A-LevelChemistryEquilibria9701/43 · 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⁺.

(a)(i)Write an expression for the Kstab of [Cu(NH₃)₄]²⁺. Kstab =[1]
(a)(ii)State the colour of the solution of [Cu(NH₃)₄]²⁺.[1]
(b)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. 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. 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]
[1] deep/dark / royal blue [1]
[1] [Cu(NH3)4]2+ + 2H2O → Cu(OH)2 + 2NH4+ + 2NH3 [1]
OR [Cu(NH3)4]2+ + 2H2O → Cu(OH)2 + 2H+ + 4NH3
[2] Cu(OH)2 + 4HCl → [CuC4]2- + 2H2O + 2H+
OR Cu(OH)2 + 4Cl- + 2H+ → [CuCl4]2⁻+ 2H2O
[CuC4]2- complex including charge [1]
rest of equation fully correct [1]
[2] Y: colour yellow, geometry tetrahedral
Z: colour blue / pale blue, geometry octahedral, formula [Cu(H2O)6]2+
one mark for any three cells [1]
two marks for all five cells [2]
[5] 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 / ΔE is different for Y and Z
AND so different frequency / wavelength of light absorbed • [1]

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