Question

[Cu(H2O)6]2+ is light blue,by adding NH3 to the hexahydro spieces , it forms [Cu(NH3)4]2+ ang the colour turns dark blue why?

Answer 1

[ Cu(H2O)6 ]^+2 (aq) + 2 NH3(aq)--> [ Cu(H2O)4(OH)2 ](s)+ 2NH4+(aq)                                                              Copper (II) hydroxide

With [Cu(H2O)4(OH)2] the substitution is incomplete:

[Cu(H2O)6]^+2(aq) + 2NH3(aq) ----> [ Cu(H2O)4(OH)2 ](s)+ 2NH4+(aq)

=>Pale blue solution to Pale blue precipitate

[Cu(H2O)4(OH)2](s) + 4NH3(aq)---> [ Cu(NH3)4(H2O)2 ]^+2(aq) + 2H2O(l) + 2OH-(aq)

=>Pale blue precipitate to Dark blue solution