Transition Elements - 2 (ligands, and sub reactions, isomers...)

Question 1

Colour of Co2+ complex hydroxide precipitate and equation

Answer 1

Blue which turns beige in the presence of air

+ 2OH- -> (H2O)4(OH)2 + 2H2O

Question 2

Colour of Fe2+ hydroxide precipitate and equation

Answer 2

Green, then turns brick red in presence of O2

Question 3

Colour of Fe3+ hydroxide precipitate and equation

Answer 3

Brick red/ rusty brown

Question 4

Colour of Cu2+ hydroxide precipitate and equation

Answer 4

Blue

Question 5

What is a bidentate ligand?

Answer 5

A ligand which can donate two lone pairs of electrons to the central metal ion to form coordinate bonds

Question 6

Ligand substitution in complexes:

Cu 2+ and ammonia, first a small amount then in excess

Answer 6

A small amount: pale blue precip. of Cu(OH)2

Excess: precip. dissolves, deep blue solution + 4NH3 [Cu(NH3)4(H2O)2]2+ + 4H2O

Question 7

Cu 2+ and hydrochloric acid

Answer 7

Initially turns green then yellow solution

+ 4Cl- equil. [CuCl4]2- + 6H2O

Question 8

Co 2+ and conc. hydrochloric acid

Answer 8

Dark blue solution

+ 4Cl- equil. [CoCl4]2- + 6H2O

Question 9

Co 2+ and ammonia

Answer 9

Small amount: = 2NH3 -> (H2O)4(OH)2 + 2NH4+ green precip

Excess: start from same complex ion, + 6NH3 -> []6NH3 2+ + 6H2O beige then black on standing

Question 10

What is a coordinate bond?

Answer 10

Bond in which both electrons in shared pair are donated by the same bonding atom

Question 11

What is a ligand?

Answer 11

A molecule or ion which donates a pair of electrons to the central metal ion to form a coordinate bond

Question 12

What is coordination number?

Answer 12

The total number of coordinate bonds formed between a central metal ion and its ligands

Question 13

Why do transition metal ions form complex ions?

Answer 13

They are relatively small and have two or more charges, therefore their high charge density makes them strongly polarizing and able to attract lone pairs of electrons or ions on molecules

Question 14

Requirement to act as a ligand

Answer 14

At least one lone pair of electrons

Question 15

Formula and charges of following ligands:

1. Water
2. Ammonia
3. Thiocyanate
4. Cyanide
5. Chloride
6. Hydroxide

Answer 15

1. :OH2 & neutral
2. :NH3 & neutral
3. :SCN- & -1
4. :CN- & -1
5. :Cl- & -1
6. :OH- & -1

Question 16

What is a monodentate ligand?

Answer 16

A ligand that donates just one pair of electrons to the central metal ion to form one coordinate bond

Question 17

Using VSEPR, describe how coordination number influences the shape of complex ions

Answer 17

Electron pairs repel each other, coordinate bonds are bonding electron pairs which will repel each other equally and as far apart as possible to minimize repulsion. Thus, number of coordinate bonds will influence the shape of the complex ion

Question 18

What type of complex ions does cis/trans isomerism occur in?

Answer 18

Octahedral complex ions that contain four of one type of ligand and two of another
Square planar complex ions

Question 19

Difference between cis and trans complex ion isomers

Answer 19

In the cis, the two ligands which are the same are at adjacent corners and 90~ to one another
In the trans, the two ligands which are the same are at opposite corners and 180~ to one another

Question 20

What is a bidentate ligand?

Answer 20

A ligand that can donate two lone pairs of electrons to the central metal ion to form two coordinate bonds.

Question 21

What is a hexadentate ligand?

Answer 21

A ligand with 6 lone pairs of electrons, each forming a coordinate bond to a metal ion in a complex ion

Question 22

Which complex ions form optical isomers?

Answer 22

Octahedral complexes that contain multidentate ligands

Question 23

Requirements for optical isomers

Answer 23

(No plane of symmetry)

• A complex with three molecules or ions of a bidentate ligand
• A complex with two molecules or ions of a bidentate ligand, and two molecules or ions of a monodentate ligand
• A complex with one hexadentate ligand

Question 24

What is a ligand substitution?

Answer 24

A reaction in which one ligand in a complex ion is replaced by another ligand

Question 25

Reaction between copper (II) ions and ammonia

Answer 25

Small amount of ammonia: ammonia acts as base and blue precipitate of Cu(OH)2 is formed (from blue solution)
When excess ammonia is added, the pale blue precip dissolves and a deep blue solution is formed. Four of water ligands are replaced by ammonias.
[Cu(H2O)6]2+ + 4NH3 [Cu(NH3)4(H2O)2]2+ + 4H2O

Question 26

Reaction between copper (II) ions and conc. HCl

Answer 26

Blue solution turns green then yellow. When green, both the blue and yellow, aqua and the chloro complexes are present.
[Cu(H2O)6]2+ + 4Cl- [CuCl4]2- + 6H2O

Question 27

Cobalt (II) ions and conc. HCl

Answer 27

Pale pink solution turns dark blue.

[Co(H2O)6]2+ 4Cl- CoCl4 2- + 6H2O

Question 28

Cobalt (II) ions and ammonia

Answer 28

Small amount: Green precip of hydroxide
Excess: Straw coloured solution is formed
[Co(H2O)6]2+ + 6NH3 [Co(NH3)6]2+ + H2O

Question 29

What is the equation for Kstab?

Answer 29

[new complex]/[aqua complex][new ligands]^no. of moles of ligand in eqn.

Question 30

What does large Kstab value show about equilibrium?

Answer 30

The position of equilibrium lies to the right

Question 31

What does a large Kstab value show about stability and formation of ion?

Answer 31

Complex ions with larger Kstab values are more stable than those with lower ones, large stability shows that the ion is easily formed