CCEA A Level Chemistry
CCEA A Level Chemistry 5.5 Transition Metals Exam Questions
Free CCEA A Level Chemistry revision for 5.5 Transition Metals: what you need to know, common mistakes and exam-style questions with answers. Covers properties, configurations, complexes and ligands, ligand replacement, the chelate effect, ion colours, hydroxides and vanadium chemistry.
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About CCEA A Level Chemistry 5.5 Transition Metals
Section 5.5 of Unit A2 2 is a large descriptive section. You need the characteristic properties of transition metals (incomplete d subshell, variable oxidation states, catalytic activity and coloured complexes), electronic configurations of atoms and ions, and the language of complexes: ligand, co-ordination number and oxidation state. CCEA requires monodentate, bidentate (en) and polydentate (edta) ligands, ligand replacement of hexaaquacopper(II) with concentrated HCl and ammonia, and the entropy explanation of the chelate effect. You must learn the colours of many aqueous ions, the hydroxide precipitates with sodium hydroxide and ammonia, and the reduction of vanadium(V) by zinc, using emf values to choose reagents.
What you need to know
- Definition and properties A transition metal forms at least one stable ion with an incomplete d subshell. They show variable oxidation states, catalytic activity and form coloured complexes. Remove 4s electrons before 3d when forming ions, so Fe3+ is [Ar] 3d5.
- Complexes and ligands A ligand donates a lone pair to the metal ion through a co-ordinate bond. Monodentate: H2O, NH3, Cl-. Bidentate: ethane-1,2-diamine (en). Polydentate: edta, which forms six bonds. Co-ordination number is the number of co-ordinate bonds.
- Copper ligand replacement [Cu(H2O)6]2+ (blue, octahedral) with concentrated HCl gives [CuCl4]2- (yellow, tetrahedral), with a green mixture in between. With excess ammonia it gives [Cu(NH3)4(H2O)2]2+ (deep blue).
- Chelate effect [Cu(H2O)6]2+ + 3en gives [Cu(en)3]2+ + 6H2O: four particles become seven, so entropy increases and the chelate complex is more stable.
- Colours and hydroxides Cr3+ green, Mn2+ pale pink, Fe2+ pale green, Fe3+ yellow-brown, Co2+ pink, Ni2+ green, Cu2+ blue. With NaOH: Cu(OH)2 blue, Fe(OH)2 green, Fe(OH)3 orange-brown, Ni(OH)2 green, Cr(OH)3 green (dissolves in excess NaOH). Cu(OH)2 dissolves in excess ammonia to a deep blue solution.
- Vanadium Zinc and acid reduce VO2+ (yellow, +5) to VO2+ (blue, +4), V3+ (green, +3) and V2+ (violet, +2). Use electrode potentials to predict whether a reagent can carry out each step.
Common mistakes in 5.5 Transition Metals
- Removing 3d electrons before 4s when forming ions. The 4s electrons are lost first.
- Saying zinc is a transition metal. Zn2+ has a full 3d10 subshell, so zinc does not meet the definition.
- Writing the copper chloride complex as octahedral. Chloride ions are large, so [CuCl4]2- is tetrahedral.
- Explaining the chelate effect with stronger bonds only. The key reason is the increase in entropy as more particles form.
- Mixing up the vanadium colours. Learn the order yellow, blue, green, violet for +5, +4, +3, +2.
Try three 5.5 Transition Metals exam-style questions
1. Write the electronic configuration of Cu2+. (1 mark)
Show the mark scheme answer
[Ar] 3d9 (1s2 2s2 2p6 3s2 3p6 3d9).
2. Describe what is seen when concentrated hydrochloric acid is added to aqueous copper(II) sulfate and give the shape of the new complex. (3 marks)
Show the mark scheme answer
Blue solution turns green then yellow; [CuCl4]2- formed; tetrahedral.
3. Explain, in terms of entropy, why edta readily replaces water ligands in [Cu(H2O)6]2+. (3 marks)
Show the mark scheme answer
[Cu(H2O)6]2+ + edta4- gives [Cu(edta)]2- + 6H2O; two particles form seven; large increase in entropy so delta G is negative and the reaction is favourable.
5.5 Transition Metals FAQs
What does CCEA A Level Chemistry 5.5 Transition Metals cover?
It covers transition metal properties and configurations, complexes and ligands, ligand replacement, the chelate effect, ion colours, hydroxide precipitates and vanadium chemistry.
Which unit is 5.5 Transition Metals examined in?
It is examined in Unit A2 2: Analytical, Transition Metals, Electrochemistry and Organic Nitrogen Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
What is the best way to learn the colours?
Build a table of ions with their aqueous colour, the precipitate with a little NaOH and with a little NH3, and what happens in excess of each, then test yourself regularly.
