AQA Chemistry A-level: Transition Metals

3.2.5.1 Transition Metal Properties

• Location: d-block of periodic table, partially filled d-orbital.
• Ion Formation: Lose s-orbital electrons to form positive ions.
• Physical Properties: Similar atomic radius, high densities, melting and boiling points.
• Chemical Properties:
  • Form complexes
  • Form coloured ions
  • Exhibit variable oxidation states
  • Act as good catalysts

Complexes

• Central metal ion/atom surrounded by ligands.
• Represented using square brackets.

Ligands

• Molecules/ions with a lone electron pair forming coordinate bonds.
• Common ligands: Cl-, H2O, NH3.
• Coordination Number: Number of coordinate bonds around the central metal ion.
  • Silver: 2, Platinum: 4.

3.2.5.2 Ligand Substitution Reactions

• Ligands NH3 and H2O: Similar size, uncharged, exchangeable without coordination number change.
• Example: Copper complexes with NH3 form deep blue solutions.
• Cl- Ligand: Larger, changes coordination number, forms tetrahedral complexes with coordination number four.

Bidentate Ligands

• Form two coordinate bonds due to two lone pairs.
• Examples: Ethanedioate ions, Ethane-1,2-diamine.

Multidentate Ligands

• Form up to six coordinate bonds.
• Example: EDTA, forms stable complexes (Chelate Effect).
• Haem: Transports oxygen, forms six bonds around Fe2+ ion.

Chelate Effect

• Favorable Entropy: Positive entropy change means more stable complex.
• Substitution of unidentate with bidentate/multidentate ligands increases stability.
• Enthalpy Change: Near zero, as bond energies of formed and broken bonds are similar.

3.2.5.3 Shapes of Complex Ions

• Octahedral Complexes: Common with H2O and NH3 ligands, bond angle 90°.
• Cis-trans Isomerism: Occurs in octahedral complexes with different ligands.
• Optical Isomerism: Occurs in octahedral complexes with bidentate ligands.
• Tetrahedral Complexes: Larger ligands like Cl-, bond angle 109.5°.
• Square Planar Complexes: Platinum and Nickel, bond angle 90°.
• Linear Complexes: Silver, bond angle 180°, present in Tollens reagent.
• Cisplatin: Square planar platinum complex, used in cancer therapy.

3.2.5.4 Coloured Ions

• Identification by Colour: Depends on coordination number, type of ligand, oxidation state.
• Colour Observation: Related to light absorption/reflection.
• d-orbital Electrons: Energy transitions correspond to observable colour.
• Colorimetry: Technique to determine concentration by measuring absorbance.

3.2.5.5 Variable Oxidation States

• Vanadium: Exhibits multiple oxidation states (+5 to +2) with distinct colours.
• Redox Conditions: pH affects oxidation or reduction of transition metals.
• Tollens Reagent: Used to test for aldehydes, forms silver mirror.

3.2.5.6 Catalysts

• Catalyst Definition: Increases reaction rate without being consumed.
• Types:
  • Heterogeneous Catalysts: Different phase from reactants (e.g., Iron in Haber Process).
  • Homogeneous Catalysts: Same phase as reactants.
• Functioning:
  • Adsorption: Molecules adsorbed on catalyst surface increase reaction rate.
  • Catalyst Poisoning: Impurities can block active sites, requiring replacement/cleaning.
  • Autocatalysis: Product acts as a catalyst, increasing reaction rate over time.