2019 HCI Prelim H2 Chem P3 ANS

2019 HCI C2 H2 Chemistry Preliminary Exam / Paper 3 Paper 3 1 (a) (i) MgCO3(s)  MgO(s) + CO2(g) [1] (ii) +- +- qqL.E α r +r [1] Both MgCO3 and MgO have the same charges and cationic radius [0.5] Anionic radius for CO32– is bigger than O2– [0.5] Magnitude of L.E. of MgCO3 is smaller than that of MgO. (iii) S is positive as gaseous CO2 is evolved [1]. Hence, –TS term is negative. G = H –TS Since the decomposition of MgCO3 is endothermic, H is positive. So for the decomposition to be spontaneous, for G to be negative, the decomposition should take place at high temperature [1]. (iv) MgCO3 has the lower decomposition temperature. [0.5] Both Mg2+ and Ba2+ have the same charge. The ionic radius of Mg2+ is smaller than Ba2+ (0.5). So Mg2+ has a higher charge density (0.5) and a greater polarizing power and it can distort the electron cloud of the CO32– to a greater extent (0.5 for either point), weakening the C−O covalent bonds in CO32– to a greater extent (0.5), hence less energy is needed for decomposition (0.5). (b) (i) Type of reaction (reaction 1): Nucleophilic Addition [1] Type of reaction (reaction 2): Elimination/Dehydration [1] (ii) [1] (iii) The geometry about the center C is trigonal planar [1]. Hence, there is equal probability for the CN – ion to attack from either side of the plane [1] , giving rise to an equimolar mixture of two stereoisomers. (iv) [1] HWA CHONG INSTITUTION 2019 C2 H2 CHEMISTRY PRELIMINARY EXAM SUGGESTED SOLUTIONS

(c) (i) [1] (ii) Reagents and Conditions: Step 1: LiAlH4 in dry ether [1] Step 2: excess concentrated H2SO4, heat [1] Step 3: steam, concentrated H3PO4, high T high P [1] Step 4: [1] J: [1] 2 (a) (i) C3H4O3 [1] (ii) [1] (b) constitutional/structural/functional group isomerism [1] (c) The LF molecule is polar and can form strong/favourable permanent-dipole permanent-dipole interactions/hydrogen bonding/ion -dipole interactions with water molecules. [1] (d) (i) When the onion is cooled, less LF will vaporise and come into contact with the eyes. [1] (ii) Heating the onion can denature the enzyme LF synthase so that LF will not be formed. [1] (e) (i) condensation [1] (ii) C9H16OS3 + 17O2  9CO2 + 8H2O + 3SO3 [1] (iii) n(cepaene) = 5 x 10–3 / 236.3 = 2.116 x 10–5 mol [½] n(O2) = 2.116 x 10–5 x 17 = 3.597 x 10–4 = 3.60 x 10–4 mol [½] ecf from ii volume of O2 mixture at r.t.p. = 3.597 x 10–4 x 24000 = 8.63 cm3 [½] volume of mixture = 8.63 x 100/40 = 21.6 cm3 [1] (iv) Role of copper: reducing agent [1] Cu + ½O2  CuO [1] (v) The blue crystals are CuSO4 [1], which can be formed from the reaction of CuO with SO3. [1]

2019 HCI C2