VJC Prelim P3 Ans

1 2009 H2 Chemistry 9746/3 Suggested Answers 1 Vanadium, a transition metal, is commercially important. (a) (i) Write down the electronic configur ation of V, hence, write down the electronic configuration of V 2+ ion. 23V 1s 22s22p63s23p63d34s2 23V2+ 1s 22s22p63s23p63d3 [1] (ii) Explain why V 2+(aq) is coloured whereas Ca2+(aq) is colourless. In the presence of ligands , the 3d orbitals are split into two groups of slightly different energy levels. V 2+ has a partially filled d orbitals , transition of electrons between the two different levels takes place with absorption of energy in the visible spectrum . The wavelengths that are transmitted correspond to the complementary colour observed. In Ca 2+, there is no d-d transition due to absence of 3d electrons (or absence of partially filled 3d orbitals or electron transition takes place outside the visible region), hence it appears colourless. [ 3 ] (b) Sketch the general trends of the first ionization energy and the third ionization energy of the elements across the first series of tr ansition metals; including Ca to Cu on the same axes. Explain the trends shown for these two properties. ionization energy/ kJ mol -1 M 2+ → M 3+ + e M → M + + e Ca Sc Ti V Cr Mn Fe Co Ni Cu The first ionization energy of the elements from scandium to copper remains relatively invariant as it involves the removal of 4s electrons. The significant screening effect of 3d electrons in the penul timate shell nullifies the increase in nuclear charge.  VJC 2009 9746/03/PRELIM/ANS/09 [Turn over

2 The transition elements have higher 1 st ionization energy than Ca since they have higher effective nuclear charge than Ca due to the fact that d electrons are poorly shielding. The increasing trend in third ioni zation energy is due to the increase in the effective nuclear charge and relatively constant shielding effect as 3d electrons are now in valence shell. Third ionization energy of calcium is much higher than that of the other transition elements as the elec tron to be removed is from inner 3p orbital which is closer to the nucleus and has lower energy , held more strongly by the nucleus (or electron is removed from a full octet to form Ca3+). Therefore, larger amount of energy is needed to remove it. The 3 rd electron from Fe 2+ is removed from an orbi tal containing a pair of electrons, less energy is expected due to inter-electronic repulsion. [ 5 ] (c) Both Ca and Zn play an important role bi ologically in the body system. Vanadium ions are poisonous as they can cause bone porosity by replacing the calcium ions in bones. One treatment for vanadium poisoni ng involves administering a solution of edta, which forms hexacoordinate complexes with many divalent metal ion