Precipitation Reactions of Transition Metals

Question 1

Cu2+ (aq) and NaOH

Answer 1

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

From pale blue solution to blue precipitate

Question 2

Fe2+ (aq) and NaOH

Answer 2

[Fe(H2O)6]2+(aq) + 2OH-(aq) –> [Fe(H2O)4(OH)2] + 2H2O
Pale green solution forms green precipitate
Darkens to a rusty brown on standing

Question 3

Fe3+(aq) and NaOH

Answer 3

[Fe(H2O)6]3+(aq) + 3OH-(aq) –> Fe(H2O)3(OH)3 + 3H2O(l)
yellow solution to orange/brown precipitate
Darkens on standing

Question 4

Mn2+(aq) and NaOH

Answer 4

[Mn(H2O)6]2+(aq) + 2OH-(aq) –> Mn(H2O)4(OH)2 + 2H2O(l)

Pale pink solution to buff/brown precipitate which darkens on standing

Question 5

Cr3+(aq) and NaOH

Answer 5

[Cr(H2O)6]3+(aq) + 3OH-(aq) –> Cr(H2O)3(OH)3 + 3H2O(l)
green solution to grey green precipitate
With excess NaOH the precipitate redissolves:
Cr(H2O)3(OH)3 + 3OH- (aq) –> [Cr(OH)6]3-(aq) + 3H2O(l)
Forms a dark green solution

Question 6

Why does the green precipitate turn rusty brown on standing for iron?

Answer 6

Green Fe2+ ions are readily oxidised to rusty brown Fe3+ ion

Question 7

Cu2+(aq) with NH3(aq)

Answer 7

[Cu(H2O)6]2+ (aq) + 2NH3(aq) –> Cu(H2O)4(OH)2 + 2NH4+(aq)
pale blue solution to blue precipitate which redissolves to a dark blue solution

Question 8

Fe2+(aq) and NH3(aq)

Answer 8

[Fe(H2O)6]2+ (aq) + 2NH3(aq) –> Fe(H2O)4(OH)2 + 2NH4+(aq)
pale green solution to dark green precipitate

Question 9

Fe3+(aq) and NH3(aq)

Answer 9

[Fe(H2O)6]3+ (aq) + 3NH3(aq) –> Fe(H2O)3(OH)3 + 3NH4+(aq)
yellow solution to rusty brown precipitate

Question 10

Mn2+(aq) and NH3(aq)

Answer 10

[Mn(H2O)6]2+ (aq) + 2NH3(aq) –> Mn(H2O)4(OH)2 + 2NH4+(aq)
pale pink solution to brown precipitate which darkens on standing

Question 11

Cr3+(aq) and NH3(aq)

Answer 11

[Cr(H2O)6]3+ (aq) + 3NH3(aq) –> Cr(H2O)3(OH)3 + 3NH4+(aq)
Green solution to (grey-)green precipitate
With excess NH3:
Cr(H2O)3(OH)3 + 6NH3(aq) –> [Cr(NH3)6]3+(aq) + 3H2O(l) + 3OH-(aq)
forms a purple solution

Question 12

Equation of oxidation of precipitate formed by Mn2+

Answer 12

4Mn(OH)2 + O2 –> 2Mn2O3 + 4H2O