

Question 6

Test for a chloroalkane

Answer 6

Warm with silver nitrate solution (AgNO3)

AgCl (s) -> white precipitate
AgBr (s) -> cream precipitate
AgI (s) -> yellow precipitate

Question 7

Test for group 2 metal cations

Answer 7

Add dilute sodium hydroxide

MgCl2 + 2NaOH → Mg(OH)2 + 2NaCl
White precipitate formed

CaBr2 + 2NaOH → Ca(OH)2 + 2NaBr
White precipitate formed

SrCl2 + 2NaOH → Sr(OH)2 + 2NaCl
White precipitate

BaCl2 + 2NaOH → Ba(OH)2 + 2NaCl
Colourless solution formed

Question 8

Test for group 2 metal cations

Answer 8

Add dilute sulphuric acid

MgCl2 + H2SO4 → MgSO4 + 2HCl
Colourless solution

CaBr2 + H2SO4 → CaSO4 + 2HBr
White precipitate

SrCl2 + H2SO4 → SrSO4 + 2HCl
White precipitate

BaCl2 + H2SO4 → BaSO4 + 2HCl
White precipitate formed

Question 9

Test for ammonium ions

Answer 9

Warm with sodium hydroxide in a fume cupboard

NH4Cl + NaOH → NH3 + NaCl + H2O

Test ammonia gas produced with damp litmus paper the paper should turn blue

Litmus detects alkaline or acidic , Ammonia gas is alkaline therefore it turns blue

Question 10

Test for carbonate ions in aqueous solution

Answer 10

Add calcium hydroxide (limewater) to sodium carbonate

Limewater solution goes from
colourless -> cloudy

Question 11

Test for sulphate ions

Answer 11

Add dilute HCl and the barium chloride solution

Solution will turn cloudy and a white precipitate is formed

Question 12

Test for halide ions in aqueous solution

Answer 12

Add dilute nitric acid followed by silver nitrate

Potassium chloride- white precipitate
Potassium bromide- cream precipitate
Potassium iodide- yellow precipitate

To distinguish between similar colours add ammonia solution

Potassium chloride- dissolves forming colourless solution

Potassium bromide- only dissolves in concentrated ammonia solution

Potassium iodide- doesn’t dissolve at all reminds a yellow precipitate

Question 13

Testing halide ions using concentrated. H2SO4

Answer 13

Potassium chloride- solution remains white produces effervescence

Potassium bromide- solution turns dark brown
Brown gas /effervescence

Potassium iodide - solution turns bright orange
Effervescence

Question 14

Period II oxides

Answer 14

Reaction in air Flame

Na2O vigorous Yellow
MgO. vigorous Brilliant white
Al2O3. Slow N/A
SiO2. Slow N/A
P4O10 Spontaneous Brilliant white
Combustion
SO2. Burns steadily. Blue

Question 15

In aqueous solution

Fe(H2O)6 2+

Answer 15

Green solution

Question 16

In aqueous solution

Cu(H2O)6 2+

Answer 16

Blue solution

Question 17

In aqueous solution

Fe(H2O)6 3+

Answer 17

Pale violet but appears orange due to hydrolysis to Fe(H2O)5(OH) 2+

Question 18

In aqueous solution

Al(H2O)6 3+

Answer 18

Colourless solution

Question 19

Reaction of Fe(H2O)6 2+ with sodium hydroxide

Answer 19

Fe(H2O)4(OH)2
Green precipitate

Darkens on standing as its oxidised to Fe(H2O)3(OH)3

In excess= no further reaction

Question 20

Reaction of Cu(H2O)6 2+ with sodium hydroxide

Answer 20

Cu(H2O)4(OH)2

Blue precipitate formed

In excess= no further reaction

Question 21

Reaction of Fe(H2O)6 3+ with sodium hydroxide

Answer 21

Fe(H2O)3(OH)3

Brown precipitate formed

In excess = no further reaction

Question 22

Reaction of Al(H2O)6 3+ with sodium hydroxide

Answer 22

Al(H2O)3(OH)3

White precipitate formed

In excess = Al(H2O)2(OH)4 1-
Redissolves to give a colourless solution

Question 23

Reaction of Metal Aqua ions with NH3

Answer 23

Same as the relations with sodium hydroxide

Except in excess Cu(H2O)4(OH)2 forms Cu(H2O)2(NH3)4 2+
Which is a dark blue solution

Question 24

Reaction of Fe(H2O)6 2+ with CaCO3

Answer 24

FeCO3

Green precipitate

Question 25

Reaction of Cu(H2O)6 2+ with CaCO3

Answer 25

CuCO3 | Blue-green precipitate

Question 26

Reaction of Fe(H2O)6 3+ with CaCO3

Answer 26

Fe(H2O)3(OH)3 Brown precipitate and bubbles of gas (CO2)

Question 27

Reaction of Al(H2O)6 3+ with CaCO3

Answer 27

Al(H2O)3(OH)3 White precipitate and bubbles of gas (CO2)

Question 28

Substitution reaction of Fe(H2O)6 2+ with HCl

Answer 28

FeCl4 2- Yellow solution

Question 29

Substitution reaction of Cu(H2O)6 2+ with HCl

Answer 29

CuCl4 2- Green solution

Question 30

Substitution reaction of Fe(H2O)6 3+ with HCl

Answer 30

FeCl4 1- Yellow solution

Question 31

Substitution reaction of Al(H2O)6 3+ with HCl 11

Answer 31

AlCl4 1- Colourless solution

Question 32

Distinguishing between halogens

Answer 32

Add an aqueous solution of chlorine Displaces elements that are Lower Halogen In water With cyclohexane Cl2 Green. Green Br2. Orange. Orange I2. Brown. Violet Cl2 + 2Br- → Br2 + 2Cl- Cl2 + 2I- → I2 + 2Cl-

Question 33

NaF and NaCl with sulfuric acid

Answer 33

NaCl (s) +H2SO4 (l) → NaHSO4 (s) +HCl (g) Produces steamy HCl fumes NaF (s) +H2SO4 (l) → NaHSO4 (s) +HF (g) produces steamy HF fumes HF & HCl aren't strong enough reducing agents to reduce sulfuric acid so reaction stops Not a redox reaction, oxidation states remain the same

Question 34

NaBr with sulfuric acid

Answer 34

NaBr (s) +H2SO4 (l) → NaHSO4 (s) +HBr (g) Produces steamy HBr fumes HBr strong enough to reduce sulfuric acid so it reacts further 2HBr(aq) + H2SO4 (l) → Br2 (g) + SO2 (g) + 2H2O (l) Choking fumes of SO2 Orange fumes Br2

Question 35

NaI with sulfuric acid

Answer 35

NaI (s) +H2SO4 (l) → NaHSO4 (s) +HI (g) Produces steamy HI fumes HI strong enough to reduce sulfuric acid so it reacts further 2HI(aq) + H2SO4 (l) → I2 (g) + SO2 (g) + 2H2O (l) HI strongest reducing agent so further reduces SO2 6HI(g) + SO2 (g) → H2S (g) + 3I2 (s) + 2H2O (l) H2S toxic eggy smell I2 purple gas/ grey solid

Question 36

Halogen properties

Answer 36

Colour. State F2. Pale yellow. Gas Cl2. Green. Gas Br2 Red-brown Liquid I2. Grey Solid

Question 37

Displacement Reaction with chlorine water

Answer 37

Potassium chloride - no reaction Potassium bromide - colourless -> orange solution of Br2 formed Potassium iodide - colourless -> brown solution I2 formed

Question 38

Displacement reaction with bromine water

Answer 38

Potassium chloride - no reaction Potassium bromide - no reaction Potassium iodide - colourless -> brown solution of I2

Question 39

Displacement reaction with iodine solution

Answer 39

Potassium chloride - no reaction Potassium bromide - no reaction Potassium iodide - no reaction