2025 ASRJC Prelim H2Chem P3_MS

1 9729/03/H2 ANDERSON SERANGOON JUNIOR COLLEGE 2025 JC 2 PRELIMINARY EXAMINATION PAPER 3 SOLUTIONS Section A 1 (a) Transition elements have characteristic physical and chemical properties. (i) Explain what is meant by the term transition element. [1] A transition element is a d block element that can form one or more stable ions with partially filled d subshells. [1] (ii) Explain why the melting point and density of nickel is higher than that of calcium. [3] • [A] In Ni, both the 3d and 4s electrons are available for metallic bonds since the energy level difference between the 3d and 4s orbitals is small. In Ca, only two 3s electrons are available for metallic bonding. • [B] The greater number of 3d and 4s delocalised electrons available for metallic bonding in Ni results [A or B - 1m] • in stronger electrostatic attraction between the cations and the ‘sea’ of delocalised electrons. More energy is required to overcome the stronger metallic bonds in transition elements as compared to Ca. [1m] • Ni has smaller atomic radii and higher atomic mass compared to Ca. • Also, Ni has more closely packed [1m] structure due to their stronger metallic bonding. • Thus, transition elements are denser than Ca. (b) (i) A solution containing the [Ni(H2O)6]2+ complex ion is green. When 1,2 -diaminoethane, en, H 2NCH2CH2NH2, is added, the colour of the solution changes to purple. This is due to the formation of the [Ni(en)2(H2O)2]2+ complex ion. Explain why the two solutions are coloured, and why the colours are different. [3] In the presence of the ligands, the partially filled 3d orbitals of Ni2+ are split into two levels with a small energy gap, E (d orbital splitting). When a 3d electron absorbs energy from the visible light region corresponding to E, this electron is promoted from the d orbital of a lower energy level to a d orbital of a higher energy level (d–d transition). The colour observed is the complementary of the colour absorbed. The colours are different as different ligands split the partially filled 3d orbitals to different extents with different E. Red is absorbed for [Ni(H2O)6]2+ complex and yellow is absorbed for [Ni(en)2(H2O)2]2+ complex. [1]: presence of ligand; partially filled 3d–orbital for Fe2+(aq); small energy gap [1]: d–d transition explanation; colour is the complement of the colour absorbed [1]: different ligands, different E

2 9729/03/H2 (ii) [Ni(en)2(H2O)2]2+ complex ion can exist in three different forms where the ions differ in the spatial arrangement of the ligands around the central metal ion. Fig. 1.1 shows one of the isomers. Ni N N OH2 N H2O N 2+ H2 H2 H2 H2 Fig. 1.1 Draw another isomer and state the type of isomerism.