A-LevelChemistryChemical bonding9701/41 · May/June 2018 · Q517 Marks

Copper is a transition element with atomic number 29.

(a)Complete the electronic configurations of a Cu atom and a Cu⁺ ion. Cu atom 1s22s22p6 Cu⁺ ion 1s22s22p6[2]
(b)Cu⁺ ions form a linear complex with Cl⁻ ions, which are monodentate ligands. Draw the structure of this complex and include its overall charge.[2]
(c)Cu²⁺ ions exist as [Cu(H2O)6]²⁺ complex ions in aqueous solution. Complete a three-dimensional diagram to show the shape of this complex. Name its shape. Label and state the value of one bond angle. [Figure for diagram][2]
(d)When NH3(aq) is added to Cu²⁺(aq), dropwise at first and then in excess, two chemical reactions occur as shown. dropwise NH3(aq) | excess NH3(aq) --- | --- [Cu(H2O)6]²⁺ A | B reaction 1 | reaction 2 For each reaction, describe what you would see and write an equation.[4]
(e)(i)EDTA⁴⁻ is a polydentate ligand. When a solution of EDTA⁴⁻ is added to a solution containing [Cu(H2O)6]²⁺ a new complex is formed. The formula of this complex is [CuEDTA]²⁻. Name the type of reaction occurring here.[1]
(e)(ii)Write an expression for the stability constant, Kstab, of [CuEDTA]²⁻ in this reaction.[1]
(e)(iii)The numerical value of the Kstab of [CuEDTA]²⁻ is 6.3 × 10¹⁹ at 298K. State what this tells us about the [CuEDTA]²⁻ complex ion.[1]
(f)(i)Ethanedioate ions, C₂O4²⁻, can act as a bidentate ligand. Explain what is meant by the term bidentate ligand.[2]
(f)(ii)When ethanedioate ions are added to a solution of zirconium ions, Zr⁴⁺, a complex ion containing four C2O4²⁻ ions and one Zr⁴⁺ ion is formed. All four ethanedioate ions act as bidentate ligands in this complex. Give the formula of this complex ion and explain why this complex is not octahedral.[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 2018). The mark or mark type is shown in brackets.

[1] 3s²3p63d104s1
[1] 3s²3p63d10
[1] Cl-Cu-Cl
[1] one minus charge
[1] [diagram with Cu at center, 6 H2O ligands with dative bonds, correct 3D representation like an octahedron with bonds shown to O atoms of water]
[1] octahedral and 90° or 180° labelled correctly on diagram as appropriate
[1] reaction 1: blue ppt / blue solid
[1] [Cu(H2O)6]2+ + 2OH− → Cu(OH)2 + 6H2O or [Cu(H2O)6]2+ + 2OH− → Cu(OH)2(H2O)4 + 2H2O or [Cu(H2O)6]2+ + 2NH3 → Cu(OH)2(H2O)4 + 2NH4+
[1] reaction 2: deep / dark / royal blue solution
[1] Cu(OH)2 + 4NH3 + 2H2O → [Cu(NH3)4(H2O)2]2+ + 2OH− or Cu(OH)2 + 4NH3 → [Cu(NH3)4]2+ + 2OH− or Cu(OH)2(H2O)4 + 4NH3 → [Cu(NH3)4(H2O)2]2+ + 2OH− + 2H2O
[1] ligand exchange / displacement / replacement / substitution
[1] Kstab = [[CuEDTA]2−] / [[Cu(H2O)6]2+][[EDTA]4−]
[1] stable / more stable than [Cu(H2O)6]2+
[1] donates lone pairs / forms dative / co-ordinate bonds to (central) metal atom / metal ion
[1] donates two lone pairs / forms two (dative or coordinate) bonds
[1] [Zr(C2O4)4]4−
[1] not octahedral because 8 dative bonds to Zr or not octahedral because not 6 dative bonds to Zr or not octahedral because co-ordination number is 8 / is not 6