3.2.6 Reactions of ions in aqueous solution

Question 1

What are the colours of the following metal aqua ions before and after hydrolysis:
a) [Cu(H20)6]^2+
b) [Fe(H20)6]^2+
c) [Fe(H20)6]^3+
d) [Al(H20)6]^3+

Answer 1

a) Blue solution → blue ppt
b) Green solution → green ppt
c) Yellow solution → brown ppt
d) Colourless solution → white ppt

Question 2

Explain why an aqueous solution containing 3+ metal ions has a lower pH than an aqueous solution containing 2+ ions.

Answer 2

• The 3+ ion has a larger charge and is a smaller ion
• The 3+ ions are more polarising
• More O-H bonds in the water ligands break so more hydrogen ions are released

Question 3

How do the metal aqua ions react with NaOH?

Answer 3

2OH- + [M(H2O)6]^2+ → [M(H2O)4(OH)2] + 2H2O

3OH- + [M(H2O)6]^3+ → [M(H2O)3(OH)3] + 3H2O

Question 4

What happens when you add excess NaOH to the neutral complex ions and why?

Answer 4

Cu, Fe = no visible change
Al = white ppt dissolves

Aluminum neutral complex ion can act as a acid, continues to be hydrolysed as it is amphoteric.

Question 5

What happens to the neutral complex ion [Fe(H2O)4(OH)2]?

Answer 5

• Darkens over time (brown solid forms)
• Fe2+ oxidised to Fe3+

Question 6

How do you reverse the hydrolysis?

Answer 6

Add acid (H+)

Question 7

How do the metal aqua ions react with ammonia solution?

Answer 7

2NH3 + [M(H2O)6]^2+ → [M(H2O)4(OH)2] + 2NH4+

3NH3 + [M(H2O)6]^3+ → [M(H2O)3(OH)3] + 3NH4+

Question 8

What happens when you add excess NH3 to the neutral complex ions?

Answer 8

Al/Fe = no visible change
Cu2+ = dark blue solution (incomplete ligand substitution)

4NH3 + [Cu(H2O)4(OH)2] → 2H2O + 2OH- +[Cu(H2O)2(NH3)4]^2+

Question 9

How do the metal aqua ions react with sodium carbonate solution?

Answer 9

Precipitation reaction:
CO3^2- + [M(H2O)6]^2+ → MCO3 + 6H2O

Hydrolysis:
3CO3^2- + 2[M(H2O)6]^3+ → 2[M(H2O)3(OH)3] + 3CO2 + 3H2O

Question 10

What are the oservations when the 3+ metal aqua ions react with sodium carbonate and why?

Answer 10

Effervescene
- Carbon dioxide produced
- 3+ ions are stronger Lewis acids

Question 11

What are the observations when Na2CO3 is added to the hexaque ions?

Answer 11

[Cu(H2O)6]^2+ = green-blue ppt
[Fe(H2O)6]^2+ =
[Fe(H2O)6]^3+ = effervescence
[Al(H2O)6]^3+ = effervescence

Question 12

What is the observations when HCl is added to the hexaque ions?

Answer 12

[Cu(H2O)6]^2+ = yellow/green solution
[Fe(H2O)6]^2+
[Fe(H2O)6]^3+
[Al(H2O)6]^3+