A-LevelChemistryChemical bonding9701/42 · Feb/Mar 2021 · Q411 Marks

The transition elements are able to form stable complexes with a wide range of molecules and ions.

(a)State the meaning of transition element.[1]
(b)(i)Complete the diagram to show the arrangement of d orbital energy levels in octahedral and in tetrahedral complexes. [Figure 4.1][1]
(b)(ii)Sketch the shape of two d orbitals: • one d orbital from the lower energy level in an octahedral complex • one d orbital from the higher energy level in an octahedral complex. Use the axes below. [Figure 4.2][2]
(c)(i)Edds⁴⁻ and edta⁴⁻ are polydentate ligands that form octahedral complexes with Fe³⁺(aq). [Figure 4.3] The formulae of the complexes are [Fe(edds)]⁻ and [Fe(edta)]⁻ respectively. On the diagram of edds⁴⁻, circle each atom that forms a bond to the Fe³⁺ ion in [Fe(edds)]⁻.[1]
(c)(ii)[Fe(edds)]⁻ is red and [Fe(edta)]⁻ is yellow. Explain why the two complexes have different colours.[2]
(c)(iii)When edds⁴⁻(aq) is added to Fe³⁺(aq), the following reaction occurs. [Fe(H₂O)₆]³⁺(aq) + edds⁴⁻(aq) ⇌ [Fe(edds)]⁻(aq) + 6H₂O(l) State the type of reaction that occurs.[1]
(c)(iv)Write an expression for the stability constant, Kstab, of [Fe(edds)]⁻(aq). Kstab = ..........................[1]
(c)(v)The table shows the values for the stability constants, Kstab, of both complexes. complex Kstab / mol⁻¹ dm³ [Fe(edds)]⁻ 3.98 × 10²⁰ [Fe(edta)]⁻ 1.26 × 10²⁵ Predict which of the [Fe(edds)]⁻ and [Fe(edta)]⁻ complexes is more stable. Explain your answer with reference to the Kstab value for each complex.[1]
(c)(vi)When an excess of edta⁴⁻(aq) is added to [Fe(edds)]⁻(aq), the following equilibrium is established. [Fe(edds)]⁻(aq) + edta⁴⁻(aq) ⇌ [Fe(edta)]⁻(aq) + edds⁴⁻(aq) Calculate the equilibrium constant, Kc, for this equilibrium, using the Kstab values given in the table in (c)(v). Kc = ..........................[1]

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 (Feb/Mar 2021). The mark or mark type is shown in brackets.

[1] (element that forms one or more stable) ions with incomplete / partially filled 3d-orbitals / d-subshell
[1] Correct d-orbital splitting diagrams for octahedral and tetrahedral complexes, showing the correct relative energy levels (Oh: t2g lower, eg higher; Td: eg lower, t2g higher).
[2] M1: Sketch of a d-orbital from the lower energy level in an octahedral complex (e.g., dxy, dxz, dyz), showing electron density primarily between the axes.
[2] M2: Sketch of a d-orbital from the higher energy level in an octahedral complex (e.g., dx²-y² or dz²), showing electron density primarily along the axes.
[1] Circles around both nitrogen atoms and all four oxygen atoms that are part of the carboxylate groups on the edds⁴⁻ ligand.
[2] M1: The d-d energy gap / ΔE between the split d-orbitals is different for the two complexes.
[2] M2: This causes them to absorb different frequencies / wavelengths of visible light.
[1] ligand exchange / substitution / displacement / replacement
[1] Kstab = [Fe(edds)⁻] / ([Fe(H₂O)₆³⁺][edds⁴⁻])
[1] [Fe(edta)]⁻ is more stable as it has the higher Kstab value (1.26 × 10²⁵ > 3.98 × 10²⁰).
[1] Kc = Kstab([Fe(edta)]⁻) / Kstab([Fe(edds)]⁻) = (1.26 × 10²⁵) / (3.98 × 10²⁰) = 3.17 × 10⁴ (allow 3.1658 × 10⁴, minimum 2 sf)