Transition metals (A Level) flashcards

Topic 17: Transition metals

What gives transition metals (Ti–Cu) their characteristic properties, and what are the four properties?

An incomplete d sub-level in atoms or ions. Properties: complex formation, coloured ions, variable oxidation state and catalytic activity.

Ligand

A molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons.

Co-ordination number

The number of co-ordinate bonds to the central metal atom or ion.

Why does replacing H₂O by Cl⁻ change the co-ordination number from 6 to 4?

Cl⁻ is larger than H₂O, so fewer fit round the metal: e.g. [Co(H₂O)₆]²⁺ + 4Cl⁻ → [CoCl₄]²⁻ + 6H₂O (octahedral to tetrahedral).

The chelate effect

Multidentate ligands replace monodentate ones. ΔH is small (similar bonds broken and made) but the number of particles increases, so ΔS is positive and ΔG = ΔH − TΔS is negative.

Why is carbon monoxide toxic?

CO replaces the O₂ that is co-ordinately bonded to Fe(II) in haem, so oxygen cannot be transported in the blood.

Why are transition metal complexes coloured?

Some visible wavelengths are absorbed, moving d electrons from the ground state to an excited state (ΔE = hν = hc/λ). The colour seen is the light transmitted or reflected.

Colours of vanadium oxidation states when vanadate(V) is reduced by zinc in acid

VO₂⁺ (+5) yellow → VO²⁺ (+4) blue → V³⁺ (+3) green → V²⁺ (+2) violet.

Manganate(VII) titrations: mole ratio with Fe²⁺ and with ethanedioate

MnO₄⁻ : Fe²⁺ = 1 : 5. MnO₄⁻ : C₂O₄²⁻ = 2 : 5 (the mixture is warmed). The end point is the first permanent pale pink.

Heterogeneous vs homogeneous catalyst, with an example of each

Heterogeneous: a different phase; works at surface active sites (V₂O₅ in the Contact process, Fe in the Haber process). Homogeneous: the same phase; works through an intermediate (Fe²⁺ in S₂O₈²⁻ + I⁻).

Autocatalyst

A product of the reaction that catalyses it, e.g. Mn²⁺ in the reaction of MnO₄⁻ with C₂O₄²⁻.