A-LevelChemistryChemical periodicity9701/41 · Oct/Nov 2020 · Q512 Marks

A solution is made by dissolving CuSO4•5H2O in an excess of aqueous ammonia. This solution contains the copper complex [Cu(NH3)4]2+. The solution of [Cu(NH3)4]2+ is heated gently in a fume cupboard so that NH3 is released. Some NH3 remains in solution and some forms NH3 gas. The colour of the solution changes; a precipitate of Cu(OH)2 forms and is collected. A sample of Cu(OH)2 is added to concentrated hydrochloric acid. A reaction takes place forming a coloured copper complex, Y. A sample of Cu(OH)2 is added to dilute sulfuric acid. A reaction takes place forming a coloured copper complex, Z. [Cu(NH3)4]2+, Y and Z are different colours.

(a)(i)Write an expression for the Kstab of [Cu(NH3)4]2+.[1]
(a)(ii)State the colour of the solution of [Cu(NH3)4]2+.[1]
(b)Suggest an equation for the reaction of [Cu(NH3)4]2+ to form Cu(OH)2 as the aqueous solution of [Cu(NH3)4]2+ is heated.[1]
(c)Suggest an equation for the reaction of Cu(OH)2 with concentrated hydrochloric acid, forming Y.[2]
(d)Complete the table with the colour and geometry of complex Y and the colour, geometry and formula of complex Z. Y Z colour of complex geometry of complex formula of complex[2]
(e)Explain why complexes Y and Z are coloured and why their colours are different.[5]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[1] Kstab = [(Cu(NH3)4)2+] / ([(Cu(H2O)6)2+][NH3]4)
[1] deep / dark / royal blue
[1] [Cu(NH3)4]2+ + 2H2O → Cu(OH)2 + 2NH4+ + 2NH3 OR [Cu(NH3)4]2+ + 2H2O → Cu(OH)2 + 2H+ + 4NH3
[1] [CuCl4]2- complex including charge
[1] rest of equation fully correct
[2] (c) Cu(OH)2 + 4HCl → [CuCl4]2- + 2H2O + 2H+ OR Cu(OH)2 + 4Cl- + 2H+ → [CuCl4]2- + 2H2O
[1] one mark for any three cells
[1] two marks for all five cells
[2] (d) Y: colour of complex = yellow, geometry of complex = tetrahedral, formula of complex = [CuCl4]2-; Z: colour of complex = blue / pale blue, geometry of complex = octahedral, formula of complex = [Cu(H2O)6]2+
[1] M1: d orbitals splits into two sets of energy levels of different energy
[1] M2: wavelength / frequency / light / photon / hv absorbed
[1] M3: electron(s) promoted / excited
[1] M4: colour seen is complementary (to colour absorbed)
[1] M5: d-d energy gap / ∆E is different for Y and Z AND so different frequency / wavelength of light absorbed

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