Transition metalsEdexcel A-Level Chemistry: Topic test

1. 1
   Chromium (Z = 24) and nickel (Z = 28) are added to iron to make stainless steel. Both metals form stable ions in their compounds.

   (a)

   What is the electronic configuration of a Ni²⁺ ion?

   [1 mark]

   • A[Ar] 3d⁶ 4s²
   • B[Ar] 3d⁸
   • C[Ar] 3d⁸ 4s²
   • D[Ar] 3d⁷ 4s¹

   (b)

   Why do transition metals show variable oxidation numbers?

   [1 mark]

   • ATheir atoms have partially filled p orbitals
   • BAll their atoms are the same size
   • CTheir ions always have the same charge
   • DThe 3d and 4s electrons are close in energy, so different numbers of electrons can be lost to form stable ions

   (c)

   Deduce the electronic configuration of a chromium atom and explain why Cr³⁺ is classed as a transition metal ion.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The complex ion [Co(en)₂Cl₂]⁺, where en is the ligand NH₂CH₂CH₂NH₂, is one of many cobalt complexes studied by a student.

   (a)

   Which row correctly describes the ligand en?

   [1 mark]

   • AMonodentate, forming one dative bond
   • BBidentate, forming one dative bond
   • CBidentate, forming two dative bonds
   • DHexadentate, forming six dative bonds

   (b)

   What are the oxidation number of cobalt and the coordination number in [Co(en)₂Cl₂]⁺?

   [1 mark]

   • A+3 and 6
   • B+3 and 4
   • C+1 and 6
   • D+2 and 6

   (c)

   Explain how a molecule of en can bond to a cobalt ion as a ligand.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student adds aqueous sodium hydroxide dropwise, and then in excess, to a solution containing the hexaaqua chromium(III) ion, [Cr(H₂O)₆]³⁺.

   (a)

   Describe what the student sees, and write an ionic equation for the first change.

   [3 marks]

   (b)

   Chromium(III) hydroxide is described as amphoteric. Explain this, using ionic equations, and state how the behaviour differs from ligand exchange.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Copper forms compounds in two oxidation states. Aqueous copper(II) sulfate contains the complex ion [Cu(H₂O)₆]²⁺ and is blue, whereas solid copper(I) chloride, CuCl, is white.

   (a)

   Explain why [Cu(H₂O)₆]²⁺ is coloured but copper(I) chloride is white.

   [6 marks]

   (b)

   The colour of a transition metal complex ion can change when the ligand, the coordination number or the oxidation number changes. For each of these three changes, give one example of a colour change and explain why the colour changes.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Aqueous potassium chromate(VI) is yellow. When it is acidified the solution turns orange as the equilibrium 2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l) is established.

   (a)

   Which statement about the change from chromate(VI) to dichromate(VI) is correct?

   [1 mark]

   • AIt is a reduction because chromium changes from +6 to +3
   • BIt is an oxidation because chromium changes from +6 to +7
   • CIt is a redox reaction because the hydrogen ions are reduced
   • DIt is not a redox reaction because the oxidation number of chromium stays at +6

   (b)

   Which reagent converts Cr³⁺(aq) into chromate(VI) ions?

   [1 mark]

   • AZinc and dilute acid
   • BHydrogen peroxide in alkaline solution
   • CDilute sulfuric acid
   • DAqueous sodium chloride

   (c)

   Explain, using the equilibrium, why adding aqueous sodium hydroxide to the orange solution turns it yellow.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Acidified potassium manganate(VII) is added to a warm solution of sodium ethanedioate, Na₂C₂O₄: 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O. The purple colour fades very slowly at first, and then the mixture decolourises faster and faster.

   (a)

   Which term describes the role of Mn²⁺ in this reaction?

   [1 mark]

   • AAn autocatalyst
   • BA heterogeneous catalyst
   • CAn inhibitor
   • DA reactant

   (b)

   Which statement about homogeneous catalysis is correct?

   [1 mark]

   • AThe catalyst is in a different phase from the reactants and the reaction occurs on its surface
   • BThe catalyst is permanently used up in the reaction
   • CThe catalyst is in the same phase as the reactants and the reaction proceeds via an intermediate species
   • DThe catalyst changes the position of equilibrium

   (c)

   Explain why the reaction is slow at first and then speeds up, even though the concentrations of the reactants are falling.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   EDTA⁴⁻ is added to foods and cleaning products to bind traces of copper(II) ions. In water the copper is present as [Cu(H₂O)₆]²⁺, and EDTA⁴⁻ replaces all six water ligands: [Cu(H₂O)₆]²⁺ + EDTA⁴⁻ → [Cu(EDTA)]²⁻ + 6H₂O.

   (a)

   Explain why EDTA⁴⁻ is described as a multidentate ligand, and state the shape of the complex ion [Cu(EDTA)]²⁻.

   [3 marks]

   (b)

   Explain, in terms of entropy, why replacing the water ligands by EDTA⁴⁻ is favourable and gives a more stable complex.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Margarine is made by hydrogenating liquid vegetable oils, which contain C=C double bonds, with hydrogen gas at about 150 °C. A finely divided nickel catalyst is stirred into the oil.

   (a)

   Explain how nickel acts as a heterogeneous catalyst in this reaction, and why finely divided nickel is used.

   [6 marks]

   (b)

   Evaluate the use of solid nickel rather than a soluble transition metal catalyst for this process.

   [6 marks]

   Total for question 8: 12 marks

End of questions