6. Ligand substitution and precipitation reactions (reactions of different ions with NaOH and NH3)

Question 1

Cu2+(aq) with NaOH

colour change

Answer 1

Blue precipitate

gelatinous

Question 2

Fe2+(aq) with NaOH

colour change

Answer 2

Pale green -> green precipitate turning a rusty brown at its surface on standing air

Question 3

Fe3+(aq) with NaOH

colour change

Answer 3

Pale yellow -> rusty brown precipitate

Question 4

Mn2+(aq) with NaOH

colour change

Answer 4

Brown precipitate which darkens in air as it oxidises

Question 5

Cr3+(aq) with NaOH

colour change

Answer 5

Green precipitate

re-dissolves in excess hydroxide

Question 6

Cu2+(aq) with ammonia

colour change

Answer 6

Blue precipitate (gelatinous) 
re-dissolves to a blue solution

Question 7

Fe2+(aq) with ammonia

colour change

Answer 7

Dark green precipitate

Question 8

Fe3+(aq) with ammonia

colour change

Answer 8

Rusty brown precipitate

Question 9

Mn2+(aq) with ammonia

colour change

Answer 9

Brown precipitate which darkens in air as it oxidises

Question 10

Cr3+(aq) with ammonia

colour change

Answer 10

Green precipitate

re-dissolves in excess ammonia

Question 11

Cu2+(aq) with NaOH

ionic equation

Answer 11

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

Question 12

Fe2+(aq) with NaOH

ionic equation

Answer 12

Fe2+(aq) + 2OH-(aq) -> Fe(OH)2(s)

Question 13

Fe3+(aq) with NaOH

ionic equation

Answer 13

Fe3+(aq) + 3OH-(aq) -> Fe(OH)3(s)

Question 14

Mn2+ with NaOH

ionic equation

Answer 14

Mn(H2O)6 + 2OH-(aq) -> Mn(OH)2(H2O)4 + 2H2O
oxidation can be summarised by:
Mn(OH)2 + O2 -> 2Mn2O3 + 4H2O

Question 15

Cr3+(aq) with NaOH

ionic equation

Answer 15

[Cr(H2O)6}3+(aq) + 3OH-(aq) -> Cr(OH)3(H2O)3 + 3H2O
Then
Cr(OH)3(H2O)3 + 3OH-(aq) -> [Cr(OH)6]3-(aq) + 3H2O

Question 16

Cu2+(aq) with ammonia

ionic equation

Answer 16

[Cu(H2O)6]2+(aq) + 2NH3(aq) -> Cu(OH)2(H2O)4 + 2NH4+(aq)

Question 17

Fe2+(aq) with ammonia

ionic equation

Answer 17

[Fe(H2O)6]2+(aq) + 2NH3(aq) -> [Fe(OH)2(H2O)4] + 2NH4+(aq)

Question 18

Fe3+(aq) with ammonia

ionic equation

Answer 18

[Fe(H2O)6]3+(aq) + 3NH3(aq) -> [Fe(OH)3(H2O)3}(s) + 3NH4+(aq)

Question 19

Mn2+(aq) with ammonia

ionic equation

Answer 19

[Mn(H2O)6]2+(aq) + 2NH3(aq) -> Mn(OH)2(H2O)4 + 2NH4+(aq)

Question 20

Cr3+(aq) with ammonia

ionic equation

Answer 20

[Cr(H2O)6]3+(aq) + 3NH3(aq) -> Cr(OH)3(H2O)3 + 3NH4(aq)
Then
Cr(OH)3(H2O)3 + 6NH3(aq) -> [Cr(NH3)6]3+(aq) + 3H2O + 3OH-(aq)