2024 Prelims HCI H2 Chem P1 (Ans)

2024 HCI C2 H2 Chemistry Prelims / Paper 3 HWA CHONG INSTITUTION 2024 C2 H2 CHEMISTRY PRELIMINARY EXAMINATION SUGGESTED SOLUTIONS Paper 1 1 2 3 4 5 6 7 8 9 10 A D C B C B A A C D 11 12 13 14 15 16 17 18 19 20 A D B B D C C D C A 21 22 23 24 25 26 27 28 29 30 D D C A B B B A B C Comments 1 A 2H has 1 proton, 1 electron and 1 neutron. 16O has 8 protons, 8 electrons and 8 neutrons. For D3O+, Number of neutrons = 1 + 1 + 1 + 8 = 11 Since D3O+ is cationic, it is short of 1 electron overall. Number of electrons = 1 + 1 + 1 + 8 – 1 = 10 2 D Here are the electron-in-box diagrams of the valence orbitals in each species. Only sulfur has 2 unpaired electrons in its electronic configuration. 3 C Based on the number of bond pairs and lone pairs listed in the question, these are the corresponding geometries and bond angles: A: Bent, 118° B: Bent, 104.5° C: Trigonal pyramidal, 107° D: T-shaped: 88°

4 B Since electrons are delocalised in the carbonate ion, there is partial double bond character, and the bond length is likely to be between a C –O bond and a C=O bond. From Data Booklet , the average C –O bond energy is 360 kJ mol –1 while the average C=O bond energy is 740 kJ mol –1 thus it is likely that the C –O bond energy in carbonate is 485 kJ mol–1. 5 C bonding type physical properties 1 giant covalent high melting point, conducts electricity when in solution but not when solid does not conduct electricity in any state. 2 simple covalent low melting point, does not conduct electricity in any state 3 metallic variety of melting points, conducts electricity when solid and when molten 4 ionic low melting point high melting point, conducts does not conduct electricity in any state. 6 B The ordinate (i.e. y-coordinate) is PV = nRT = mRT/Mr = constant / Mr Hence the y-value of the graph is inversely proportional to the molar mass of the gas. In order of increasing molar mass, the gases are helium (Mr = 4.0), methane (Mr = 16.0), nitrogen ( Mr = 28.0) and chlorine ( Mr = 71.0). Hence the graph for methane should be the second highest horizontal line (i.e. second largest y - value). 7 A The gas is compressed to a pressure 4 times its original. If the gas were an ideal gas, its new volume would be expected to be one -quarter of its original, i.e. 19.0 cm 3. However, its new volume was 20.5 cm 3, so the gas did not behave ideally. The gas could not have dimerised, as the number of moles of gas would have decreased and the final volume would have been smaller than 19.0 cm3. Option 3 also does not explain the observation, as the presence of significant intermolecular forces of attraction would have also caused the actual volume to be smaller than 19.0 cm3. 8 A First, use the Data Booklet to find the first ionisation energies of the elements. 1st IE / KJ mol–1 Mg 736 Al 577 Si 786 P 1060 From this data, A l will