2023 RI H2 Chem Prelims P2 Answers

1 2023 Y6 H2 Chemistry Preliminary Examinations Paper 2 Suggested Solutions 1 (a) The volatilities of the halogens decrease from chlorine to iodine. From C l2 to I2, the electron clouds of the halogens become larger and more polarisable. Hence, more energy is required to overcome the increasing strength of the instantaneous dipole –induced dipole interactions between the halogen molecules down the group, leading to decreasing volatility. (b) (i) X Y Z nucleon number 140 103 37 charge +1 +1 0 (ii) (iii) nucleon number = 103 = 31 + 35 + 37 + 37 There is 1 35Cl atom in Y. Y is PCl2+. Its shape is bent. (c) (i) P(g) + e– ⎯→ P–(g) (ii) 1s 2s 2p 3s 3p (iii)  Nuclear charge increases from A l to C l but shielding effect remains Y X

2 effectively constant. Effective nuclear charge increases. Electrostatic attraction between the nucleus and the incoming electron increases, resulting in an increase in the energy released.  The attraction between the nucleus and incoming electron is weakened due to inter-electronic repulsion between this electron and the one already present in the 3p orbital. Hence, less energy is released as the first electron affinity. 2 (a) Au has a smaller atomic size and thus have more atoms per unit volume . It also has a larger atomic mass than Ba. Hence it has a greater mass per unit volume (i.e. higher density) as compared to Ba. (b) max % of Au in amalgam = 100% − 40% = 60% max mass of Au in 250 g of amalgam = 0.6  250 = 150 g (c) conc of Hg vapour = 1 g of Hg / 1 m3 of air = 1 g of Hg / 1.19 kg of air = 1 g of Hg / 1190 g of air = ( 1 1190 ) g of Hg / 1 g of air = ( 1 1190  106) g of Hg / 106 g of air = 840.3 g of Hg / 106 g of air  840 ppm (d) (i) Water prevents / reduces the vaporisation / evaporation of the collected mercury. (ii) When heated, the mercury in the amalgam vaporises and then condenses on the cooler inner surface of the glass bowl . Then it slides back down to accumulate on the sand to be collected and recovered. (iii) advantage • low cost/convenient to set up because of the use of simple household items or no need special equipment (e.g. metal apparatus) • no mercury-contaminated wastewater produced disadvantage • the mercury collected will mix with the sand and hence be harder to recover • the setup is not as airtight as Method I, i.e. mercury vapour has higher chance to escape back into the atmosphere • when the bowl is removed, mercury can vaporise again whilst collecting the mercury from the sand • mercury poisoning from re -use of cooking pot used in the extraction

3 for cooking purposes (e) (i) O2 + 2H2O + 4e– ⎯→ 4OH– (ii) mol ratio 4Au : 1O2 : 4e– Each Au atom loses 1 e– to form Au+. The oxidation state of Au is +1. (iii) CN– is the ligand in D. The comple