24. Reactions of inorganic compounds in aqueous solutions

Question 1

What is an aqua ion

Answer 1

When a transition metal forms a complex ion with 6 water molecules as ligands

Question 2

Why is a solution of Fe3+ acidic and Fe2+ not

Answer 2

• Fe3+ ion is smaller and more highly charged than Fe2+
• it is therefore more strongly polarising
• in the [Fe(H2O)6] 3+ (aq) ion the iron strongly attracts electrons from the oxygen atoms
• weakening O-H bonds in the water molecules
• this complex ion will then readily release an H+ ion
• Fe2+ is less polarising so fewer O-H bonds break in solution

Question 3

What is a hydrolysis reaction

Answer 3

A reaction in which O-H bonds of water are broken and new species are formed

Question 4

Write an equation for the hydrolysis of [Fe(H2O)6] 3+

Answer 4

[Fe(H2O)6] 3+ (aq) + H2O (l) —-> [Fe(H2O)5(OH)] 2+ (aq) + H3O+ (aq)

Question 5

The reaction of [Fe(H2O)6] 3+ releasing H+ ions can be described as what reaction

Answer 5

Hydrolysis

Question 6

Write an equation for the release of H+ ions from [Fe(H2O)6] 3+

Answer 6

[Fe(H2O)6] 3+ (aq)—-> [Fe(H2O)5(OH)] 2+ (aq) + H+ (aq)

Question 7

In the reaction [Fe(H2O)6] 3+ (aq) + H2O (l) —-> [Fe(H2O)5(OH)] 2+ (aq) + H3O+ (aq) what is the Fe3+ complex acting as

Answer 7

A Bronsted Lowry acid as it is donating a proton (H+) to a water molecule

Question 8

What are the two examples of [M(H2O)6] 3+

Answer 8

[Fe(H2O)6] 3+

[Al(H2O)6] 3+

Question 9

What are the two examples of [M(H2O)6] 2+

Answer 9

[Fe(H2O)6] 2+

[Cu(H2O)6] 2+

Question 10

What happens if you add a base (e.g OH-) to a M2+ or M3+ aqueous complex

Answer 10

It will remove protons from the aqueous complex

Question 11

What happens if you add a base (e.g OH-) to a M3+ aqueous complex

Answer 11

[M(H2O)6] 3+ (aq) + OH- (aq) —> [M(H2O)5(OH)] 2+ (aq) + H2O (l)
[M(H2O)5(OH)] 2+ (aq) + OH- (aq) —> [M(H2O)4(OH)2] + (aq) + H2O (l)
[M(H2O)4(OH)2] + (aq) + OH- (aq) —-> M(H2O)3(OH)3 (s) + H2O
It becomes a neutral metal (III) hydroxide precipitate

Question 12

What happens if you add a base (e.g OH-) to a M2+ aqueous complex

Answer 12

[M(H2O)6] 2+ (aq) + OH- (aq) —> [M(H2O)5(OH)] + (aq)
+ H2O (l)
[M(H2O)5(OH)] + (aq) + OH- —> M(H2O)4(OH)2 (s) + H2O (l)
It becomes a neutral metal (II) hydroxide precipitate

Question 13

What is the precipitate formed when a base (OH-) is added to a M3+ aqueous complex

Answer 13

M(H2O)3(OH)3

Question 14

What is the precipitate formed when a base (OH-) is added to a M2+ aqueous complex

Answer 14

M(H2O)4(OH)2

Question 15

Why does iron (III) carbonate not exist but iron (II) carbonate does

Answer 15

• because Fe3+ (aq) is more acidic than Fe2+ (aq)
• carbonate ion is able to remove protons from [Fe(H2O)6] 3+ to form hydrated iron (III) hydroxide
• cannot do this with [Fe(H2O)6] 2+

Question 16

What is the overall equation for the reaction between iron (III) aqua complex and carbonate ions

Answer 16

2[Fe(H2O)6] 3+ (aq) + 3CO3 2- (aq) —-> 2[Fe(H2O)3(OH)3] (aq) + 3CO2 (g) + 3H2O

Question 17

What is the overall equation for the reaction between iron (II) aqua complex and carbonate ions

Answer 17

[Fe(H2O)6] 2+ (aq) + CO3 2- (aq) —-> FeCO3 (s) +6H2O

Question 18

What is the general rule for the formation of carbonates with M2+ and M3+ aqua complexes

Answer 18

In general, carbonates of transition metal ions in oxidation state 2+ exist, whilst those of ions in the +3 state do not

Question 19

What is the colour of [Fe(H2O)6] 2+

Answer 19

pale green

Question 20

What is the colour of [Fe(H2O)6] 3+

Answer 20

pale brown

Question 21

Describe a test to distinguish [Fe(H2O)6] 2+ and [Fe(H2O)6] 3+

Answer 21

• add dilute alkali to both
• the precipitates of both of these complexes are far more distinguishable
• Fe(H2O)3(OH)3 (s) is brown
• Fe(H2O)4(OH)2 is green

Question 22

What does amphoteric mean

Answer 22

Showing both acidic and basic properties and can react with both acids and bases

Question 23

What is an example of an amphoteric hydroxide

Answer 23

Aluminium hydroxide

Question 24

Write an equation for the reaction between HCl and Al(H2O)3(OH)3

Answer 24

Al(H2O)3(OH)3 + 3HCl —> Al(H2O)6 3+ + 3Cl-

Question 25

Write an equation for the reaction between OH- and Al(H2O)3(OH)3

Answer 25

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

Question 26

Name two reasons other ligands would replace water ligands in an aqua ion complex

Answer 26

- the other ligand forms stronger co-ordinate bonds | - the other ligand are present in higher concentration and equilibrium is displaced

Question 27

List the properties of things that can replace water as a ligand

Answer 27

- water molecules may be replaced by other neutral ligands (e.g ammonia) - water molecules may be replaced by negatively charged ligands, such as chloride ions - water molecules may be replaced by bi-or multidentate ligands (chelation)

Question 28

What happens when a M2+ aqua ion by ammonia

Answer 28

- water molecules are replaced one at a time by ammonia - [M(H2O)6] 2+ +NH3 -----> [M(NH3)(H2O)5] 2+ + H2O - [M(NH3)(H2O)5] 2+ + NH3 ----> [M(NH3)2(H2O)4] 2+ H2O - and so on until - [M(NH3)6] 2+ is formed

Question 29

Write the overall equation for the reaction of | [M(H2O)6] 2+ and ammonia

Answer 29

[M(H2O)6] 2+ + 6NH3 [M(NH3)6] 2+ + 6H2O

Question 30

What is the complication in the reaction between [M(H2O)6] 2+ and ammonia

Answer 30

- ammonia is a base as well a ligand - so the OH- ions that are in ammonia (aq) might react with the [M(H2O)6] 2+ - a precipitate might form and dissolve

Question 31

Write the reaction where ammonia reacts as a base with [M(H2O)6] 2+

Answer 31

[M(H2O)6] 2+ (aq) + 2OH- (aq) ---> M(H2O)4(OH)2 (s) + 2H2O | M(H2O)4(OH)2 (s) + 6NH3 (aq) ----> [M(NH3)6] 2+ (aq) +4H2O (l) + 2OH- (aq)

Question 32

Write the equation for the reaction of [Co(H2O)6] 2+ and ammonia (conc.)

Answer 32

[Co(H2O)6] 2+ (aq) + 2NH3 (aq) ---> Co(H2O)4(OH)2 + 2NH4+ (aq) -here ammonia is acted as a base

Question 33

What is the colour of the precipitate formed when ammonia reacts as a base with [Co(H2O)6] 2+

Answer 33

Blue ppt.

Question 34

Write an equation if you add more excess ammonia to the products of the reaction between [Co(H2O)6] 2+ and ammonia (conc.)

Answer 34

Co(H2O)4(OH)2 (s) + 6NH3 (aq) ---> [Co(NH3)6] 2+ (aq) | 4H2O (l) + 2OH- (aq)

Question 35

Why does ammonia replace water as ligands in Co(H2O)4(OH)2 (s)

Answer 35

1. ammonia is a better ligand than water | 2. high concentration of ammonia displaces equilibria (displacing both (OH- and H2O))

Question 36

Describe the observations in the reaction between [Co(H2O)6] 2+ and ammonia

Answer 36

First blue ppt. which then dissolves to form a pale yellow solution.

Question 37

When aqueous copper ions react with ammonia in aqueous solution ligand replacement is only...

Answer 37

Partial

Question 38

How many ligands are replace when aqueous copper ions react with ammonia in aqueous solution

Answer 38

4

Question 39

What is the overall reaction between aqueous copper ions react with ammonia in aqueous solution

Answer 39

[Cu(H2O)6] 2+ + 4NH3 ---> [Cu(NH3)4(H2O)2] 2+ + 4H2O | -here ammonia is acting as a base

Question 40

What is the colour of [Cu(H2O)6] 2+ and [Cu(NH3)4(H2O)2] 2+

Answer 40

[Cu(H2O)6] 2+ is pale blue | [Cu(NH3)4(H2O)2] 2+ is deep blue

Question 41

What happens in the reaction between ammonia and [Cu(H2O)6] 2+

Answer 41

- ammonia first acts as a base - the precipitate [Cu(OH)2(H2O)4] (s) is produced - which is pale blue ppt. - when more ammonia is added the ppt. dissolves to form [Cu(NH3)4(H2O)2] 2+ (deep blue)

Question 42

Write the equation for when the ppt. [Cu(OH)2(H2O)4] dissolves in ammonia

Answer 42

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

Question 43

What is the shape of [Cu(NH3)4(H2O)2] 2+

Answer 43

- octahedral (as expected for six co-ordinate ion) | - the 4 ammonia molecules exist in square planar

Question 44

When aqueous copper ions react with concentrated hydrochloric acid there is a change in both...

Answer 44

- charge | - co-ordination number

Question 45

Write the equation for the reaction between chloride ions and [Cu(H2O)6] 2+

Answer 45

[Cu(H2O)6] 2+ + 4Cl- ---> [CuCl4] 2- + 6H2O

Question 46

What is the colour of [CuCl4] 2-

Answer 46

yellow

Question 47

Why does [Cu(H2O)6] 2+ have coordination number six and become [CuCl4] 2- with coordination number of only 4

Answer 47

Cl- is larger than H2O so fewer ligands can physically fit around the central copper ion

Question 48

What is chelation

Answer 48

Formation of complexes with multidentate ligand

Question 49

What are multidentate ligands

Answer 49

Ligands with more than one lone pair so they can form more than one co-ordinate bond

Question 50

Give 3 examples of multidentate ligands

Answer 50

- ethylene diamine - benzene- 1,2-diol - EDTA 4-

Question 51

Why are complexes usually more stable with multidentate ligands than monodentate ligands

Answer 51

There is a positive entropy change of the reaction

Question 52

What can ethylene diamine be represented as

Answer 52

en

Question 53

Write the equation for the reaction between en and [Cu(H2O)6] 2+

Answer 53

[Cu(H2O)6] 2+ (aq) + 3en ---> [Cu(en)3] 2+ (aq) + 6H2O | 4 entities to 7 meaning big increase in entropy

Question 54

Write the equation for the reaction between en and [Cu(H2O)6] 2+ (aq) and EDTA 4-

Answer 54

[Cu(H2O)6] 2+ (aq) + EDTA 4- (aq) ----> [CuEDTA] 2- (aq) + 6H2O

Question 55

Reaction between OH- and [Fe(H2O)6] 2+ (aq)

Answer 55

- green ppt. | - [Fe(H2O)4(OH)2]

Question 56

Reaction between OH- and [Cu(H2O)6] 2+ (aq)

Answer 56

- pale blue ppt. | - [Cu(H2O)4(OH)2]

Question 57

Reaction between NH3 and [Fe(H2O)6] 2+ (aq)

Answer 57

- green ppt. | - [Fe(H2O)4(OH)2]

Question 58

Reaction between little NH3 and [Cu(H2O)6] 2+ (aq)

Answer 58

- pale blue ppt. | - [Cu(H2O)4(OH)2]

Question 59

Reaction between excess NH3 and [Cu(H2O)6] 2+ (aq)

Answer 59

- deep blue solution | - [Cu(NH3)4(H2O)2] 2+

Question 60

Reaction between CO3 2- and [Fe(H2O)6] 2+ (aq)

Answer 60

- green ppt. | - FeCO3

Question 61

Reaction between CO3 2- and [Cu(H2O)6] 2+ (aq)

Answer 61

- green/blue ppt. | - CuCO3

Question 62

Reaction between OH- and [Fe(H2O)6] 3+ (aq)

Answer 62

- brown ppt. | - [Fe(H2O)3(OH)3]

Question 63

Reaction between little OH- and [Al(H2O)6] 3+ (aq)

Answer 63

- white ppt. | - [Al(H2O)3(OH)3]

Question 64

Reaction between excess OH- and [Al(H2O)6] 3+ (aq)

Answer 64

- colourless | - [Al(OH)4-]

Question 65

Reaction between NH3 and [Fe(H2O)6] 3+ (aq)

Answer 65

- brown ppt. | - [Fe(H2O)3(OH)3]

Question 66

Reaction between NH3 and [Al(H2O)6] 3+ (aq)

Answer 66

- white ppt. | - [Al(H2O)3(OH)3]

Question 67

Reaction between CO3 2- and [Fe(H2O)6] 3+ (aq)

Answer 67

- brown ppt. - [Fe(H2O)3(OH)3] - and CO2 bubbles

Question 68

Reaction between CO3 2- and [Al(H2O)6] 3+ (aq)

Answer 68

- white ppt. - [Al(H2O)3(OH)3] - and bubbles of CO2