Transition metalsAQA A-Level Chemistry: Topic test

1. 1
   Manganese is a d-block element with atomic number 25 and the ground-state electron configuration [Ar]3d⁵4s². It forms compounds in which its oxidation state is +2, +3, +4, +6 or +7, for example MnCl₂, MnO₂ and the purple manganate(VII) ion, MnO₄⁻.

   (a)

   What is the electron configuration of a Mn²⁺ ion?

   [1 mark]

   • A[Ar]3d³4s²
   • B[Ar]3d⁵
   • C[Ar]3d⁷
   • D[Ar]3d⁵4s²

   (b)

   Which statement best explains why manganese is classed as a transition metal?

   [1 mark]

   • AIt is in the d block of the Periodic Table
   • BIt forms coloured compounds
   • CIt has more than one oxidation state
   • DIt forms at least one ion with an incomplete d sub-level

   (c)

   Explain why manganese shows so many different oxidation states.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Iron(III) ions in aqueous solution exist as [Fe(H₂O)₆]³⁺. When potassium ethanedioate solution is added, the green complex ion [Fe(C₂O₄)₃]³⁻ forms. The ethanedioate ion, ⁻OOC–COO⁻, is written C₂O₄²⁻ and is bonded to the iron through oxygen atoms.

   (a)

   What is the co-ordination number of iron in [Fe(C₂O₄)₃]³⁻?

   [1 mark]

   • A3
   • B9
   • C6
   • D12

   (b)

   What is the oxidation state of iron in [Fe(C₂O₄)₃]³⁻?

   [1 mark]

   • A+3
   • B+6
   • C+2
   • D−3

   (c)

   Explain why the ethanedioate ion acts as a bidentate ligand.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Hydrated chromium(III) chloride, CrCl₃·6H₂O, can contain the green octahedral complex ion [Cr(H₂O)₄Cl₂]⁺, in which the two chloride ligands may be either next to each other or on opposite sides of the chromium ion. The violet ion [Cr(H₂O)₆]³⁺ and the yellow ion [Cr(NH₃)₆]³⁺ are also known.

   (a)

   Name the type of isomerism shown by [Cr(H₂O)₄Cl₂]⁺ and describe the two isomers, stating the Cl–Cr–Cl bond angle in each.

   [3 marks]

   (b)

   Ammonia replaces all six water ligands in [Cr(H₂O)₆]³⁺ without any change in co-ordination number. In contrast, concentrated hydrochloric acid converts octahedral [Fe(H₂O)₆]³⁺ into the tetrahedral ion [FeCl₄]⁻. Explain the difference.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A plant-feed tablet of mass 2.75 g contains iron(II) sulfate heptahydrate, FeSO₄·7H₂O (Mr = 278.0), as its only source of iron. To check the label, a technician dissolves the tablet in dilute sulfuric acid, makes the solution up to 250.0 cm³ and titrates 25.0 cm³ portions against 0.0100 mol dm⁻³ potassium manganate(VII). The mean titre is 18.60 cm³. The half-equations are MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O and Fe²⁺ → Fe³⁺ + e⁻. Plant-feed manufacturers also add EDTA⁴⁻ because iron in soil is mainly present as Fe³⁺ ions, which exist as [Fe(H₂O)₆]³⁺.

   (a)

   Calculate the percentage by mass of FeSO₄·7H₂O in the tablet, and explain why dilute sulfuric acid, rather than dilute hydrochloric acid, is used to acidify the titration mixture.

   [6 marks]

   (b)

   Explain why Fe³⁺ ions are held more strongly as [Fe(EDTA)]⁻ than as [Fe(H₂O)₆]³⁺ when EDTA⁴⁻ is added. Your answer should refer to the number of co-ordinate bonds, the entropy change and the enthalpy change in the substitution.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Aqueous nickel(II) sulfate is green because the hexaaquanickel(II) ion, [Ni(H₂O)₆]²⁺, absorbs light strongly at a wavelength of 650 nm, in the red region of the spectrum. Use h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹.

   (a)

   What is the energy gap, ΔE, between the d levels in one [Ni(H₂O)₆]²⁺ ion?

   [1 mark]

   • A3.06 × 10⁻¹⁹ J
   • B4.62 × 10¹⁴ J
   • C3.06 × 10⁻²⁸ J
   • D1.84 × 10⁵ J

   (b)

   What happens to the [Ni(H₂O)₆]²⁺ ion when it absorbs light of wavelength 650 nm?

   [1 mark]

   • AAn electron is lost from the ion, which is oxidised
   • BA lone pair on a water ligand moves into the nucleus
   • CA d electron is promoted to a higher energy d orbital
   • DA 4s electron is promoted to the 3d sub-level

   (c)

   A green solution containing [Ni(H₂O)₆]²⁺ turns blue-violet when excess ammonia is added and [Ni(NH₃)₆]²⁺ forms. Explain why the colour changes.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Zinc is added to an acidified green solution containing chromium(III) ions under an atmosphere of nitrogen. The solution turns blue as [Cr(H₂O)₆]²⁺ forms. If the blue solution is exposed to air it quickly turns green again. Standard electrode potentials: E° for Zn²⁺/Zn = −0.76 V; E° for [Cr(H₂O)₆]³⁺/[Cr(H₂O)₆]²⁺ = −0.41 V.

   (a)

   What happens to the blue solution when it is exposed to air and turns green?

   [1 mark]

   • AThe chromium(II) ions are reduced to chromium metal
   • BThe chromium(III) ions are reduced to chromium(II) ions
   • CThe chromium(II) ions are oxidised to chromium(IV) ions
   • DThe chromium(II) ions are oxidised to chromium(III) ions

   (b)

   What is the electron configuration of the Cr²⁺ ion? (The chromium atom is [Ar]3d⁵4s¹.)

   [1 mark]

   • A[Ar]3d³
   • B[Ar]3d⁴
   • C[Ar]3d⁴4s²
   • D[Ar]3d⁵4s¹

   (c)

   Write an ionic equation for the reaction of chromium(III) ions with zinc, and use the electrode potentials to explain why the reaction is feasible.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Vegetable oils are hardened to make margarine by reacting the liquid unsaturated oil with hydrogen gas in the presence of finely divided nickel metal supported on an inert ceramic material, at about 150 °C. If the hydrogen is contaminated with traces of sulfur compounds, the nickel quickly loses its activity.

   (a)

   Explain why nickel is a heterogeneous catalyst in this reaction and why it is used finely divided on a support.

   [3 marks]

   (b)

   Explain why sulfur impurities in the hydrogen reduce the efficiency of the process, the cost implication of this, and how the plant could avoid it.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Chromium(III) forms several octahedral complex ions, including violet [Cr(H₂O)₆]³⁺, yellow [Cr(NH₃)₆]³⁺ and orange-yellow [Cr(en)₃]³⁺, where en is the bidentate ligand H₂NCH₂CH₂NH₂. The Cr³⁺ ion has the electron configuration [Ar]3d³.

   (a)

   Explain why these chromium(III) complexes are coloured and why their colours are different.

   [6 marks]

   (b)

   The complex [Cr(en)₃]³⁺ exists as two isomers, whereas [Cr(H₂O)₆]³⁺ does not. Explain the isomerism of [Cr(en)₃]³⁺. Explain also why [Cr(H₂O)₆]³⁺ + 3en → [Cr(en)₃]³⁺ + 6H₂O is feasible although six Cr–O bonds are replaced by six Cr–N bonds of similar strength.

   [6 marks]

   Total for question 8: 12 marks

End of questions