OxfordAQA International A Level Chemistry 3.2.5 Transition Metals Exam Questions

About OxfordAQA International A Level Chemistry 3.2.5 Transition Metals

Transition metals is a substantial A2 inorganic topic examined in Unit 3. It covers titanium to copper and their characteristic properties: complex formation, coloured ions, variable oxidation states and catalytic activity. You need to explain ligand substitution with H2O, NH3 and Cl-, bidentate and multidentate ligands and the chelate effect, the shapes of complexes including isomerism, and the origin of colour. You also need vanadium oxidation states, redox titrations with manganate(VII), and heterogeneous and homogeneous catalysis, including autocatalysis. It links to electrode potentials, bonding and the reactions of aqueous ions in 3.2.6.

What you need to know

• Complexes and ligands A ligand donates a lone pair to a metal ion to form a co-ordinate bond. The co-ordination number is the number of co-ordinate bonds. H2O, NH3 and Cl- are monodentate; ethanedioate and 1,2-diaminoethane are bidentate; EDTA4- is hexadentate.
• Substitution H2O and NH3 are similar in size, so substitution does not change the co-ordination number, though it can be incomplete, as in [Cu(NH3)4(H2O)2]2+. Cl- is larger, so substitution can change the co-ordination number to 4, as in [CuCl4]2-.
• Chelate effect Replacing monodentate ligands with multidentate ligands increases the number of particles in solution, so entropy increases and the complex is more stable. Haem is an iron(II) complex; carbon monoxide replaces oxygen, which is why CO is toxic.
• Shapes Octahedral complexes can show cis-trans and optical isomerism. Tetrahedral complexes form with Cl-. Square planar complexes show cis-trans isomerism, and cisplatin is the cis isomer. [Ag(NH3)2]+ in Tollens' reagent is linear.
• Colour Ligands split the d orbitals; electrons absorb visible light to move to a higher level, and ΔE = hν = hc/λ. Colour depends on oxidation state, ligand and co-ordination number, and colorimetry measures concentration.
• Redox and catalysis Zinc in acid reduces vanadate(V) to V(IV), V(III) and V(II). Manganate(VII) titrations measure Fe2+ and ethanedioate. V2O5 catalyses the Contact process and iron the Haber process; Fe2+ catalyses the reaction of I- with S2O8 2-; Mn2+ autocatalyses the reaction of ethanedioate with manganate(VII).

Common mistakes in 3.2.5 Transition Metals

• Saying the colour seen is the light absorbed. The colour seen is the complementary colour, made of the wavelengths not absorbed.
• Explaining the chelate effect by bond strength alone. The key point is the increase in entropy as more particles form.
• Forgetting the change in co-ordination number with Cl-. Cl- is larger, so only four fit around the metal ion.
• Using the wrong mole ratio in manganate(VII) titrations. MnO4- reacts with Fe2+ in a 1:5 ratio and with ethanedioate in a 2:5 ratio.

Try three 3.2.5 Transition Metals exam-style questions