Commomly Tested Eg

Question 1

When conc HCl added to CuSO4(aq), solution turns from blue to green to yellow.

Further addition of NH3(aq) causes yellow solution to turn deep blue

Answer 1

Ligand exchange; water molecules in [Cu(H2O)6]2+ are replaced by Cl- ions to form more stable [CuCl4]2-

Mixture of blue and yellow causes the solution to appear green;
Solution gradually turns yellow when [CuCl4]2- conc increases;

Addition of NH3 causes replacement of Cl- ions in [CuCl4]2- by NH3 molecules to form a more stable

[CuCl4]2- + 4NH3 <=> deep blue + 4Cl-

Question 2

Addition of NH3(aq) to CuSO4(aq) causes a blue ppt to form which dissolves in Xs NH3 to give a deep blue sol

Answer 2

NH3 undergoes hydrolysis to px OH-
NH3 + H2O <=> NH4+ + OH-

When a small amt of NH3 is added, a blue ppt of Cu(OH)2 is formed
Cu2+ + 2OH- <=> Cu(OH)2

Addition of Xs NH3 causes the formation of the more stable deep blue complex through ligand exchange reaction

[Cu(H2O)6]2+ + 4NH3 <=> [Cu(H2O)2(NH3)4]2+ + 4H2O