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 450 oC. (accept value between 360 – 820). [1] Cu2+ has a charge density between that of Mg2+ and Ca2+. Hence, it has and weaker polarising power than Mg2+ but stronger than that of Ca 2+. The electron cloud of the carbonate is distorted and the C–O bond is weakened to a smaller extent than by Mg2+ but greater extent than by Ca 2+. Hence, a temperature higher than 350 oC but lower than 840 oC is required. [1] (d) The chemistry and coordination of cobalt( II) compounds are similar to those of copper( II) compounds with similar ligands. (i) When cobalt(II) nitrate, Co(NO3)2, is added to water, a pink solution of G is formed. When aqueous sodium hydroxide is added to solution G, a blue precipitate H is observed. Precipitate H dissolves when excess aqueous ammonia is added to form a brown solution I. Nitrogen and hydrogen are the only other elements in the octahedral cobalt-containing complex in I. Identify the complex species in G, H and I. For each of the reactions to form H and I, identify the type of reaction and give relevant chemical equations. [4] G – [Co(H2O)6]2+ H – [Co(H2O)4(OH)2] I – [Co(NH3)6]2+ [2] All three species correctly identified Co(NO3)2 added to water (FYI) Co(NO3)2(s) + 6H2O(l) → [Co(H2O)6]2+(aq) + 2NO3–(aq) Aqueous sodium hydroxide added to G: Acid-base reaction [Co(H2O)6]2+(aq) + 2OH–(aq) [Co(H2O)4(OH)2](s) + 2H2O(l)

3 ASRJC JC2 PRELIM 2024 9729/03/H2 [Turn over Aqueous ammonia added to precipitate H: Ligand exchange reaction [Co(H2O)6]2+(aq) + 6NH3(aq) + 6H2O(l) [2] for types of reaction and equations (ii) Addition of concentrated hydrochloric acid to pink solution G forms a blue solution J. Solution J turns pink again when water is added to it. With the aid of a balanced equation, explain these observations. [2] [Co(H2O)6]2+(aq) + 4Cl– (aq) [CoCl4]2–(aq) + 6H2O(l) There’s an equilibrium between G and J. Addition of HCl causes concentration of Cl– to increase and position of equilibrum shifts to the right and form J. / [Co(H2O)6]2+ undergoes ligand exchange reaction with Cl– ligands to form the blue complex, [CoCl42–](aq). [1] for equation and explanation. When water is added, concentration of ions decreases, and position of equilibrium shifts to the left to form more ions and forms the pink [Co(H2O)6]2+ [1] (iii) Salt K has the molecular formula CoN5H17OCl3 (Mr = 268.4). It has one or more ligands which is similar to those found in complex species G, H and J. When 1.00 g of salt K is dissolved in water, the resulting solution required 22.40 cm3 of 0.50 mol dm–3 silver nitrate solution for complete reaction. Calculate the number of moles of free chloride ions per mole of K and give the structural formula of the complex ion in K. [2] Number of moles of AgNO3 = 22.4/1000 × 0.50 = 0.0112 mol Number of moles of free Cl– = 0.0112 mol Number of moles of K = 1/268.4 = 0.00372 mol Number of moles of free Cl– per mole of K = 0.0112/0.00372 = 3 [1] Since there are 3 free Cl– ion, there is no Cl– coordinated to the Co center. Structure of the complex ion in K is [Co(NH3)5(H2O)]3+. [1] (e) The ethanedioate ion, C2O42–, can act as a bidentate ligand. (i) Explain what is meant by a ligand. [1] A ligand is a neutral molecule or an anion that has at least one lone pair of electrons to be used in forming a dative bond to the central metal atom or ion.

4 ASRJC JC2 PRELIM 2024 9729/03/H2 (ii) The complex [Co(H2O)2(C2O4)BrCl]– exists as stereoisomers. Isomer L has the following structure. The C2O42– ligand is represented using O O. Co H2O O Br O. III Cl H2O Isomer L Draw an isomer that has an equal and opposite effect on plane polarised light compared to isomer L. [1] Co OH2O BrO. III Cl OH2 Mirror image of isomer L [1] [Total: 21]

5 ASRJC JC2 PRELIM 2024 9729/03/H2 [Turn over 2 (a) A sample of sulfur contains four stable isotopes as shown in Table 2.1. Table 2.1 relative isotopic mass of S percentage abundances / % 31.97 94.99 32.97 0.75 w 4.25 35.97 0.01 The relative atomic mass of sulfur in this sample is 32.06. Use the data in Table 2.1 to calculate a value for w to 2 decimal places. [1] (31.97 × 94.99) + (32.97 × 0.75) + (w × 4.25) + (35.97 × 0.01)32.06 = 100 3206 = 3205.9303 + 4.25w w = 33.90 [1] answer to 2 dp and no units. (b) Table 2.2 lists the boiling points of different sulfur containing compounds. Table 2.2 compound formula Mr boiling point / oC thiourea C S NH2H2N 76.1 150 thionyl fluoride S O F F