A - Aqueous Solutions

Question 1

How do transition metal ions become hydrated in water?

Answer 1

Transition metal compounds dissolve in water and the water molecules form co-ordinate bonds with the metal ions. This forms metal-aqua complex ions.

Question 2

In general, how many water molecules form co-ordinate bonds with each metal ion?

Answer 2

6

Question 3

The overall charge of the metal-aqua complex is the same as what? Why?

Answer 3

The charge of the metal ion because water molecules are neutral.

Question 4

In aqueous solution, what metal-aqua ions do Fe, Cu and Al form?

Answer 4

[M(H2O)6]2+ for Fe and Cu.

[M(H2O)6]3+ for Al and Fe.

Question 5

Why is the acidity of [M(H2O)6]3+ greater than that of [M(H2O)6]2+?

Answer 5

Metal 3+ ions are relatively small but have a big charge giving them a high charge/size ratio. The metal 2+ ions have a much lower charge/size ratio.

This makes the 3+ ions more polarising than the 2+ ions. More polarising power means they attract electrons from the oxygen atoms of the co-ordinated water molecules more strongly, weakening the O-H bond.

Therefore, it is more likely that a H+ ion will be released, making the solution more acidic.

Question 6

Some metal hydroxides show amphoteric character. What does this mean?

Answer 6

They can dissolve in both acids and bases.

Question 7

Give an example of an amphoteric metal hydroxide.

Answer 7

Aluminium hydroxide.

Question 8

What colour solutions do the following metal-aqua ions make?

1. [Cu(H2O)6]2+
2. [Fe(H2O)6]2+
3. [Al(H2O)6]2+
4. [Fe(H2O)6]3+

Answer 8

1. Blue solution
2. Green solution
3. Colourless solution
4. Yellow solution

Question 9

Give the complex that forms and the colour of the precipitate when each of the following metal-aqua ions react with OH- or NH3:

1. [Cu(H2O)6]2+
2. [Fe(H2O)6]2+
3. [Al(H2O)6]3+
4. [Fe(H2O)6]3+

Answer 9

1. Cu(OH)2(H2O)4 - blue precipitate
2. Fe(OH)2(H2O)4 - green precipitate
3. Al(OH)3(H2O)3 - white precipitate
4. Fe(OH)3(H2O)3 - brown precipitate

Question 10

What happens to the Cu2+, Fe2+, Al3+ and Fe3+ metal-aqua ions when dissolve in excess OH-? Why is Al3+ different?

Answer 10

No change other than the Aluminium metal-aqua ion which dissolves to form a colourless solution [Al(OH)4(H2O)2]-, as it is amphoteric.

Question 11

What happens to the four metal-aqua ions I need to know about when reacted with excess NH3?

Answer 11

No change other than the copper metal-aqua ion where the precipitate dissolves to form a deep blue solution, [Cu(NH3)4(H2O)2]2+. Only the copper hydroxide will dissolve in excess ammonia because it undergoes a ligand exchange reaction.

Question 12

What happens to the four metal-aqua ions I need to know about when they react with sodium carbonate?

Answer 12

All form precipitates. CuCO3 (green-blue), FeCO3 (green), Al(OH)3(H2O)3 (white) and Fe(OH)3(H2O)3 (brown).

With M3+ ions, bubbles will also form (CO2). This can help to distinguish between Fe2+ and Fe3+ ions.