Reactions of Ions in Aqueous Solution 3.2.6

Question 1

How are metal-aqua complexes formed

Answer 1

When we add a transition metal compound to water

Question 2

what are metal aqua ions in solution .

Answer 2

Acidic in solution
The charge of the ion has an effect on how acidic it is

Question 3

What are 2 generic examples of acidity or hydrolysis reactions

Answer 3

[M(H2O)6]2+ + H2O (reverse sign) [M(H2O)5(OH)]+ + H3O+

[M(H2O)6]3+ + H2O (reverse sign) [M(H2O)5(OH)]2+ + H3O+

Question 4

Why are [M(H2O)6] 3+ more acidic than [M(H2O)6] 2+

Answer 4

• +3 metal ions have a higher charge density than 2+ ions . This is because they have a larger charge and they’re small
• +3 metal ions are more polarising . This means that they attract electrons in the O-H bond in the water ligands more strongly
• This weakens O-H bond so it breaks more easily releasing H+ ions

Question 5

Where does equilibrium lie when we add OH- ions to a hydrolysis reaction of metal aqua ions

Answer 5

If we add OH- they remove H3O+ and equilibrium shifts to the right

Question 6

How can we hydrolyse a metal aqua ion further

Answer 6

Use OH- ions to make insoluble metal hydroxides

Question 7

what are the equations on further hydrolysing a metal aqua ion using OH- to form a neutral complex that is solid and forms a precipitate in solution for +3 ions

Answer 7

Start from :
[M(H2O)6]3+ (aq) + H2O(l) (reverse sign) [M(H2O)5(OH)]2+ (aq) + H3O+ (aq)

Further hydrolysed to :

[M(H2O)5(OH)]2+ (aq) + H2O(l) (reverse sign) [M(H2O)4(OH)2]+ (aq) + H3O+ (aq)

Further hydrolysed to :

[M(H2O)4(OH)2]+ (aq) + H2O(l) (reverse sign) M(H2O)3(OH)3 (s) + H3O+ (aq)

Question 8

what are the equations for further hydrolysis of metal aqua ions using OH- to form a neutral , insoluble solid complex from 2+ metal aqua ions

Answer 8

Start from :
[M(H2O)6]2+ (aq) +H2O(l) (reverse sign) [M(H2O)5(OH)]+ (aq) +H3O+ (aq)

further hydrolyse to :

[M(H2O)5(OH)]+ (aq) +H2O(l) (reverse sign) M(H2O)4(OH)2 (s) +H3O+ (aq)

Question 9

what does amphoteric mean and what’s an example

Answer 9

Metal hydroxides can act as an acid or a base

Question 10

what does it mean when metal hydroxides are amphoteric

Answer 10

Will dissolve in both excess acid and base

Question 11

how can we reverse the reaction of amphoteric metal hydroxides precipitates

Answer 11

Add an acid (H+ ions) to the metal hydroxide precipitate and it will dissolve .
They do this by accepting a H+ ion and are bronsted lowry bases

Question 12

what’s an example of an amphoteric metal hydroxide

Answer 12

Aluminium hydroxide Al(H2O)3(OH)3

Question 13

what happens when we add a base to aluminium hydroxide (amphoteric molecule)

Answer 13

It acts as a bronsted lowry acid and donates H+ ions to react with the OH- and it dissolves

Question 14

what’s the reaction of adding a base to aluminium hydroxide (amphoteric)

Answer 14

Al(H2O)3(OH)3 (s) + OH- (aq) => [Al(H2O)2(OH)4]- (aq) + H2O(aq)

Question 15

what happens when we add an acid to aluminium hydroxide (amphoteric)

Answer 15

It acts as a bronsted lowry base by accepting H+ ions to form H3O+ in solution and dissolves

Question 16

what’s the reaction of adding an acid to aluminium hydroxide (amphoteric )

Answer 16

Al(H2O)3(OH)3 (s) + 3H+ (aq) => [Al(H2O)6] 3+ (aq)

Question 17

what happens when we add ammonia to metal aqua ions

Answer 17

They form metal hydroxide precipitates

Question 18

what happens when we add excess ammonia to metal aqua ions

Answer 18

A ligand substitution reaction occurs

Question 19

what’s the reaction of Cu(OH)2(H2O)4 when excess ammonia is added

Answer 19

Cu(OH)2(H2O)4(aq) + 4NH3(aq) (reverse sign) [Cu(NH3)4(H2O)2]2+ (aq) + 2OH-(aq) + 2H2O(l)

Question 20

whats the reaction when you add a 2+ metal aqua ion complex with a small amount of ammonia

Answer 20

[M(H2O)6]2+ (aq) + 2NH3(aq) => M(H2O)4 (OH2) (s) + 2NH4+ (aq)

Question 21

whats the reaction when you add a small amount of ammonia to a 3+ metal aqua complex

Answer 21

[M(H2O)6]3+ (aq) + 3NH3(aq) => Fe(H2O)3(OH)3 (s) + 3NH4+ (aq)

Question 22

what happens when you add sodium carbonate to a 2+ metal aqua ion

Answer 22

forms metal carbonates precipitates

Question 23

what’s the reaction of adding carbonates to a 2+ metal aqua ions

Answer 23

[M(H2O)6]2+ (aq) + CO3 2- (aq) (reverse sign) MCO3(s) + 6H2O(l)

Question 24

what’s the reaction with carbonates and a 3+ metal aqua ions

Answer 24

2 [M(H2O)6]3+ (aq) + 3CO3 2- (aq) (reverse sign) 2M(OH)3(H2O)3 (s) + 3CO2 (g) + 3 H2O (l)

Question 25

How do we perform test tube reactions to identify metal ion solutions

Answer 25

1) add a sample of the unknown metal ion solution into 3 test tubes
2) Drop by drop add NaOH to tube 1 and observe any changes .
Then add more to see if excess NaOH results in a further change .
3) Repeat step 2 except use ammonia solution in test tube 2
4) In tube 3 add sodium carbonate solution Na2CO3

Question 26

what is the observation when you add NaOH and excess NaOH to aluminium hydroxide

Answer 26

Al3+ ions in solution + NaOH = Precipitate formed
Add excess NaOH = precipitate dissolves

Question 27

what’s the observation when you add NH3 and NH3 in excess to copper hydroxide

Answer 27

Cu2+ ions in solution + NH3 = precipitate formed
Add excess NH3 = precipitate dissolves dark blue colour formed

Question 28

What is the observation with Al3+ and Fe3+ (3+ metal ions) reacting with carbonates

Answer 28

Form a brown precipitate and carbon dioxide so bubbles will be observed

Question 29

what’s the observation with Cu2+ and Fe2+ (2+ metal ions) reacting with carbonates

Answer 29

forms a green precipitate only

Question 30

why do solutions of Fe2+ forming Fe3+ need to be made fresh

Answer 30

Because Fe2+ oxidises readily with air to form Fe3+

Question 31

what’s the colour of Cu2+ in aqueous solution

Answer 31

Blue

Question 32

what’s the colour when you add small amount of OH- or NH3 to aqueous solution of Cu2+

Answer 32

Pale blue precipitate

Question 33

what’s the colour when you add excess NH3 to the precipitate of Cu2+

Answer 33

Dark blue solution

Question 34

what’s the colour when you add Na2CO3 to Cu2+

Answer 34

Green blue precipitate

Question 35

what’s the colour when you add excess OH- to the precipitate of Cu2+

Answer 35

Insoluble in excess NaOH
(no change )

Question 36

what’s the colour of Fe2+ in aqueous solution

Answer 36

Pale green

Question 37

what’s the colour of Fe3+ in aqueous solution

Answer 37

Yellow

Question 38

what’s the colour of Al3+ in aqueous solution

Answer 38

Colourless

Question 39

what’s the colour when you add small amount of OH- or NH3 to aqueous solution of Fe2+

Answer 39

Dirty Green precipitate

Question 40

what’s the colour when you add small amount of OH- or NH3 to aqueous solution of Fe3+

Answer 40

Orange precipitate

Question 41

what’s the colour when you add small amount of OH- or NH3 to aqueous solution of Al3+

Answer 41

White precipitate

Question 42

what’s the colour when you add excess OH- to the precipitate of Fe2+

Answer 42

Insoluble in excess NaOH
(No change)

Question 43

what’s the colour when you add excess OH- to the precipitate of Fe3+

Answer 43

Insoluble in excess NaOH
(No change)

Question 44

what’s the colour when you add excess OH- to the precipitate of Al3+

Answer 44

Colourless solution

Question 45

what’s the colour when you add excess NH3 to the precipitate of Fe2+

Answer 45

Insoluble in excess NH3
(No change)

Question 46

what’s the colour when you add excess NH3 to the precipitate of Fe3+

Answer 46

Insoluble in excess NH3
(no change)

Question 47

what’s the colour when you add excess NH3 to the precipitate of Al3+

Answer 47

No change

Question 48

what’s the colour when you add Na2CO3 to Fe2+

Answer 48

Green precipitate

Question 49

what’s the colour when you add Na2CO3 to Fe3+

Answer 49

Brown precipitate

Question 50

what’s the colour when you add Na2CO3 to Al3+

Answer 50

white precipitate