Topic 28: Chemistry of transition elements

Question 1

Definitions I:
(a) Transition element
(b) Ligand
(c) Complex ion

Answer 1

Definitions I:
(a) d-block element forming ions with incomplete d orbitals, e.g., Cu²⁺.
(b) Species donating lone pair to metal, e.g., NH₃.
(c) Ion with central metal surrounded by ligands, e.g., [Cu(H₂O)₆]²⁺.

Question 2

Definitions II:
(a) Coordination number
(b) Degenerate d orbitals
(c) Stability constant

Answer 2

Definitions II:
(a) Number of ligands around a metal, e.g., 6 in [Co(NH₃)₆]²⁺.
(b) d orbitals of equal energy before splitting, e.g., in free ion.
(c) Equilibrium constant for complex formation, e.g., Kstab = [MLₙ]/[M][L]ⁿ.

Question 3

Sketch the shapes of:
(a) 3dxy orbital [1 Mark]
(b) 3dz² orbital [1 Mark]

Answer 3

(a) 3dxy: 4 lobes in xy plane, between axes (1 mark)
(b) 3dz²: 2 lobes along z-axis, ring in xy plane (1 mark)

Question 4

Why does Fe exhibit +2 and +3 oxidation states in its compounds? [2 Marks]

Answer 4

3d and 4s sub-shells close in energy (1 mark)
Electrons easily lost from both, e.g., Fe²⁺, Fe³⁺ (1 mark)

Question 5

In a lab, Cu²⁺(aq) turns from blue to deep blue with excess NH₃. Explain the reaction. [2 Marks]

Answer 5

[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O (1 mark)
Ligand exchange, NH₃ stronger ligand (1 mark)

Question 6

Predict the geometry and coordination number of [Co(NH₃)₆]²⁺. [2 Marks]

Answer 6

Octahedral, 120°/90° bond angles (1 mark)
Coordination number = 6 (1 mark)

Question 7

Calculate ΔG° for MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O (E°cell = 0.74 V, F = 96500). [2 Marks]

Answer 7

ΔG° = -nFE°cell, n = 5 (1 mark)
ΔG° = -5 × 96500 × 0.74 = -357050 J/mol = -357 kJ/mol (1 mark)

Question 8

Describe the splitting of d orbitals in:
(a) Octahedral [Co(H₂O)₆]²⁺ [1 Mark]
(b) Tetrahedral [CoCl₄]²⁻ [1 Mark]

Answer 8

(a) Octahedral: 3 lower (t₂g), 2 higher (eg) (1 mark)
(b) Tetrahedral: 2 lower (e), 3 higher (t₂) (1 mark)

Question 9

Explain how Fe catalyses reactions like the Haber process. [2 Marks]

Answer 9

Fe has multiple oxidation states, e.g., +2/+3 (1 mark)
Vacant d orbitals adsorb N₂, weaken bonds (1 mark)

Question 10

In a test, [Cu(H₂O)₆]²⁺ turns green with Cl⁻. Why does the colour change? [2 Marks]

Answer 10

[Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ + 6H₂O (1 mark)
Cl⁻ increases ΔE, absorbs different light (1 mark)

Question 11

Deduce the feasibility of Cu²⁺ + 2I⁻ → CuI + ½I₂ (E° Cu²⁺/Cu⁺ = +0.15, I₂/I⁻ = +0.54 V). [2 Marks]

Answer 11

E°cell = 0.54 - 0.15 = 0.39 V (>0, feasible) (1 mark)
Cu²⁺ reduced, I⁻ oxidised (1 mark)

Question 12

Identify cis/trans isomers in [Pt(NH₃)₂Cl₂] and predict polarity. [3 Marks]

Answer 12

Cis: NH₃ adjacent, polar; Trans: NH₃ opposite, non-polar (1 mark)
Square planar, 90° angles (1 mark)
Two isomers identified (1 mark)

Question 13

Calculate Kstab for [Cu(NH₃)₄]²⁺ if [Cu²⁺] = 10⁻⁵, [NH₃] = 0.1, [Cu(NH₃)₄]²⁺ = 0.01 M. [2 Marks]

Answer 13

Kstab = [Cu(NH₃)₄]²⁺/[Cu²⁺][NH₃]⁴ (1 mark)
Kstab = 0.01 / (10⁻⁵ × 0.1⁴) = 10¹¹ M⁻⁴ (1 mark)

Question 14

Suggest a mechanism for MnO₄⁻ + C₂O₄²⁻ → Mn²⁺ + 2CO₂ and calculate moles MnO₄⁻ for 0.1 mol C₂O₄²⁻. [3 Marks]

Answer 14

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺, C₂O₄²⁻ → 2CO₂ + 2e⁻ (1 mark)
2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O (1 mark)
0.04 mol MnO₄⁻ (2:5 ratio) (1 mark)

Question 15

Why does [Ni(en)₃]²⁺ show optical isomerism but not [Ni(en)₂(H₂O)₂]²⁺? [2 Marks]

Answer 15

[Ni(en)₃]²⁺: 3 bidentate, octahedral, non-superimposable (1 mark)
[Ni(en)₂(H₂O)₂]²⁺: plane of symmetry, no optical isomers (1 mark)