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5.3.1 Transition elements
(a) the electron configuration of atoms and ions of the d-block elements of Period 4 (Sc–Zn), given the atomic number and charge
(b) the elements Ti–Cu as transition elements i.e. d-block elements that have an ion with an incomplete d-sub-shell
(c) illustration, using at least two transition elements, of:
(i) the existence of more than one oxidation state for each element in its compounds (see also 5.3.1 k)
(ii) the formation of coloured ions (see also 5.3.1 h, j–k)
(iii) the catalytic behaviour of the elements and their compounds and their importance in the manufacture of chemicals by industry
(d) explanation and use of the term ligand in terms of coordinate (dative covalent) bonding to a metal ion or metal, including bidentate ligands
(e) use of the terms complex ion and coordination number and examples of complexes with:
(i) six-fold coordination with an octahedral shape
(ii) four-fold coordination with either a planar or tetrahedral shape
(f) types of stereoisomerism shown by complexes, including those associated with bidentate and multidentate ligands:
(i) cis–trans isomerism e.g. Pt(NH3)2Cl2
(ii) optical isomerism e.g. [Ni(NH2CH2CH2NH2)3]2+
(g) use of cis-platin as an anti-cancer drug and its action by binding to DNA preventing cell division
(h) ligand substitution reactions and the accompanying colour changes in the formation of:
(i) [Cu(NH3)4(H2O)2]2+ and [CuCl4]2– from [Cu(H2O)6]2+
(ii) [Cr(NH3)6]3+ from [Cr(H2O)6]3+(see also 5.3.1 j)
(i) explanation of the biochemical importance of iron in haemoglobin, including ligand substitution involving O2 and CO
(j) reactions, including ionic equations, and the accompanying colour changes of aqueous Cu2+, Fe2+, Fe3+, Mn2+ and Cr3+ with aqueous sodium hydroxide and aqueous ammonia, including:
(i) precipitation reactions
(ii) complex formation with excess aqueous sodium hydroxide and aqueous ammonia
(k) redox reactions and accompanying colour changes for:
(i) interconversions between Fe2+ and Fe3+
(ii) interconversions between Cr3+ and Cr2O72–
(iii) reduction of Cu2+ to Cu+ and disproportionation of Cu+ to Cu2+ and Cu
(l) interpretation and prediction of unfamiliar reactions including ligand substitution, precipitation, redox
In this topic learners will use their prior knowledge to understand the characteristic properties of transition metal elements and their compounds. They will need to understand the concepts of subshells and orbitals in order to see the difference between d-block elements and transition metals, and link this to ionisation energies to understand the formation of compounds with variable oxidation states. Learners’ understanding of bonding and shapes of molecules will also be expanded upon as they explore ligands and complex formation, and they will need to have a basic understanding of protein structure in order to gain a full appreciation of the biological importance of these elements.
As learners approach the end of their course, the key to their success in the subject is to start seeing the ‘bigger picture’ and to make links between different interrelated concepts. Regular consolidation of past material should be used to build strong foundations for new content and learners should also use concept mapping techniques to bring together ideas about energy, entropy and equilibria. All of the serious calculations have been covered by this stage and it is worth including examples of different calculations (equilibrium constants, entropy changes, Born-Haber cycles, titrations) throughout the topic so that these skills are reinforced.
By this stage learners should also have covered a range of practical activities and there is little in this topic to challenge them in terms of the manipulation of apparatus. Practical activities should therefore be as open-ended as possible to allow learners the freedom to work out how to carry out and interpret qualitative tests. At all times, they should be encouraged to make as many observations as possible and to link these to their understanding across a range of topics – for example, the microscale activity in which ammonia gas is generated and allowed to diffuse into droplets of different substances.
The ‘similarities and differences’ exercise mentioned can be used very effectively to challenge learners’ understanding of a range of closely related terms and enable the teacher to very quickly see where there are misconceptions, gaps in knowledge or links that have not been made.
Common misconceptions or difficulties students may have:
Bonding and structure
This is always a common area of misconceptions for learners, in particular because of their tendency to compartmentalise ideas about bonding as being ionic, covalent, metallic or intermolecular. Similarly, learners will tend to see the hydration of s-block ions (covered in a previous topic when related to enthalpy of solution) as a completely separate ‘other’ process to the formation of transition metal complexes.
To address these issues, it is worth spending some time reminding learners about the wide range of bonding types they have encountered so far in their studies. When learning about shapes of molecules, they may have come across AlCl3 as an example of a metal forming covalent bonds; they should also have seen examples such as PF5 and SF6 where the octet is ‘expanded’. Examples of dative bonding outside of transition metal complexes should be given and the move from ionic bonding to covalent should be seen as a broad spectrum where polarisation of ions or the formation of permanent dipoles produce ‘intermediate’ bonding. Displaying and discussing the Van Arkel-Ketelaar triangle may be beneficial.
When more transition metal chemistry has been encountered, learners should be encouraged to make links between the polarising ability of the transition metal ion and the ease of hydrolysis of surrounding water molecules. The strong attraction of oxygen atoms in water molecules to Cr3+ can be used as an explanation for why solid chromium(III) hydroxide is soluble excess sodium hydroxide. The metal ion makes surrounding water molecules sufficiently acidic to allow deprotonation in the presence of a base, forming a negatively charged metal complex. Learners could also investigate the acidity of some transition metal ion solutions and link this to the polarising ability of the cation and the resultant deprotonation of water even in the absence of a base.
Variable oxidation states and ionic charges
To help explain the high oxidation states achieved by some transition metals such as manganese and chromium, learners should be encouraged to use bond polarity to help explain the overall oxidation state. Other more common oxyanions such as sulfates, nitrates and carbonates can be used to illustrate this idea before introducing those containing transition metals.
Although higher oxidation states can be explained by the number of bonds formed to oxygen in oxyanions, learners may struggle with the idea of transition metals being able to form more than one stable cation. This is usually caused by an over-reliance on the octet rule and ‘full outer shell stability’ to explain ion formation. By this stage, learners should be familiar with Born-Haber cycles and these can be a useful prompt to encourage learners to think about the energy required for ion formation as being offset by the exothermic processes of either lattice formation or hydration of the metal ion.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course:
Prior knowledge: This topic is traditionally taught quite late in the course; as such it relies heavily on prior knowledge and understanding. Remember that learners, unlike their teachers, are not fully immersed in the subject and have no doubt assimilated a large amount of content in a relatively short period of time. Before building upon ideas covered earlier in the course (such as electronic configurations or redox), it is worth using short straightforward exercises to consolidate these ideas and bring them to the forefront of learners’ minds. This can be as simple an exercise as re-using a question sheet from a previous topic and setting it as a homework exercise before the lesson. Sometimes the pressure to forge ahead with new content, for fear of not covering it all in the time available, can actually make progress much slower than if a fraction of the lesson were dedicated to eliciting prior knowledge and eradicating misconceptions.
Stereoisomerism: Learners should have a good understanding of geometric isomerism by this stage but may not have been introduced to optical isomerism. If this is the case, it is probably more effective to introduce the idea of a tetrahedral chiral centre, using examples and molecular modelling kits, before moving on to the much more demanding concept of octahedral complexes with bidentate ligands.
Amines: Learners will not necessarily have covered the chemistry of amines before they are introduced as examples of mono- and bidentate ligands in this topic. They will, however, be familiar with the reactions of ammonia as both a base and a nucleophile, so comparisons can be drawn here.
Approaches to teaching the content:
The percentage of copper in a brass tack or penny can be calculated either by colorimetry or by a ‘back titration’ involving oxidation of the copper to Cu2+ with concentrated nitric acid, followed by reduction of the copper with iodide ions and titration of the resulting iodine with thiosulfate solution. This practical incorporates acid-base, redox and precipitation reactions and there are many accompanying observations as well as some more complex calculations.
The interconversions between different oxidation states of vanadium always provide a vivid illustration of the existence of more than one oxidation state for transition metals. To extend learners’ knowledge, they could use electrode potential data at the start of the practical to select suitable oxidising and reducing agents for these interconversions.
Bonding and structure
This topic provides many opportunities to consolidate and extend learners’ understanding of dative covalent bonding, molecular ions and polarisation of ions.
Ligand substitution reactions provide opportunities for learners to practice writing expressions for equilibrium constants, and to predict the effects of addition of ligands or of temperature changes. The equilibrium between chloro and hexaaqua complexes of cobalt (II) are particularly useful.
The size of the equilibrium constant for the displacement of monodentate ligands by multidentate ligands can be related (either mathematically or qualitatively) to the free energy change associated with the process and the change to the total entropy of the system.
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