2019 HCI Prelim H2 Chem P1 ANS

2019 HCI C2 H2 Chemistry Preliminary Exam / Paper 1 Paper 1 ANSWERS: Comments 1 D The definition for atomic mass is the ratio of the average mass of one atom of an element to one -twelfth the mass of one atom of 12C. In option D, the mass of one mole of atoms of an element has already taken into account all the isotopes and their relative abundances. 2 A The electronegativity increases across Period 3 as the effective nuclear charge increases. The electrical conductivity of the metals Na to A l increases while Si is a semiconductor and the non-metals P, S and Cl are non-conductors. The atomic radii across Period 3 shows a generally decreasing trend due to the increase in effective nuclear charge across the period. The melting po int increases from Na to A l on account of the increasing metallic bonding strength; the trend peaks at Si which has a giant covalent structure with extensive strong covalent bonding, before showing a decreasing trend for the simple covalent compounds which only have weak dispersion forces between molecules. 3 D Ice has a simple covalent structure. There is hydrogen bonding between water molecules, and covalent bonding between atoms of H and O in each molecule of water. HWA CHONG INSTITUTION 2019 C2 H2 CHEMISTRY PRELIMINARY EXAM SUGGESTED SOLUTIONS 1 2 3 4 5 6 7 8 9 10 D A D B C C A B C B 11 12 13 14 15 16 17 18 19 20 B B B A C D A B C C 21 22 23 24 25 26 27 28 29 30 D D D A C B A C D C

Iodine has a simple covalent str ucture. There are dispersion forces between iodine molecules, and covalent bonding between atoms of iodine in each molecule of iodine. Aluminium chloride has an ionic lattice structure with a large amount of covalency in the solid state at room temperature. Its structure becomes simple covalent just below its melting point resulting in dispersion forces between A l2Cl6 molecules. Hence permanent dipole interactions are not found in the solid state. Graphite has a giant covalent structure with dispersion forces holding the sheets of carbon atoms together. 4 B Species NH4+ XeF4 Shape Tetrahedral Square planar Bond Angle 109.5o 90o Species NF3 NH3 Shape Trigonal pyramidal Trigonal pyramidal Bond Angle 102o 107o F is more electronegative than N and so the electron cloud of the N–F bond is distorted towards F. This causes the electron density about N to decrease. Thus, bond pair-bond pair repulsion is weaker around the central N atom in NF 3 than in NH3. Hence, bond angle in NF3 is smaller. Species NO2+ SO2 Shape Linear Bent Bond Angle 180o <120o Species BCl3 PCl 3 Shape Trigonal planar Trigonal pyramidal Bond Angle 120o 107o 5 C Since the number of moles of gas in the three containers did not change before and after the taps are opened, the number of moles of gas found in the individual containers before and after the taps are opened should be the same. gas