24.4 Ligand Substitution & Precipitation

Question 1

Appearance of aqueous Cu2+

Answer 1

Blue solution

Question 2

Appearance of aqueous Fe2+

Answer 2

Pale green solution

Question 3

Appearance of aqueous Fe3+

Answer 3

Pale yellow solution

Question 4

Appearance of aqueous Mn2+

Answer 4

Pale pink solution

Question 5

Appearance of aqueous Cr3+

Answer 5

Violet solution

Question 6

Observation when NaOH or NH3 is added DROPWISE to Cu2+

Answer 6

Blue precipitate

Question 7

Observation when NaOH or NH3 is added DROPWISE to Fe2+

Answer 7

Green precipitate (turns orange-brown on standing - oxidises to Fe3+)

Question 8

Observation when NaOH or NH3 is added DROPWISE to Fe3+

Answer 8

Orange brown precipitate

Question 9

Observation when NaOH or NH3 is added DROPWISE to Mn2+

Answer 9

Light brown precipitate (darkens on standing)

Question 10

Observation when NaOH or NH3 is added DROPWISE to Cr3+

Answer 10

Grey-green precipitate

Question 11

Observation when EXCESS NH3 is added to Cu2+

Answer 11

Precipitate dissolves to form dark blue solution

Question 12

Observation when EXCESS NH3 is added to Fe2+

Answer 12

Precipitate is insoluble

Question 13

Observation when EXCESS NH3 is added to Fe3+

Answer 13

Precipitate is insoluble

Question 14

Observation when EXCESS NH3 is added to Cr3+

Answer 14

Precipitate dissolves to form purple solution

Question 15

Observation when EXCESS NH3 is added to Mn2+

Answer 15

Precipitate is insoluble

Question 16

Observation when EXCESS NaOH is added to Cu2+

Answer 16

Precipitate is insoluble

Question 17

Observation when EXCESS NaOH is added to Fe2+

Answer 17

Precipitate is insoluble

Question 18

Observation when EXCESS NaOH is added to Fe3+

Answer 18

Precipitate is insoluble

Question 19

Observation when EXCESS NaOH is added to Mn2+

Answer 19

Precipitate is insoluble

Question 20

Observation when EXCESS NaOH is added to Cr3+

Answer 20

Precipitate dissolves to form dark green solution

Question 21

Equation for ligand substitution of [Cr(H2O)6]3+ with NH3, and associated colour changes

Answer 21

[Cr(H2O)6]3+ (aq) + 6NH3 (aq) ⇌ [Cr(NH3)6]3+ (aq) + 6H2O (l)

Octahedral violet solution → octahedral purple solution

Question 22

Equation for ligand substitution of [Cr(H2O)5 SO4]+ with NH3 and associated colour changes

Answer 22

[Cr(H2O)5 SO4]+ (aq) + 6NH3 (aq) ⇌ [Cr(NH3)6]3+ + 5H2O (l) + (SO4)2- (aq)

Octahedral green → octahedral purple

Question 23

Equation for ligand substitution of [Cu(H2O)6]2+ with NH3 and associated colour changes

Answer 23

For copper a partial exchange occurs:

[Cu(H2O)6]2+ (aq) + 4NH3 (aq) ⇌ [Cu(NH3)4 (H2O)2]2+ + 4H2O (l)

Pale blue → dark blue

Question 24

Equation for ligand substitution of [Cu(H2O)6]2+ with Cl- (from conc. HCl)

Answer 24

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

Octahedral pale blue → tetrahedral yellow

Question 25

Structure of haemoglobin

Answer 25

• 4 protein chains held together by weak intermolecular forces
• Each one contains a haem molecule
• Within this, central metal atom is Fe2+ which can bind to O2

Question 26

How does haemoglobin transport O2/CO2 around the body?

Answer 26

• O2 forms a weak dative bond with Fe2+ in the lungs
• Releases it at the body cells and binds to CO2
• CO2 carried back to lungs and is exhaled

Question 27

What happens if CO is breathed in?

Answer 27

• Ligand substitution takes place
• O2 replaced with CO, forming carboxyhaemoglobin
• CO binds very strongly so it prevents O2 from being carried
• Leads to death if [CO] becomes too high