[24.4] ligand substitution and precipitation

Question 1

what is a ligand substitution reaction?

Answer 1

when one ligand in a complex ion is replaced by another ligand

Question 2

what is a precipitation reaction?

Answer 2

when two aqueous solutions containing ions react together to form an insoluble ionic solid called a precipitate

Question 3

precipiation reaction with some ammonia: [Cu(H₂O)₆]²⁺

Answer 3

• [Cu(H₂O)₆]²⁺ (aq) + 2NH₃ (aq) -> [Cu(H₂O)₄(OH)₂] (s) + 2NH₄⁺ (aq)
• pale blue solution -> blue precipitate

Question 4

reaction with excess ammonia: [Cu(H₂O)₆]²⁺

Answer 4

• [Cu(H₂O)₄(OH)₂] (s) + 4NH₃ (aq) -> [Cu(H₂O)₂(NH₃)₄]²⁺ (aq) + 2H₂O (l) + 2OH⁻ (aq)
• blue precipitate -> dark blue solution

Question 5

ligand subsitution with ammonia overall: [Cu(H₂O)₆]²⁺

Answer 5

• [Cu(H₂O)₆]²⁺ (aq) + 4NH₃ (aq) -> [Cu(NH₃)₄(H₂O)₂]²⁺ (aq) + 4H₂O (l)
• pale blue solution -> dark blue solution

Question 6

precipitation reaction with ammonia: [Fe(H₂O)₆]²⁺

Answer 6

• [Fe(H₂O)₆]²⁺ (aq) + 2NH₃ (aq) -> [Fe(H₂O)₄(OH)₂] (s) + 2NH₄⁺ (aq)
• green solution -> green precipitate
• insoluble with excess ammonia

Question 7

precipitation reaction with ammonia: [Mn(H₂O)₆]²⁺

Answer 7

• [Mn(H₂O)₆]²⁺ (aq) + 2NH₃ (aq) -> [Mn(H₂O)₄(OH)₂] (s) + 2NH₄⁺ (aq)
• pale pink solution -> pale brown precipitate
• insoluble in excess ammonia

Question 8

precipitation reaction with ammonia: [Fe(H₂O)₆]³⁺

Answer 8

• [Fe(H₂O)₆]³⁺ (aq) + 3NH₃ (aq) - > [Fe(H₂O)₃(OH)₃] (s) + 3NH₄⁺ (aq)
• yellow solution -> brown precipitate
• insoluble in excess ammonia

Question 9

precipitation reaction with some ammonia: [Cr(H₂O)₆]³⁺

Answer 9

• [Cr(H₂O)₆]³⁺ (aq) + 3NH₃ (aq) -> [Cr(H₂O)₃(OH)₃] (s) + 3NH₄⁺ (aq)
• violet solution -> grey-green precipitate

Question 10

reaction with excess ammonia: [Cr(H₂O)₆]³⁺

Answer 10

• [Cr(H₂O)₃(OH)₃] (s) + 6NH₃ (aq) -> [Cr(NH₃)₆]³⁺ (aq) + 3H₂O (l) + 3OH⁻ (aq)
• grey-green precipitate -> purple solution

Question 11

ligand substitution with ammonia overall: [Cr(H₂O)₆]³⁺

Answer 11

• [Cr(H₂O)₆]³⁺ + 6NH₃ -> [Cr(NH₃)₆]³⁺ + 6H₂O (l)
• violet -> purple

Question 12

precipitation reaction with NaOH: [Cu(H₂O)₆]²⁺

Answer 12

• [Cu(H₂O)₆]²⁺ (aq) + 2OH⁻ (aq) -> [Cu(H₂O)₄(OH)₂] (s) + 2H₂O (l)
• pale blue solution -> blue precipitate
• insoluble in excess NaOH

Question 13

precipitation reaction with NaOH: [Fe(H₂O)₆]²⁺

Answer 13

• [Fe(H₂O)₆]²⁺ (aq) + 2OH⁻ (aq) -> [Fe(H₂O)₄(OH)₂] (s) + 2H₂O (l)
• green solution -> green precipitate
• insoluble in excess NaOH

Question 14

precipitation reaction with NaOH: [Mn(H₂O)₆]²⁺

Answer 14

• [Mn(H₂O)₆]²⁺ (aq) + 2OH⁻ (aq) -> [Mn(H₂O)₄(OH)₂] (s) + 2H₂O (l)
• pale pink solution -> pale brown precipitate
• insoluble in excess NaOH

Question 15

precipitation reaction with NaOH: [Fe(H₂O)₆]³⁺

Answer 15

• [Fe(H₂O)₆]³⁺ (aq) + 3OH⁻ (aq) -> [Fe(H₂O)₃(OH)₃] (s) + 3H₂O (l)
• yellow solution -> brown precipitate
• insoluble in excess NaOH

Question 16

precipitation reaction with NaOH: [Cr(H₂O)₆]³⁺

Answer 16

• [Cr(H₂O)₆]³⁺ (aq) + 3OH⁻ (aq) -> [Cr(H₂O)₃(OH)₃] (s) + 3H₂O (l)
• violet solution -> green precipitate

Question 17

reaction of [Cr(H₂O)₆]³⁺ with excess NaOH

Answer 17

• [Cr(H₂O)₃(OH)₃] (s) + 3OH⁻ (aq) -> [Cr(OH)₆]³⁻ (aq) + 3H₂O (l)

Question 18

ligand substitution with chloride ions: [Cu(H₂O)₆]²⁺

Answer 18

• [Cu(H₂O)₆]²⁺ (aq) + 4Cl⁻ (aq) ⇌ [CuCl₄]²⁻ (aq) + 6H₂O (l)
• pale blue solution -> yellow solution

Question 19

how and why does the shape / coordination number change in ligand substitution with chloride ions?

Answer 19

• octahedral shape -> tetrahedral shape
• 6 -> 4
• chloride ligands are bigger than water ligands so fewer chloride ligands can fit around the central Cu²⁺ ion

Question 20

explain the biochemical importance of ligand substitution involving oxygen and CO

Answer 20

• CO can bind to the Fe²⁺ ion in haemoglobin, forming carboxyhaemoglobin
• if CO is breathed in, a ligand substitution reaction takes palce where the oxygen in haemoglobin is replaced by CO
• if the concentration is carboxyhaemoglobin becomes too high, oxygen transport is prevented