24.4 Ligand Substitution And Precipitation

Question 1

What is ligand substitution?

Answer 1

A reaction in which one ligand in a complex ion is replaced by another

Question 2

What is a precipitation reaction?

Answer 2

It is a reaction in which two aqueous solutions containing ions react to form an INSOLUBLE IONIC SOLID, called a precipitate.

Question 3

What are the colour observations for the precipitation of transition metal ions with aqueous NaOH(aq)?

Answer 3

1. Cu2+ + 2OH- -> Cu(OH)2
   PALE BLUE solution to PALE BLUE precipitate.
2. Fe2+ + 2OH- -> Fe(OH)2
   GREEN solution to GREEN precipitate.
3. Fe3+ + 3OH- -> Fe(OH)3
   YELLOW solution to ORANGE-BROWN precipitate.

(Fe2+ also oxidises in air to form the same ORANGE-BROWN PRECIPITATE of Fe3+.

Fe(OH)2 -> Fe(OH)3)

4. Mn2+ + 2OH- -> Mn(OH)2
   PALE PINK solution to LIGHT BROWN precipitate which darkens on standing in air.

All of these precipitates, except Fe2+, is INSOLUBLE in excess sodium hydroxide.

Question 4

What are the colour observations for the precipitation of Cr3+ ions with NaOH(aq)?

Answer 4

1. Cr3+ + 3OH- -> Cr(OH)3

VIOLET solution to GREY-GREEN precipitate.

2. The GREY-GREEN precipitate is further soluble in excess sodium hydroxide to form a DARK GREEN SOLUTION.

Cr(OH)3 + 3OH- -> [Cr(OH)6]3-

Question 5

How do Fe2+, Fe3+ and Mn2+ react with excess of aqueous ammonia?

Answer 5

They all react in the same way as they do with NaOH(aq), and form precipitates of:

Fe(OH)2, Fe(OH)3 and Mn(OH)2.

These precipitates do NOT dissolve in further ammonia.