Colour in transition metal complexesEdexcel International A Level Chemistry: Mind map

Exam questions on Colour in transition metal complexes

1. A teacher compares two aqueous solutions of the same concentration. The solution containing Cu²⁺(aq) ions is blue, whereas the solution containing Sc³⁺(aq) ions is colourless.
   Explain why the Cu²⁺(aq) solution appears blue.2 marks
2. Aqueous copper(II) ions form the pale blue complex [Cu(H₂O)₆]²⁺. Adding excess aqueous ammonia gives the deeper blue complex [Cu(NH₃)₄(H₂O)₂]²⁺, and adding concentrated hydrochloric acid gives the yellow-green complex [CuCl₄]²⁻.
   Explain why replacing water ligands by ammonia ligands changes the shade of blue.2 marks
3. Complexes of copper(I), such as [CuCl₂]⁻, are colourless, whereas complexes of copper(II), such as [Cu(H₂O)₆]²⁺, are blue. Cobalt(II) forms the pink complex [Co(H₂O)₆]²⁺ and the blue complex [CoCl₄]²⁻.
   Explain, with reference to electronic configurations, why copper(II) complexes are coloured but copper(I) complexes are colourless.3 marks