Reactions of Inorganic Compounds in Aqueous Solution

Question 1

Give the colour of the metal-aqua ion Fe^(2+).

Answer 1

Green solution.

Question 2

Give the colour of the metal-aqua ion Co^(2+).

Answer 2

Pink solution.

Question 3

Give the colour of the metal-aqua ion Cu^(2+).

Answer 3

Blue solution.

Question 4

Give the colour of the metal-aqua ion Al^(3+).

Answer 4

Colourless solution.

Question 5

Give the colour of the metal-aqua ion Cr^(3+).

Answer 5

Violet/green solution.

Question 6

Give the colour of the metal-aqua ion Fe^(3+).

Answer 6

Yellow solution.

Question 7

Give the equation for the equilibrium that occurs between 2+ metal-aqua ions and water.

Answer 7

[M(H2O)6]^(2+) + H2O –> [M(H2O)5(OH)]^(+) + H3O^(+)

Question 8

Give the equation for the equilibrium that occurs between 3+ metal-aqua ions and water.

Answer 8

[M(H2O)6]^(3+) + H2O –> [M(H2O)5(OH)]^(2+) + H3O^(+)

Question 9

Are solutions containing 3+ or 2+ metal-aqua ions more acidic, and why?

Answer 9

3+, because they have a higher charge density (charge/size ratio). This means they are more polarising, and are better at attracting the electrons from the coordinate bond, which makes hte O-H bond weaker, meaning it is more likely a hydrogen ion will be released.

Question 10

Give the equation and describe any observations of the reaction between Fe^(2+) metal-aqua ions and OH^(-) ions.

Answer 10

[Fe(H2O)6]^(2+) + 2OH^(-) –> [Fe(H2O)4(OH)2] + 2H2O

This changes from a green solution to a green precipitate.

Question 11

Give the equation and describe any observations of the reaction between Co^(2+) metal-aqua ions and OH^(-) ions.

Answer 11

[Co(H2O)6]^(2+) + 2OH^(-) –> [Co(H2O)4(OH)2] + 2H2O

This changes from a pink solution to a blue-green precipitate.

Question 12

Give the equation and describe any observations of the reaction between Cu^(2+) metal-aqua ions and OH^(-) ions.

Answer 12

[Cu(H2O)6]^(2+) + 2OH^(-) –> [Cu(H2O)4(OH)2] + 2H2O

This changes from a blue solution to a blue precipitate.

Question 13

Give the equation and describe any observations of the reaction between Al^(3+) metal-aqua ions and OH^(-) ions.

Answer 13

[Al(H2O)6]^(3+) + 3OH^(-) –> [Al(H2O)3(OH)3] + 3H2O

This changes from a colourless solution to a white precipitate.

Question 14

Give the equation and describe any observations of the reaction between Cr^(3+) metal-aqua ions and OH^(-) ions.

Answer 14

[Cr(H2O)6]^(3+) + 3OH^(-) –> [Cr(H2O)3(OH)3] + 3H2O

This changes from a violet/green solution to a green precipitate.

Question 15

Give the equation and describe any observations of the reaction between Fe^(3+) metal-aqua ions and OH^(-) ions.

Answer 15

[Fe(H2O)6]^(3+) + 3OH^(-) –> [Fe(H2O)3(OH)3] + 3H2O

This changes from a yerllow solution to a brown precipitate.

Question 16

Which metal-aqua ions react further than an insoluble precipitate with excess OH- ions, and what complexes do they form? Give the colour of these complexes.

Answer 16

Al^(3+) and Cr^(3+).
[Al(H2O)2(OH)4]^(-) is a colourless solution.
[Cr(OH)6]^(3-) is a green solution.

Question 17

Define a Lewis acid.

Answer 17

An electron pair acceptor.

Question 18

Define a Lewis base.

Answer 18

An electron pair donor.

Question 19

What must be formed for a reaction to be a Lewis acid-base reaction?

Answer 19

A coordinate bond.

Question 20

What must a molecule or ion have to be a Lewis base?

Answer 20

A lone pair of electrons.

Question 21

What is formed when we put a transition metal ion in water?

Answer 21

A hexaaquaion.

Question 22

If add small amounts of ammonia to a metal-aqua ion solution, what type of reaction will we see?

Answer 22

The same reaction we would see when adding small amounts of OH- ions, because ammonia forms an equilibrium with water to form these.

Question 23

Which metal-aqua ions react further than an insoluble precipitate with excess ammonia?

Answer 23

Co^(2+), Cu^(2+) and Cr^(3+).

Question 24

Give the equations and any observations for the reaction of Co^(2+) metal-aqua ions with excess ammonia.

Answer 24

[Co(H2O)6]^(2+) + 2NH3 –> [Co(H2O)4(OH)2] + 2NH4^(+)
This changes from a pink solution to a blue-green precipitate.
[Co(H2O)4(OH)2] + 6NH3 –> [Co(NH3)6]^(2+) + 4H2O + 2OH^(-)
This changes colour from a blue-green precipitate to a straw coloured solution.

Question 25

Give the equations and any observations for the reaction of Cr^(3+) metal-aqua ions with excess ammonia.

Answer 25

[Cr(H2O)6]^(3+) + 3NH3 –> [Cr(H2O)3(OH)3] + 3NH4^(+)
This changes from a violet/green solution to a green precipitate.
[Cr(H2O)3(OH)3] + 6NH3 –> [Cr(NH3)6]^(3+) + 3H2O + 3OH^(-)
This changes from a green precipitate to a purple solution.

Question 26

Give the equations and any observations for the reaction of Cu^(2+) metal-aqua ions with excess ammonia.

Answer 26

[Cu(H2O)6]^(2+) + 2NH3 –> [Cu(H2O)4(OH)2] + 2NH4^(+)
This changes from a blue solution to a blue precipitate.
[Cu(H2O)4(OH)2] + 4NH3 –> [Cu(H2O)2(NH3)4]^(2+) + 2H2O + 2OH^(-)
This changes from a blue precipitate to a deep blue solution.

Question 27

Give the equation and any observations for the reaction of Fe^(2+) metal-aqua ions with ammonia.

Answer 27

[Fe(H2O)6]^(2+) + 2NH3 –> [Fe(H2O)4(OH)2] + 2NH4^(+)

This changes from a green solution to a green precipitate.

Question 28

Give the equation and any observations for the reaction of Fe^(3+) metal-aqua ions with ammonia.

Answer 28

[Fe(H2O)6]^(3+) + 3NH3 –> [Fe(H2O)3(OH)3] + 3NH4^(+)

This changes from a yellow solution to a brown precipitate.

Question 29

Give the equation and any observations for the reaction of Al^(3+) metal-aqua ions with ammonia.

Answer 29

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

This changes from a colourless solution to a white precipitate.

Question 30

Give the equation and any observations for the reaction of Fe^(3+) metal-aqua ions with sodium carbonate.

Answer 30

2[Fe(H2O)6]^(3+) + 3CO3^(2-) –> 2[Fe(H2O)3(OH)3] + 3CO2 + 3H2O
This changes from a yellow solution to a brown precipitate.

Question 31

Give the equation and any observations for the reaction of Cr^(3+) metal-aqua ions with sodium carbonate.

Answer 31

2[Cr(H2O)6]^(3+) + 3CO3^(2-) –> 2[Cr(H2O)3(OH)3] + 3CO2 + 3H2O
This changes from a violet/green solution to a green precipitate.

Question 32

Give the equation and any observations for the reaction of Al^(3+) metal-aqua ions with sodium carbonate.

Answer 32

2[Al(H2O)6]^(3+) + 3CO3^(2-) –> 2[Al(H2O)3(OH)3] + 3CO2 + 3H2O
This changes from a colourless solution to a white precipitate.

Question 33

Give the equation and any observations for the reaction of Fe^(2+) metal-aqua ions with sodium carbonate.

Answer 33

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

This changes from a green solution to a green precipitate.

Question 34

Give the equation and any observations for the reaction of Cu^(2+) metal-aqua ions with sodium carbonate.

Answer 34

[Cu(H2O)6]^(2+) + CO3^(2-) –> CuCO3 + 6H2O

This changes from a blue solution to a green-blue precipitate.

Question 35

Give the equation and any observations for the reaction of Co^(2+) metal-aqua ions with sodium carbonate.

Answer 35

[Co(H2O)6]^(2+) + CO3^(2-) –> CoCO3 + 6H2O

This changes from a pink solution to a to a pink precipitate.

Question 36

Why is it that only MCO3 is formed, and not M2(CO3)3?

Answer 36

Because metal 2+ ions are much weaker acids, they aren’t able to give a H+ ion to the carbonate ion, react by the basic carbonate reaction.

Question 37

Which metal hydroxides show amphoteric character, and why?

Answer 37

Al^(3+) and Cr^(3+), because as well as dissolving in acids, like all metal hydroxides do, they can dissolve in a base by donating a H+ ion to the OH- ions in the base.

Question 38

Give the equation and observations for the equilibrium that forms between chromate ions and dichromate ions, and give the oxidation state of chromium in both cases.

Answer 38

Cr2O7^(-) + H2O –> 2CrO4^(2-) + 2H^(+)
The oxidation state of chromium stays at +6.
If the equilibrium lies over to the left, the solution is orange, and yellow if over to the right.

Question 39

What happens to the coordination number of a metal-aqua ion when a substitution reaction between water and ammonia occurs, and why?

Answer 39

It stays the same, because both the ligands are of a similar size and are both uncharged.

Question 40

What happens to the coordination number of a metal-aqua ion when a substitution reaction between water and choride ions occur, and why?

Answer 40

The coordination number changes (from 6 to 4 or vice versa) because the ligands are of different sizes and charges.

Question 41

Give an example of a ligand substitution reaction that does not reach completion.

Answer 41

Copper metal-aqua ions reacting with excess ammonia forms [Cu(H2O)2(NH3)4]^(2+).

Question 42

Which is more stable - a complex formed with a unidentate, bidentate or multidentate ligand, and what does this mean for the reaction?

Answer 42

Multidentate. This means that the reaction is much harder to reverse.

Question 43

Explain, using the chelate effect why some ligand substitution reactions are almost impossible to reverse.

Answer 43

When we substitute a unidentate ligand with a a ligand that can form more coordinate bonds, less multidentate ligands are needed than unidentate ligands, so there will be more particles on the right hand side of the reaction than the left. This means there is an increase in disorder, and therefore an increase in entropy. It is difficult to reverse these reactions because that would cause a decrease in entropy.