Metal Aqua Ions

Question 1

Transition Metals Dissolving in Water

What does this form?

In General

How do water molecules do this?

Answer 1

The water molecules form co-ordinate bonds with the metal ions.

Metal-aqua complex ions.

Six water molecules form co-ordinate bonds with each metal ion.

By donating a non-bonding pair of electrons from their oxygen

Question 2

What happens in a solution containing metal-aqua 2+ ions?

What’s this called?

Describe the solution formed and explain?

Answer 2

There’s a reaction between the metal-aqua ion and the water.

Hydrolysis or acidity reaction.

The metal-aqua 2+ ions release H+ ions, so an acidic solution is formed.

There’s only a slight dissociation though, so the solution is only weakly acidic

Question 3

Describe the difference between 3+ metal-aqua ions and 2+ metal-aqua ions?

Answer 3

Metal 3+ ions are small but have a big charge.

So they’ve got a high charge density (ie charge/size ratio).

The metal 2+ ions have a much lower charge density

Question 4

What are metal-aqua 3+ ions compared to metal-aqua 2+ ions?

Explain

Describe the effect on the acidity of the solution?

Answer 4

3+ ions much more polarising than the 2+ ions.

The more polarising power means that they attract electrons from the oxygen atoms of the co-ordinate water molecules more strongly, weakening the O-H bond.

It’s more likely that a H+ ion will be released, and more H+ ions means a more acidic solution

Question 5

Describe further hydrolysis of metal-aqua ions?

Answer 5

Forms precipitates.

Adding OH- ions to solutions of metal-aqua ions produces insoluble metal hydroxides

Question 6

Amphoteric

Answer 6

Metal hydroxides that can act as both acids and bases.

This means they’ll dissolve in an excess of base as well as in acids

Question 7

Give an example of an amphoteric metal hydroxide

Answer 7

Aluminium hydroxide

Question 8

Describe formation of precipitates from ammonia

Answer 8

Use of ammonia solution to add OH- ions to a solution instead of a strong alkali.

Because OH- ions are formed, adding a small amount of ammonia solution gives the same results as sodium hydroxide

Question 9

Addition of excess ammonia to eg Cu (OH)2 (H2O)4

Explain what this forms

Answer 9

The H2O and OH- ligands are displaced by NH3 ligands.

This forms a charged complex which is soluble in water, so the precipitate dissolves

Question 10

Reaction of Metal Aqua Ions with Sodium Carbonate

Answer 10

Forms insoluble metal carbonates

Question 11

Effect of metal 3+ ions compared to 2+ ions on reactions with sodium carbonate

What is the result?

Answer 11

As metal 3+ ions are stronger acids, there’s a higher concentration of H3O+ ions in solution.

Rather than displacing water from the metal ions, the carbonate ions react with H3O+, removing them from the solution and shifting the equilibrium of reactions to the right.

So the precipitate that forms is M(OH)3 (H2O)3 instead of M2 (CO3)3