Question

When aqueous ammonia is added to CuSO₄ solution, the solution turns deep blue due to the formation of tetramminecopper (II) complex, \[\ce{[Cu(H2O)6]^{2+}_{( aq)} + 4NH3_{( aq)} ⇌ [Cu(NH3)4]^{2+}_{( aq)}}\], among HO₂ and NH₃ Which is stronger Lewis base.

Answer 1

• \[\ce{[Cu(H2O)6]^{2+}_{( aq)} + 4NH3_{( aq)} ⇌ [Cu(NH3)4]^{2+}_{( aq)} + H2}\]
• Nitrogen less electronegative than oxygen and donates its lone pair of electrons readily. Hence NH₃ is a stronger Lewis base.
• If a better Lewis base (ligand) is available, a Lewis acid (central metal ion) will react (Ligand exchange reaction)
• In this reaction, H₂O is exchanged with NH₃.
• The Lewis acid Cu²⁺ exchanges the Lewis base H with better Lewis base N-H₃ to form [Cu(NH₃)₄]²⁺
• Hence NH₃ is a stronger Lewis base than H₂O in this reaction.