Aqueous Ions

Question 1

The Acid-Base Chemistry of Aqueous Transition Metal Ions: 3+ ions

Answer 1

3+ ions are more acidic
- Fe3+ is smaller and more highly charged than Fe2+
- So Fe3+ is more polarising
- This means Fe3+ strongly attracts the lone pair on the Oxygen of the water ligands.
- In turn this weakens the O-H bond strength.
- This means the Fe(III) complex more easily releases H+ ions making the solution acidic.
- The Fe(II) complex doesn’t release H+ ions

[Fe(H2O)6]3+ → [Fe(H2O)5(OH)]2+ + H+

Question 2

[Fe(H2O)6]2+(aq) With Na2CO3

Answer 2

[Fe(H2O)6]2+ + CO32- → FeCO3(s) + 6H2O

Green solution → green precipitate

Question 3

[Fe(H2O)6]2+ With NaOH

Answer 3

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

Green solution → green precipitate

green precipitate will turn brown on standing as the oxygen in the air oxidises it from Fe2+ to Fe3+.

Question 4

[Fe(H2O)6]2 with excess NaOH

Answer 4

No further change

Question 5

[Fe(H2O)6]2+ With NH3

Answer 5

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

Green solution → green precipitate

green precipitate will turn brown on standing as the oxygen in the air oxidises it from Fe2+ to Fe3+.

[Fe(H2O)4(OH)2] forms [Fe(H2O)3(OH)3]

Question 6

[Fe(H2O)6]2 with excess NH3

Answer 6

No further change

Question 7

[Cu(H2O)6]2+ With Na2CO3

Answer 7

[Cu(H2O)6]2+ + CO32- → CuCO3(s) + 6H2O

blue solution → blue green precipitate

Question 8

[Cu(H2O)6]2+ With NaOH

Answer 8

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

Blue solution → blue precipitate

Question 9

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

Answer 9

No further change

Question 10

[Cu(H2O)6]2+ With NH3

Answer 10

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

Blue solution → blue precipitate

Question 11

[Cu(H2O)6]2+ In excess NH3

Answer 11

[Cu(H2O)6]2+ + 4NH3 → [Cu(H2O)2(NH3)4]2+ + 4H2O
Blue solution → deep blue solution

As the excess is often added once the blue precipitate has formed the equation is often written:

[Cu(H2O)4(OH)2] (s) + 4NH3 → [Cu(H2O)2(NH3)4]2+ + 2H2O + 2OH-
blue precipitate → deep blue solution

Question 12

[Cu(H2O)6]2+ With HCl

Answer 12

[Cu(H2O)6]2+(aq) + 4Cl-(aq) → [CuCl4]2-(aq) + 6H2O(l)

Blue solution → Yellow solution (may look green)

The complex also goes from octahedral to tetrahedral.

Question 13

[Fe(H2O)6]3+ With Na2CO3

Answer 13

2[Fe(H2O)6]3+ + 3CO32- → 2[Fe(H2O)3(OH)3] (s) + 3CO2 + 3H2O

brown solution → brown precipitate and effervescence

Question 14

[Fe(H2O)6]3+ With NaOH

Answer 14

[Fe(H2O)6]3+ + 3OH- → [Fe(H2O)3(OH)3] (s) + 3H2O

brown solution → brown precipitate

Question 15

[Fe(H2O)6]3+ With NH3

Answer 15

[Fe(H2O)6]3+ + 3NH3→ [Fe(H2O)3(OH)3] (s) + 3NH4+

Yellow brown solution → brown precipitate

Question 16

[Al(H2O)6]3+ With Na2CO3

Answer 16

2[Al(H2O)6]3+ + 3CO32- → 2[Al(H2O)3(OH)3] (s) + 3CO2 + 3H2O

Colourless solution → White precipitate and effervescence

Question 17

[Al(H2O)6]3+ With NaOH

Answer 17

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

Colourless solution → White precipitate

Question 18

[Al(H2O)6]3+ With excess NaOH

Answer 18

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

White precipitate dissolves to form a colourless solution.

Question 19

[Al(H2O)6]3+ With NH3

Answer 19

[Al(H2O)6]3+ + 3NH3 → [Al(H2O)3(OH)3] (s) + 3NH4+

Colourless solution → white precipitate

Question 20

Aluminium hydroxide acting as a amphoteric

Answer 20

Acting as a base (HCl added)
[Al(H2O)3(OH)3] (s) + 3H+ → [Al(H2O)6]3+

Acting as an acid (NaOH added)
[Al(H2O)3(OH)3] (s) + OH- → [Al(H2O)2(OH)4]- + H2O