AQA A Level Chemistry
AQA A Level Chemistry 3.2.5 Transition Metals Exam Questions
Free AQA A Level Chemistry exam questions for 3.2.5 Transition Metals, with a mark scheme for every pack. Covers transition metal properties, ligands and complex ion shapes, the chelate effect, colour, variable oxidation states, redox titrations and catalysts.
3.2.5 Transition Metals exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA A Level Chemistry 3.2.5 Transition Metals
Transition metals is one of the largest A Level inorganic topics on the AQA course. You study the characteristic properties of the elements titanium to copper, complex ions with monodentate, bidentate and multidentate ligands, their shapes and isomerism, the chelate effect, the origin of colour, colorimetry, variable oxidation states (including vanadium and chromium chemistry), redox titrations with manganate(VII) and dichromate(VI), and heterogeneous and homogeneous catalysis. It draws on 3.1.3, 3.1.8, 3.1.11 and 3.1.7. Examiners often set redox titration calculations, ask for explanations of colour using d orbital splitting, and expect the chelate effect to be explained using entropy.
What you need to know
- Characteristic properties Transition metals have an incomplete d sub-level in their atoms or ions. They form complexes, form coloured ions, show variable oxidation states and act as catalysts.
- Ligands and complexes A ligand donates a lone pair to the metal ion through a coordinate bond. H2O, NH3 and Cl- are monodentate; ethane-1,2-diamine and ethanedioate are bidentate; EDTA4- is hexadentate. Coordination number is the number of coordinate bonds.
- Shapes Small ligands such as H2O and NH3 usually give octahedral complexes (90°). Larger Cl- ligands give tetrahedral [CuCl4]2-. Cisplatin is square planar, and [Ag(NH3)2]+ in Tollens' reagent is linear. Octahedral and square planar complexes can show cis-trans isomerism and octahedral complexes with bidentate ligands can show optical isomerism.
- Chelate effect Replacing monodentate ligands with multidentate ligands increases the number of particles in solution, so ΔS is positive. ΔH is close to zero because similar bonds are broken and made, so ΔG is negative and the complex is more stable.
- Colour Ligands split the d orbitals. Electrons absorb visible light of energy ΔE = hν = hc/λ to move to a higher orbital, and the complementary colour is seen. Colour changes with oxidation state, coordination number and ligand. Colorimetry measures concentration of coloured ions.
- Variable oxidation states Zinc in acid reduces vanadium from +5 (yellow VO2+) to +4 (blue VO2+), +3 (green V3+) and +2 (violet V2+). Dichromate(VI) is orange and is reduced to green Cr3+. Tollens' reagent is reduced from Ag+ to Ag.
- Redox titrations and catalysts MnO4- reacts with Fe2+ in a 1:5 ratio and with C2O4 2- in a 2:5 ratio, acidified with dilute sulfuric acid. Heterogeneous catalysts include V2O5 in the Contact process and Fe in the Haber process; Fe2+ catalyses S2O8 2- with I-, and Mn2+ autocatalyses MnO4- with ethanedioate.
Common mistakes in 3.2.5 Transition Metals
- Saying a complex is coloured because it absorbs the colour you see. It absorbs some frequencies of visible light and transmits or reflects the complementary colour.
- Explaining the chelate effect by stronger bonds. The driving force is the increase in entropy from more particles.
- Including scandium and zinc as transition metals. Sc3+ has no d electrons and Zn2+ has a full 3d sub-level.
- Acidifying manganate(VII) titrations with hydrochloric acid. Chloride ions would be oxidised; use dilute sulfuric acid.
- Confusing coordination number with the number of ligands. Three bidentate ligands give a coordination number of 6.
Try three 3.2.5 Transition Metals exam-style questions
1. 25.0 cm3 of an Fe2+ solution reacted with 22.50 cm3 of 0.0200 mol dm-3 potassium manganate(VII) in acidic conditions. Calculate the concentration of Fe2+. (3 marks)
Show the mark scheme answer
Amount of MnO4- = 0.0200 x 22.50 / 1000 = 4.50 x 10^-4 mol; amount of Fe2+ = 5 x 4.50 x 10^-4 = 2.25 x 10^-3 mol; concentration = 2.25 x 10^-3 / 0.0250 = 0.0900 mol dm-3.
2. Explain why [Cu(H2O)6]2+ ions are coloured. (3 marks)
Show the mark scheme answer
Ligands cause the 3d orbitals to split into different energy levels; electrons absorb energy from visible light to move to a higher energy d orbital (ΔE = hν); the remaining light is transmitted, giving the complementary colour, blue.
3. [Cu(H2O)6]2+ + 3H2NCH2CH2NH2 → [Cu(H2NCH2CH2NH2)3]2+ + 6H2O. Explain why the product complex is more stable. (3 marks)
Show the mark scheme answer
Four particles become seven, so entropy increases (ΔS positive); ΔH is approximately zero because similar Cu-N and Cu-O bonds are broken and made; ΔG = ΔH - TΔS is negative, so the reaction is feasible and the chelate complex is more stable.
3.2.5 Transition Metals FAQs
What does AQA A Level Chemistry 3.2.5 Transition Metals cover?
It covers the general properties of transition metals, ligands and complex ion shapes, the chelate effect, colour and colorimetry, variable oxidation states, redox titrations and catalysis.
Which paper is 3.2.5 Transition Metals examined in?
It is examined in Paper 1, and Paper 3 can also test it.
What colours do I need to learn?
Learn the vanadium sequence (yellow, blue, green, violet), orange dichromate(VI) and green Cr3+, purple manganate(VII) and the colours of common aqua and ammine complexes. Colour questions often appear as observations worth separate marks.
