RI Transition Elements - Answers to Self-Check Questions

-8- Suggested Answers to Transition Elements Self-check questions 1(a) A transition element is a d-block element which can form one or more stable ions with a partially filled d subshell. Note: The definition should be in terms of partially filled d subshell (not orbitals). Partially filled d orbitals refer specifically to the d5 electronic configuration, where all the five d orbitals are partially/singly filled. 1(b) A complex ion is a charged species which consists of a central metal ion linked to one or more surrounding ions or molecules (called ligands) by dative covalent bonds. 1(c) A ligand is an ion or a molecule which contains at least one atom bearing a lone pair of electrons which can be donated into a low-lying vacant orbita l of a central metal atom or ion forming a co- ordinate bond (or dative covalent bond), resulting in the formation of a complex. 1(d) In a complex, the co-ordination number of the central metal ion (or atom) is the total number of co-ordinate bonds that the central metal ion (or atom) has formed with ligands. 2a(i) Cu < Ni < V < Fe < Cr 2a(ii) Zn2+ < Ti2+ < V2+ < Co3+ < Mn2+ 2b(i) 1s2 2s2 2p6 3s2 3p6 3d5 4s1 2b(ii) 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d2 2c central metal oxidation state electronic configuration ion or atom of the transition metal atom/ion (i) TiCl2 Ti2+ +2 1s2 2s2 2p6 3s2 3p6 3d2 (ii) [V(H2O)6]3+ V3+ +3 1s2 2s2 2p6 3s2 3p6 3d2 (iii) [Cr(NH3)6]Cl3 Cr3+ +3 1s2 2s2 2p6 3s2 3p6 3d3 (iv) [Fe(SCN)(H2O)5]2+ Fe 3+ +3 1s2 2s2 2p6 3s2 3p6 3d5 (v) [Fe(C N)6]4– Fe2+ +2 1s2 2s2 2p6 3s2 3p6 3d6 (vi) [Ni(CO)4] Ni 0 1s2 2s2 2p6 3s2 3p6 3d8 4s2 3 Answer: D 4 Answer: D 5 (a) Haemoglobin (denoted by Hb) acts as a transporter of oxygen in the human body. It carries out this function by having the oxygen molecule acting as a ligand forming a dative covalent bond to Fe(II) in its haem group. The resulting compound formed is called oxyhae moglobin and it travels in the blood to places in the human body which requires oxygen. The H 2O ligand replaces the O2 ligand when the oxygen is released to the oxygen-starved sites in the human body. Hb-O2 + H2O ⇌ Hb-OH2 + O2 (b) I n haemoglobin, a H2O molecule acts as a ligand and forms a dative covalent bond with Fe(II) in the haem group. When converted into oxyhaemoglobin, a ligand exchange reaction occurs wi th the H2O ligand being replaced by the O2 ligand. There is no redox reaction and hence no change in the oxidation number of Fe(II) in the haem group. (c) T he interaction between CO and haemoglobin is that of a co-ordinate bond formed between the carbon atom of CO and the iron( II) in the haemoglobin. The CO acts as a monodentate ligand and donates the lone pair of electrons on the C atom into a vacant low-lying orbital in Fe( II) to form a co-ordinate bond. CO is so much more toxic than CO 2 probably because the CO-haemoglobin complex is much more stable than the CO2-haemoglobin complex. In other words, the CO ligand is much stronger than the CO2 ligand and forms a stronger co-ordinate bond to Fe( II) such that it is very difficult for O2 or H2O ligand to displace it. This renders the haemoglobin ineffective in transporting oxygen and will result in death of the victim concerned.

-9- (d) By using pure O 2, the equilibrium position of the reaction: Hb-CO + O2 ⇌ Hb-O2 + CO is shifted right due to the very high concentration of O 2. The formation of Hb-O 2 means that haemoglobin resumes its role as a transporter of O2 and hence the patient can be revived. Note: The pure O2 administered does not oxidise CO to CO2 but just displaces the CO. (e) CN– ion. It also forms a very stable complex with haemoglobin. It does so by donating a lone pair of electrons from the carbon atom to a vacant low-lying orbital in Fe( II) of haemoglobin to form a s trong co-ordinate bond. This renders the haemoglobin ineffective in performing its oxygen- transporting function.  6(a)(i) Fe and Cu are first-row transition metals. Although Cu has a higher nuclear charge (greater proton number) than Fe, Cu has more 3d electrons which provide more shielding between the nucleus and the outer 4s shell of electrons. This increase in shielding effect offsets the increase in nuclear charge considerably and hence the electrostatic attraction between the nucleus and outer 4s electrons increases minimally, resulting in the atomic radii of Fe and Cu to be similar. ( a)(ii) Density = mass Volu me The relative atomic mass of Fe and Cu are higher than that of Ca. Since the atomic radius of Fe and Cu are smaller than that of Ca, there are more Fe and Cu per unit volume than Ca. Hence, the densities of Fe and Cu are significantly greater than that of Ca. (a)(iii) Fe and Cu are first row transition metals with the general valence electronic configuration 3dx 4sy. Due to the close similarity in energy between the 3d electrons and the 4s electrons, Fe and Cu can make use of different number of these electrons in bond formation (ionic or covalent) when they form compounds and hence they have a tendency to vary in their oxidation states. Calcium, an s-block element, has the electronic configuration 1s 2 2s2 2p6 3s2 3p6 4s2. It can only exist in the +2 oxidation state by losing 2 valence electrons to form Ca2+. +3 ox idation state of Ca is not favourable due to the very high 3 rd ionisation energy required to remov e the third electron from the inner quantum shell, n =3. Hence, too much energy is needed for its formation, which is not offset by the mo re exothermic lattice energy of ionic compounds containing Ca3+. N ote: Ca does not exhibit +1 oxidation state where it exists as Ca +. This is because the lattice energy of an ionic compound in which calcium exists as Ca + would be much less exothermic compared to an ionic compound in which calcium exists as Ca2+. Since the 2nd IE of Ca is not very high (as the second electron is removed from the valence 4s subshell), calcium loses the two valence electrons to form Ca2+ in its ionic compounds which have much more exothermic lattice energy. (b) (i) As shown in the diagram, Ca has a giant metallic lattice structure with a regular array of positively charged Ca2+ ions surrounded by a ‘sea of delocalised electrons’. Metallic bonding arises from the electrostatic forces of attraction between the metal cations held in the lattice and the delocalised electrons. (b) ( ii) Fe has a significantly higher melting point than Ca due to its stronger metallic bonding. The stronger metallic bonding in Fe arises because both the 3d and 4s electrons of Fe can contribute to the ‘sea’ of delocalised electrons (due to the small energy difference between these electrons). For Ca, only the two valence 4s electrons per atom can contribute to the ‘sea’ of delocalised electrons. In addition, the contribution of more electrons for delocalisation makes the iron cations in the lattice smaller and more highly charged, resulting in stronger electrostatic attraction between Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ e e e e e e e e e e e e e e e e

-10- these cations and the delocalised electrons than in the case between calcium ions and the delocalised electrons. Hence Fe has a higher melting point than Ca. (c) (i) Cu has a very high density and may sag and deform when used as overhead electrical cables. (c) (ii) E lectronic configuration: Cu 1s 22s22p63s23p63d104s1 ; Cu2+ 1s 22s22p63s23p63d9 (c) (iii) A n electrolytic method is used to purify copper industrially. The impure copper to be purified is used as the anode while pure copper is used as cathode. The electrodes are immersed in an electrolyte of copper(II) sulfate solution. During electrolysis, Cu is oxidised at the anode and enters the solution as Cu2+(aq) while Cu2+ in the electrolyte is reduced to Cu at the pure copper cathode. As the reaction proceeds, the impure Cu anode decreases in size while the pure Cu