Practical 3 - Reactions of transition metal ions with hydroxide ions

Question 1

[Cr(H2O)6]^3+
Addition of some OH-
Addition of excess OH-

Answer 1

Grey-green precipitate
Precipitate dissolved to give a green solution (Cr3+ is amphoteric)

Question 2

[Fe(H2O)6]^2+
Addition of some OH-
Addition of excess OH-

Answer 2

Green gelatinous precipitate
Precipitate does not dissolve in excess NaOH

Question 3

[Fe(H2O)6]^3+
Addition of some OH-
Addition of excess OH-

Answer 3

Orange gelatinous precipitate
Precipitate does not dissolve in excess NaOH

Question 4

[Cu(H2O)6]^2+
Addition of some OH-
Addition of excess OH-

Answer 4

Blue gelatinous precipitate
Precipitate does not dissolve in excess NaOH

Question 5

[Cu(H2O)6]2+ and OH- ionic equation

Answer 5

2Cu^2+ + 2OH- —> Cu(OH)2

Question 6

[Fe(H2O)6]^2+ and OH- ionic equation

Answer 6

Fe^2+ + 2OH- —> Fe(OH)2

Question 7

[Fe(H2O)6]^3+ and OH- ionic equation

Answer 7

Fe^3+ (aq) + 3OH- (aq) —> Fe(OH)3

Question 8

[Cr(H2O)6]^3+ and OH- ionic equation

Answer 8

Cr^3+ + 3OH- —> Cr(OH)3

Question 9

Cr(OH)3 and OH- ionic equation

Answer 9

Cr(OH)3 + 3OH- —> [Cr(OH)6]^3+

Question 10

What type of ion is Cr3+ and what does this mean?

Answer 10

Amphoteric
With excess OH-, the precipitate dissolves, proving that it’s amphoteric

Question 11

With what type of ion does the precipitate dissolve with excess OH-?

Answer 11

Amphoteric
E.g - Cr3+