Reactions of aqueous ions

Question 1

What is meant by enthalpy of hydration?

Answer 1

• standard enthalpy change
• when one mole of gaseous ions
• covered into one mole of aqueous ions

Question 2

enthalpy of hydration is…

Answer 2

• always exothermic
• bonds made with water molecules

Question 3

What is a co-ordinate bond?

Answer 3

• shared pair of electrons
• that have come from the same atom

Question 4

What is a ligand?

Answer 4

• ion/molecule with lone pair of electrons
• forms co-ordinate bond with transition metal ion

Question 5

What is a complex ion?

Answer 5

central atom/ion surrounded by ligands

Question 6

What is the co-ordination number?

Answer 6

number of co-ordinate bonds formed by transition metal ion

Question 7

What are the 4 hexa aqua ions?

Answer 7

[Fe(H2O)6] 2+

[Fe(H2O)6] 3+

[Cu(H2O)6] 2+

[Al(H2O)6] 3+

Question 8

What colour solution does Cu 2+ give when hydrated?

Answer 8

[Cu(H2O)6] 2+ (aq)
Blue solution

Question 9

What colour solution does Fe 2+ give when hydrated?

Answer 9

[Fe(H2O)6] 2+ (aq)
Pale green solution

Question 10

What colour solution does Fe 3+ give when hydrated?

Answer 10

[Fe(H2O)6] 3+ (aq)
Purple solution
may look yellow/brown

Question 11

What colour solution does Al 3+ give when hydrated?

Answer 11

[Al(H2O)6] 3+ (aq)
Colourless solution

Question 12

Why are the 3+ ions more acidic?

Answer 12

• Fe3+ smaller and more highly charged than Fe2+
• so Fe3+ more polarising
• Fe3+ attracts LP on oxygen of water ligands more strongly
• weakens O-H bond strength
• Fe3+ complex more easily releases H+
• Fe2+ does not release H+

Question 13

What is the equation which represents [Fe(H2O)6] 3+ acting as an acid?

Answer 13

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

OR

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

Question 14

What is the equation which represents [Al(H2O)6] 3+ acting as an acid?

Answer 14

[Al(H2O)6] 3+ —> [Al(H2O)5(OH)] 2+ + H+

OR

[Al(H2O)6] 3+ + H2O —> [Al(H2O)5(OH)] 2+ + H3O+

Question 15

Reaction of [Fe(H2O)6] 2+ with Na2CO3

Answer 15

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

green solution becomes green ppt FeCO3

Question 16

Reaction of [Fe(H2O)6] 2+ with NaOH

Answer 16

[Fe(H2O)6] 2+ + 2OH- —> [Fe(H2O)4(OH)2] (s) + 2H2O

green solution becomes green ppt [Fe(H2O)4(OH)2] (s)

• green ppt goes brown on standing in air
• oxygen oxidises Fe2+ to Fe3+

Question 17

Reaction of [Fe(H2O)6] 2+ with NH3

Answer 17

[Fe(H2O)6] 2+ + 2NH3 —> [Fe(H2O)4(OH)2] (s) + 2NH4+

• green solution becomes green ppt [Fe(H2O)4(OH)2] (s)
• green ppt goes brown on standing in air
• oxygen oxidises Fe2+ to Fe3+

Question 18

Reaction of [Cu(H2O)6] 2+ with Na2CO3

Answer 18

[Cu(H2O)6] 2+ + CO3 2- —> CuCO3 (s) + 6H2O

• blue solution becomes blue/green ppt

Question 19

Reaction of [Cu(H2O)6] 2+ with NaOH

Answer 19

[Cu(H2O)6] 2+ + 2OH- —> [Cu(H2O)4(OH)2] (s) + 2H2O

• blue solution becomes blue ppt [Cu(H2O)4(OH)2] (s)

Question 20

Reaction of [Cu(H2O)6] 2+ with NH3

Answer 20

[Cu(H2O)6] 2+ + 2NH3 —> [Cu(H2O)4(OH)2] (s) + 2NH4+

• blue solution becomes blue ppt

Question 21

Reaction of [Cu(H2O)6] 2+ in excess NH3

Answer 21

[Cu(H2O)6] 2+ + 4NH3 —> [Cu(H2O)2(NH3)4] 2+ (aq) + 4H2O

• blue solution becomes deep blue solution [Cu(H2O)2(NH3)4] 2+ (aq)

[Cu(H2O)6] 2+ + 4NH3 —> [Cu(H2O)2(NH3)4] 2+ (aq) + 2H2O + 2OH-

• blue solution becomes deep blue solution [Cu(H2O)2(NH3)4] (aq)

Question 22

Reaction of [Cu(H2O)6] 2+ with HCl

Answer 22

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

• blue solution —> yellow solution [CuCl4] 2- (aq)
• complex changes from octahedral to tetrahedral

Question 23

Reaction of [Fe(H2O)6] 3+ with Na2CO3

Answer 23

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

yellow/brown solution —> brown ppt and effervescence
- CO2 gas evolved

Question 24

Reaction of [Fe(H2O)6] 3+ with NaOH

Answer 24

[Fe(H2O)6] 3+ + 3OH- —> Fe(H2O)3(OH)3] (s) + 3H2O
yellow/brown solution —> brown ppt

Question 25

Reaction of [Fe(H2O)6] 3+ with NH3

Answer 25

[Fe(H2O)6] 3+ + 3NH3 —> [Fe(H2O)3(OH)3] + 3NH4+
yellow/brown solution —> brown ppt

Question 26

Reaction of [Al(H2O)6] 3+ with Na2CO3

Answer 26

2 [Al(H2O)6] 3+ + 3CO3 2- —> 2[Al(H2O)3(OH)3] (s) + 3CO2 + 3H2O

colourless solution —> white ppt and effervescence

Question 27

Reaction of [Al(H2O)6] 3+ with NaOH

Answer 27

[Al(H2O)6] 3+ + 3OH- —> [Al(H2O)3(OH)3] (s) + 3H2O

colourless solution —> white ppt [Al(H2O)3(OH)3] (s)

Question 28

Reaction of [Al(H2O)6] 3+ with excess NaOH

Answer 28

[Al(H2O)6] 3+ + OH- —> [Al(H2O)2(OH)4] - (s) + H2O

white ppt dissolves to colourless solution

Question 29

Reaction of [Al(H2O)6] 3+ with NH3

Answer 29

[Al(H2O)6] 3+ + 3NH3 —> [Al(H2O)3(OH)3] + 3NH4+

colourless solution —> white ppt

Question 30

Show how aluminium hydroxide is amphoteric when HCl is added

Answer 30

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

acts as a base

Question 31

Show how aluminium hydroxide is amphoteric when NaOH is added

Answer 31

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

acts as an acid

Question 32

Explain, with the use of an equation, why a solution containing [Al(H2O)6]3+ has
a pH ˂7

Answer 32

[Al(H2O)6]3+ ⇌ [Al(H2O)5(OH)] 2+ + H+

• Al 3+ has small size and high charge
• weakens OH bond releasing H+ ions

Question 33

Reaction of [Fe(H2O)6] 2+ with excess NH3

Answer 33

No further change

Question 34

Reaction of [Fe(H2O)6] 3+ with excess NH3

Answer 34

No further change

Question 35

Explain why an aqueous solution containing
[Fe(H2O)6]3+ ions has a lower pH than
an aqueous solution containing [Fe(H2O)6]2+ ions.

Answer 35

• Fe3+ is smaller (than Fe2+)
• Fe3+ ions are more polarising
• weaken O-H bonds in ligands more (in the Fe3+ solution) / more H+ released

Question 36

Explain, with the use of an equation, why a solution containing [Al(H2O)6]3+ has
a pH ˂ 7

Answer 36

[Al(H2O)6]3+ ⇌ [Al(H2O)5(OH)]2+ + H+
M2 Al3+ has a small size and high charge
M3 Weakens the OH bond (in water) releasing H+ ions