Flashcards · 9620 · A Level Topic 17

Transition metals (A Level) flashcards

29 cards on A Level Topic 17, subtopics 3.2.5.1 to 3.2.5.6. Pick a subtopic or study them all.

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Characteristic properties of transition metals (Ti to Cu), and their cause: They form complexes, form coloured ions, have variable oxidation states and act as catalysts. The cause is an incomplete d sub-level in the atom or ion.
Ligand: A molecule or ion that donates an electron pair to a transition metal ion to form a co-ordinate bond.
Complex, and co-ordination number: Complex: a central metal atom or ion surrounded by ligands. Co-ordination number: the number of co-ordinate bonds formed by the central metal atom or ion.
Electron configurations of Cu²⁺ and V³⁺: Cu²⁺: [Ar] 3d⁹ V³⁺: [Ar] 3d² (4s electrons are lost before 3d.)

Why does the co-ordination number stay 6 when NH₃ replaces H₂O, but fall to 4 with Cl⁻?: NH₃ and H₂O are similar in size and both uncharged. Cl⁻ is larger, so only four fit round the metal ion.
Product when excess ammonia is added to [Cu(H₂O)₆]²⁺: [Cu(NH₃)₄(H₂O)₂]²⁺, deep blue: an incomplete substitution (co-ordination number still 6).
Monodentate, bidentate and multidentate ligands, with examples: Monodentate: one donor atom (H₂O, NH₃, Cl⁻). Bidentate: two (H₂NCH₂CH₂NH₂, C₂O₄²⁻). Multidentate: many (EDTA⁴⁻, six donor atoms).
Explain the chelate effect: A multidentate ligand replacing monodentate ones increases the number of particles, so ΔS is positive. Similar bonds are broken and made, so ΔH is small. So ΔG = ΔH − TΔS is negative.
Haem and carbon monoxide: Haem is an iron(II) complex with a multidentate ligand. O₂ bonds co-ordinately to Fe(II) to be transported in blood. CO replaces the O₂ and binds strongly, so it is toxic.

Shapes of complexes with small ligands (H₂O, NH₃) and with larger Cl⁻: Small ligands: octahedral (six ligands, 90°). Cl⁻: tetrahedral (four ligands, 109.5°), e.g. [CuCl₄]²⁻.
Shape of the silver complex in Tollens’ reagent: [Ag(NH₃)₂]⁺ is linear, 180°.
Cisplatin: The cis isomer of square planar [Pt(NH₃)₂Cl₂]: the two Cl ligands are at 90° to each other.
Which types of stereoisomerism are shown by octahedral complexes?: Cis–trans with monodentate ligands (a special case of E–Z isomerism), and optical isomerism with bidentate ligands.
Cis vs trans (complexes): Identical ligands at 90° vs 180°.

Why are transition metal ions coloured?: They absorb certain wavelengths of visible light, using the energy to move a d electron from the ground state to an excited state. The colour seen is the light transmitted or reflected.
Equation linking the energy absorbed to frequency and wavelength: ΔE = hν = hc/λ (λ in m, so convert nm × 10⁻⁹).
Three changes that alter the colour of a complex: Changing the oxidation state, the co-ordination number or the ligand, because each changes ΔE.
How is a colorimeter used to find the concentration of a coloured ion?: Choose a filter of the colour most absorbed, zero with solvent, measure standards to make a calibration graph, read the unknown off it, and multiply by any dilution factor.
Absorbed vs seen (colour): The colour you see is what is not absorbed.

Colours and oxidation states of vanadium species: VO₂⁺ (+5): yellow VO²⁺ (+4): blue V³⁺ (+3): green V²⁺ (+2): violet Zinc in acid reduces vanadate(V) through all four.
Tollens’ reagent: [Ag(NH₃)₂]⁺ + e⁻ → Ag + 2NH₃. Silver mirror with an aldehyde; no change with a ketone.
Ratio and equation for the titration of Fe²⁺ with MnO₄⁻: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O MnO₄⁻ : Fe²⁺ = 1 : 5
Manganate(VII) titrations: indicator and acid: No indicator: the end point is the first permanent pink. Acidify with dilute H₂SO₄, never HCl.
pH effect (on transition metal redox): Half-equations with H⁺ (such as MnO₄⁻/Mn²⁺) shift with acidity; more H⁺ makes the higher oxidation state easier to reduce.

Heterogeneous vs homogeneous catalyst: Heterogeneous: in a different phase from the reactants (e.g. Fe in the Haber process). Homogeneous: in the same phase, working through an intermediate species.
How V₂O₅ catalyses the Contact process: V₂O₅ + SO₂ → V₂O₄ + SO₃ V₂O₄ + ½O₂ → V₂O₅ Vanadium goes +5 → +4 → +5, so the catalyst is regenerated.
How Fe²⁺ catalyses the reaction between S₂O₈²⁻ and I⁻: 2Fe²⁺ + S₂O₈²⁻ → 2Fe³⁺ + 2SO₄²⁻ 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂ Each step is between oppositely charged ions, unlike the slow direct reaction between two negative ions.
Autocatalysis in the MnO₄⁻ / C₂O₄²⁻ reaction: Mn²⁺, a product, catalyses the reaction (Mn²⁺ → Mn³⁺ → Mn²⁺). It starts slowly because two negative ions repel and no Mn²⁺ is present yet.
Support medium and poisoning (heterogeneous catalysts): A support medium spreads the catalyst to give a large surface area at low cost. Poisoning: impurities block the active sites, reducing efficiency and adding cost.