2020 ACJC Prelim Paper 2 Answers

3 © ACJC2020 9729/02/Prelim/2020 [Turn over Answer all questions in the spaces provided. 1 The Periodic Table currently used is derived from that proposed in 1869 by Mendeleev who had noticed patterns in the physical and chemical properties of the elements he had studied. (a) (i) The atomic radius of the Period 3 elements decreases from Na to Cl. Explain why this is so.  There is an increasing nuclear charge due to increase in number of protons.  There is an approximately constant shielding effect as additional electrons are added to the same valence shell and provides negligible shielding effect.  Effective nuclear charge increases  Valence electrons thus experiences stronger nuclear attraction. OR Therefore, electrons are pulled closer to the nucleus. (ii) Write the electronic configurations of the sodium atom and its ion Na 1s2………………………………… Na+ 1s2………………………………… [1] Na: 1s22s22p63s1 Na+: 1s22s22p6 (iii) Explain the difference in the radius of the sodium atom and the radius of its ion, Na+. ………………………………………………………………………………….[1]  Na+ has one less shell of electrons than Na, hence Na+ is smaller. (b) (i) Complete Table 1.1 by describing the acid/base behaviour of Al2O3 and SO3. Provide equation(s) to illustrate the behaviour of each oxide. Table 1.1 oxide acid/base behaviour equation(s) illustrating behaviour of each oxide Al2O3 amphoteric Al2O3 + 6HCl  2AlCl3 + 3H2O Al2O3 + 2NaOH + 3H2O  2Na+[Al(OH)4]

4 © ACJC2020 9729/02/Prelim/2020 [Turn over SO3 acidic SO3 + 2NaOH Na2SO4 + H2O [4] (ii) State one source of SO2 pollution. ………………………………………………………………………………….[1]  Sulfur compounds in fuels / fossil fuels / biomass OR sulfide ores OR volcanoes (iii) State an environmental consequence of SO2. ………………………………………………………………………………….[1]  Formation of acid rain OR its consequences e.g. damage to buildings / crops / plants / marine life / deforestation (c) A diagonal relationship is said to exist between certain pairs of diagonally adjacent elements in the second and third Periods. The elements and their compounds in these pairs are observed to share similar properties. An example of such a pair is beryllium and aluminium. (i) Draw the dot-and-cross diagram for AlCl3 and state the Cl−Al−Cl bond angle. Dot-and-cross diagram of AlCl3 [1] Bond angle: ………………………………. [1] Cl Cl Cl Al

5 © ACJC2020 9729/02/Prelim/2020 [Turn over (ii) Unlike AlCl3 which dimerises to form Al2Cl6, BeCl2 polymerises and exists as long chains in the solid state. Draw a section of one chain with three BeCl2 monomers, showing clearly the bonding within the chain. [1] (iii) With a small amount of water, BeCl2 hydrolyse