A-LevelChemistryChemical periodicity9701/42 · May/June 2022 · Q29 Marks

Explain why transition elements have variable oxidation states.

(a)Explain why transition elements have variable oxidation states.[1]
(b)Sketch the shape of a 3dxy orbital.[1]
(c)Explain why transition elements form coloured compounds.[3]
(d)(i)Aqueous solutions of copper(II) salts contain [Cu(H2O)6]2+ ions. Equilibrium 3 and equilibrium 4 show two reactions of these ions. equilibrium 3 [Cu(H2O)6]²+(aq) + 2OH¯(aq) ⇌ Cu(OH)2(s) + 6H2O(I) equilibrium 4 [Cu(H2O)6]²+(aq) + 4NH3(aq) = [Cu(NH3)4(H2O)2]²+(aq) + 4H2O(I) State the colour of Cu(OH)2(s) and [Cu(NH3)4(H2O)2]2+(aq).[1]
(d)(ii)Use Le Chatelier's principle to explain why a precipitate is formed when NaOH(aq) is added dropwise to [Cu(H2O)6]²+(aq).[1]
(e)There are two possible stereoisomers with the formula [Cu(NH3)4(H2O)2]2+. Draw three-dimensional diagrams to show the two stereoisomers.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[1] the d and s sub-shells / orbitals are close / similar in energy
[1] [Figure showing a 3dxy orbital]
[3] M1 (in a complex / in the presence of ligands) the d-orbitals are split into two sets of orbitals
[3] M2 as an electron is excited / promoted
[3] M3 visible light is absorbed AND colour seen is complementary
[1] Cu(OH)2: (pale / light) blue
[1] [Cu(H2O)2(NH3)4]2+: dark / deep blue
[1] (addition of OH-) increases [OH-] AND shifts equilibrium to the right
[2] M1 one correct 3D diagram with the six correct ligands
[2] M2 both 3D diagrams correct

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