2024 Prelims ASRJC H2 Chem P3 (Ans)

ASRJC JC2 PRELIM 2023 9729/03/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2024 JC 2 PRELIMINARY EXAMINATION NAME:______________________________ ( ) CLASS: 24 / _____ CHEMISTRY 9729/03 Paper 3 Free Response Questions SUGGESTED SOLUTIONS 1 (a) (i) State the full electronic configuration of an Fe2+ ion. [1] 1s2 2s2 2p6 3s2 3p6 3d6 [1] (ii) Explain why Fe 2+(aq) ions are green, whereas aqueous Group 2 metal ions are not coloured. [4] In the presence of water ligands, the partially filled 3d orbitals of Fe2+ 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 green colour of Fe2+(aq) is the complement of the red colour absorbed. Aqueous Group 2 metal ions do not have electrons in the d subshell for d–d transition to occur. [1]: partially filled 3d–orbital for Fe2+(aq); small energy gap [1]: d–d transition [1]: green colour of Fe2+(aq) is the complement of the red colour absorbed [1]: no electron in 3d–orbital for group 2 metal ions and no d–d transition (b) Iron(II) carbonate can be prepared by reacting solutions of the two ions, Fe 2+(aq) and CO32– (aq). Explain why the same method cannot be used to prepare solid iron(III) carbonate. [2] Due to the smaller cation size and a larger positive charge than Fe2+, Fe3+ has a higher charge density (high polarising power). Fe3+ is able to draw electrons to itself from the oxygen atoms of the neighbouring water molecules, which further polarises the O –H bonds , thereby producing H+ in the solution. [1] H+ in the solution undergoes acid-base reaction with the carbonate ions and produce CO2 instead. [1]

2 ASRJC JC2 PRELIM 2024 9729/03/H2 (c) The minimum temperature to decompose some Group 2 carbonates increases down the group as shown in Table 1.1. Table 1.1 Group 2 carbonate Minimum temperature of decomposition / oC MgCO3 350 CaCO3 840 SrCO3 1289 BaCO3 1360 When heated, solid copper(II) carbonate behaves in a similar way to the Group 2 carbonates. (i) Write an equation for the thermal decomposition of copper(II) carbonate. [1] CuCO3 (s) → CuO(s) + CO2 (g) [1] (ii) Use data from the Data Booklet and Table 1.1 to suggest a minimum temperature at which pure copper(II) carbonate decomposes. Explain your reasoning. [3] Cu2+ has an ionic radius of 0.073 nm , which is an intermediate between that of Mg 2+ (0.065) and Ca2+ (0.099). [1] Minimum decomposition temperature of CuCO 3 could be