Colour of complexes: past paper questions

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• Question 5May/June 2026, paper 41Structured13 marks

  Cobalt is a transition element. It forms a wide range of coloured compounds that contain ${\text{Co}}^{2+}$ ions or ${\text{Co}}^{3+}$ ions. Explain why a transition element, such as cobalt, has variable oxidation states in its compounds.

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• Question 5Oct/Nov 2025, paper 41Structured14 marks

  Copper shows typical properties of transition elements, including its behaviour as a catalyst. Complete Table 5.1 to show the total number of unpaired electrons in the 3d and 4s orbitals of an isolated gaseous Cu atom and a ${\text{Cu}}^{2+}$ ion.

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• Question 4Oct/Nov 2024, paper 41Structured12 marks

  Transition metal atoms and transition metal ions form complexes by combining with ligands. Explain why transition elements form complex ions.

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• Question 4Oct/Nov 2023, paper 41Structured7 marks

  Cobalt(II) nitrate, $\mathrm{C}\mathrm{o}(\mathrm{N}{\mathrm{O}}_3{)}_2$, is a reddish-brown crystalline solid. It dissolves in water to form a solution containing $[\mathrm{C}\mathrm{o}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$ complex ions.

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• Question 4Oct/Nov 2024, paper 42Structured12 marks

  Transition metal atoms and transition metal ions form complexes by combining with species called ligands.

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• Question 5Oct/Nov 2023, paper 41Structured12 marks

  Transition elements behave as catalysts and can form complex ions. Explain why transition elements behave as catalysts.

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• Question 6May/June 2025, paper 41Structured8 marksHas a diagram

  Iron forms complex ions with the monodentate ligand, $\mathrm{C}{\mathrm{N}}^{-}$. Complex ion A contains one $\mathrm{F}{\mathrm{e}}^{3+}$ ion and six $\mathrm{C}{\mathrm{N}}^{-}$ ligands. Complex ion B contains one $\mathrm{F}{\mathrm{e}}^{2+}$ ion and six $\mathrm{C}{\mathrm{N}}^{-}$ ligands.

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• Question 2May/June 2025, paper 42Structured17 marksHas a diagram

  When cobalt(II) sulfate, $\mathrm{C}\mathrm{o}\mathrm{S}{\mathrm{O}}_4$, is dissolved in distilled water, solution A is formed.

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• Question 3May/June 2024, paper 41Structured13 marksHas a diagram

  When a sample of hydrated lithium ethanedioate, $\mathrm{L}{\mathrm{i}}_2{\mathrm{C}}_2{\mathrm{O}}_4\cdot {\mathrm{H}}_2\mathrm{O}$, is gently heated, two gaseous products are formed and a white solid residue remains. The residue is added to $\mathrm{H}\mathrm{N}{\mathrm{O}}_3(\mathrm{a}\mathrm{q})$. A gas is produced that turns limewater milky.

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• Question 6May/June 2025, paper 43Structured8 marksHas a diagram

  Iron forms complex ions with the monodentate ligand, ${\mathrm{C}\mathrm{N}}^{-}$. Complex ion A contains one ${\mathrm{F}\mathrm{e}}^{3+}$ ion and six ${\mathrm{C}\mathrm{N}}^{-}$ ligands. Complex ion B contains one ${\mathrm{F}\mathrm{e}}^{2+}$ ion and six ${\mathrm{C}\mathrm{N}}^{-}$ ligands.

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• Question 4May/June 2024, paper 42Structured14 marksHas a diagram

  Explain why transition elements have variable oxidation states.

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• Question 1May/June 2023, paper 41Structured19 marksHas a diagram

  Group 2 nitrates decompose when heated. Describe how the thermal stability of Group 2 nitrates changes with increasing proton number.

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