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LearnAQA7405First teaching 2015Paper 1 · Paper 1: Physical and inorganic chemistryTransition metals

Paper 1 · Module

Transition metals

2 subtopics≈ 105 min in total

Start the module: Transition element properties Open graphing calculator

Transition element properties
A transition element forms one or more stable ions with an incomplete d subshell (so Sc and Zn are d-block but not transition elements). Configurations: 4s fills before 3d but is emptied first on ionisation; Cr is [Ar]3d⁵4s¹ and Cu is [Ar]3d¹⁰4s¹. Properties: variable oxidation states (similar 4s and 3d energies), coloured ions, complex formation and catalysis (Fe in the Haber process, V₂O₅ in the Contact process, Fe²⁺/Fe³⁺ in I⁻/S₂O₈²⁻). Redox titrations: $\mathrm{MnO_4^-+5Fe^{2+}+8H^+\rightarrow Mn^{2+}+5Fe^{3+}+4H_2O}$ (self-indicating: purple to colourless); $\mathrm{Cr_2O_7^{2-}}$ reacts 1 : 6 with Fe²⁺.
≈ 50 min
Complex ions, ligands and colour
A complex ion is a central metal ion with ligands bonded by dative (coordinate) bonds. Coordination number = number of dative bonds: 6 → octahedral ([Cu(H₂O)₆]²⁺); 4 → tetrahedral ([CuCl₄]²⁻, with large Cl⁻) or square planar (cisplatin, Pt); 2 → linear ([Ag(NH₃)₂]⁺). Ligands may be monodentate (H₂O, NH₃, Cl⁻), bidentate (en, C₂O₄²⁻) or polydentate (EDTA⁴⁻, 6 bonds). Colour: ligands split the d orbitals; an electron absorbs visible light of energy $\Delta E=h\nu$ to jump up; the complementary colour is seen. Changing ligand, coordination number or oxidation state changes ΔE and so the colour. Ligand exchange is favoured by larger $K_\text{stab}$ and by the chelate effect (entropy increase). Complexes show cis/trans and optical isomerism.
≈ 55 min