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 hydrolyses in the same manner as AlCl3. Write an equation for the hydrolysis of BeCl2. ………………………………………………………………………………….[1]  BeCl2 + 2H2O Be(OH)2 + 2HCl [Total: 14] 2 This question concerns the chemistry of the halogens and hydrogen halides. (a) F2 and C l2 are halogens that exist as gases under room temperature and pressure. (i) With reference to their structures and bonding, explain why C l2 has a higher boiling point than F2.  Both have simple molecular structures  Cl2 has a larger, more polarisable electron cloud  Stronger instantaneous dipole -induced dipole (id – id) interactions between Cl2 molecules  Requires more energy to overcome these interactions (iii) Use data from the Data Booklet to compare the oxidising strength of F2 and Cl2. …………………………………………………………………………………[2] The electrode potential of +2.87V (F2 + 2e− 2F−) is more positive than +1.36V (Cl2 + 2e− 2Cl−), showing F2 is a stronger oxidising agent than Cl2 as it has a greater tendency for reduction.

6 © ACJC2020 9729/02/Prelim/2020 [Turn over (b) Chlorine gas was bubbled through an aqueous solution of potassium iodide. Hexane was subsequently added and the mixture was shaken in a test − tube. Table 2.1 gives the density of water and hexane. Table 2.1 compound density / g cm−3 water 1.00 hexane 0.655 (i) Write an equation for the reaction that occurs between chlorine gas and potassium iodide solution. …………………………………………………………………………………[1] Cl2 + 2I− 2Cl− + I2 (ii) Complete Fig. 2.1 by stating the colours observed in the test – tube. Fig. 2.1  Purple layer above the brown layer (c) Vapour pressure is defined as the pressure exerted by a vapour in equilibrium with its liquid in a closed system. Table 2 .2 shows the vapour pressure of HF and HCl measured at 25 °C. Table 2.2 compound vapour pressure / kPa HF 122 HCl 4660 Give a reason for the higher vapour pressure of HCl. ………………………………………………………………………………...………... ………………………………………………………………………………...………...

7 © ACJC2020 9729/02/Prelim/2020 [Turn over ……………………………………………………………………………………..…[1]  HF molecules experiences stronger hydrogen bonding, compared to HCl’s weaker permanent dipole – permanent dipole interactions, resulting in higher vapour pressure for HCl as there is more gaseous HCl than HF. [Total: 7] 3 The Sabatier reaction describes the reversible reaction between carbon dioxide and hydrogen in the following equation: CO2(g) + 4H2(g) CH4(g) + 2H2O(g) ∆H = −126 kJ mol−1 The reactants are passed through a solid support coated with ruthenium metal as a catalyst, where the reaction occurs. (a) (i) Define the term reversible reaction. ………………………………………………………………………………...…. …………………………………………………………………………………[1]  It is a reaction that proceeds in both the forward and backward directions in a closed system OR A reaction that proceeds in either direction. (ii) Write the Kp expression for this reaction. [1] 42 22 2 CH H O p 4 CO H PP K= PP (iii) State the type of catalyst used in this reaction. …………………………………………………………………………………[1] Heterogeneous

8 © ACJC2020 9729/02/Prelim/2020 [Turn over (b) 0.05 mol of carbon dioxide and 0.05 mol of hydrogen were introduced in a 1 dm3 evacuated vessel at 353 °C. The Sabatier reaction occurred and a n equilibrium was established . The equilibrium partial pressure of methane was measured to be 20 kPa. (i) Calculate the initial partial pressure, in kPa, of carbon dioxide in the vessel. [2] 3 0.05 8.31 (353 273) 1 10P      = 260.1 = 260 kPa (ii) Determine the equilibrium partial pressures, in kPa, of:  carbon dioxide  hydrogen  water [2] CO2 + 4H2 CH4 + 2H2O Initial / kPa 260 260 0 0 Change / kPa -20 -80 +20 +40 Eqm / kPa 240 180 20 40 2COP = 240 kPa 2HP = 180 kPa 2HOP = 40 kPa (c) (i) Describe and explain how the position of equilibrium will change when the volume of the vessel is decreased from 1 dm3 to 0.5 dm3. ………………………………………………………………………………...…. ………………………………………………………………………………...…. …………………………………………………………………………………[2] An decrease in volume will lead to a rise in pressure of the system. This favours the forward reaction where the number of gaseous particles is fewer. The position of equilibrium shifts to the right.

9 © ACJC2020 9729/02/Prelim/2020 [Turn over Fig. 3.1 shows how the forward and backward rates of the Sabatier reaction change with time at 353 °C. Fig. 3.1 [2] Accept for pencil for curve drawing (ii) Sketch, in Fig. 3.1, showing how:  the time at which equilibrium is reached and  f