9701 · 28.2
General characteristic chemical properties of the first set of transition elements, titanium to copper flashcards
Revision flashcards for Cambridge 9701 General characteristic chemical properties of the first set of transition elements, titanium to copper (syllabus 28.2). Flip, recall, then mark a real past-paper question.
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What is a complex ion?
A central metal ion bonded to one or more ligands by coordinate (dative covalent) bonds.
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Define 'ligand'.
A molecule or ion that donates a pair of electrons to a central metal ion to form a coordinate bond. It acts as a Lewis base.
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What is the coordination number of a complex ion?
The number of coordinate bonds from the ligands to the central metal ion.
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Give the formula, coordination number, and shape of the hexaquacopper(II) ion.
Formula: $[Cu(H_2O)_6]^{2+}$. Coordination number: 6. Shape: Octahedral.
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Why are many transition metal complexes coloured?
Ligands cause the d-orbitals to split into different energy levels. Electrons absorb photons of visible light to jump to a higher level, and the complementary colour is transmitted or reflected.
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What is a ligand substitution reaction?
A reaction in which one or more ligands in a complex ion are replaced by other ligands.
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Describe the reaction of aqueous copper(II) ions with excess concentrated HCl.
The pale blue solution of $[Cu(H_2O)_6]^{2+}$ turns yellow-green due to the formation of $[CuCl_4]^{2-}$. The equation is: $[Cu(H_2O)_6]^{2+}(aq) + 4Cl^-(aq) \rightleftharpoons [CuCl_4]^{2-}(aq) + 6H_2O(l)$.
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Why do transition elements exhibit variable oxidation states?
The 4s and 3d subshells are very close in energy, so electrons can be lost from both shells without a prohibitively large increase in successive ionisation energies.
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What are the products of the reduction of manganate(VII) ions, $MnO_4^-$, in acidic solution?
Manganese(II) ions, $Mn^{2+}$, and water. The purple $MnO_4^-$ is decolourised.
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Distinguish between homogeneous and heterogeneous catalysis.
In homogeneous catalysis, the catalyst is in the same phase as the reactants (e.g., all aqueous). In heterogeneous catalysis, the catalyst is in a different phase (e.g., a solid catalyst for gaseous reactants).
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How does iron act as a catalyst in the Haber process?
This is heterogeneous catalysis. Gaseous $N_2$ and $H_2$ adsorb onto the surface of the solid iron catalyst, which weakens their bonds and lowers the activation energy for the formation of ammonia.
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Why are aqueous solutions of transition metal ions, like Cu²⁺(aq), often coloured?
Ligands (H₂O) split the d-orbitals. Electrons absorb energy from visible light to move to a higher d-orbital (d-d transition). The colour seen is the complement of the light absorbed. Cu²⁺ has a partially filled d-subshell (3d⁹), allowing this.
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Why are compounds containing Sc³⁺ or Zn²⁺ ions colourless?
Sc³⁺ has an empty d-subshell (3d⁰) and Zn²⁺ has a full d-subshell (3d¹⁰). As there are no partially filled d-orbitals, d-d electronic transitions cannot occur, so no visible light is absorbed.
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Define 'ligand substitution' and give an example.
A reaction where one or more ligands in a complex ion are replaced by other ligands. Example: [Cu(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CuCl₄]²⁻(aq) + 6H₂O(l). The colour changes from pale blue to yellow-green.
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Explain how iron acts as a heterogeneous catalyst in the Haber process.
Solid iron provides an active surface. N₂ and H₂ molecules are adsorbed onto the surface, which weakens their covalent bonds (chemisorption) and increases their proximity, lowering the activation energy for the reaction to form ammonia.
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Explain why transition elements like manganese exhibit variable oxidation states.
The energy levels of the 4s and 3d sub-shells are very close. This means that variable numbers of electrons can be removed from these sub-shells with relatively small successive increases in ionisation energy.