2020 ACJC Prelim Paper 2 Qns

3 © ACJC2020 9729/02/Prelim/2020 [Turn over Answer all the 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 C l. Explain why this is so. ………………………………………………………………………………...….. ………………………………………………………………………………...….. ………………………………………………………………………………...….. ………………………………………………………………………………...….. ……………………………………………………………………………….…[2] (ii) Write the electronic configurations of the sodium atom and its ion. Na 1s2………………………………… Na+ 1s2………………………………… [1] (iii) Explain the difference in the radius of the sodium atom and the radius of its ion, Na+. ………………………………………………………………………………….[1] (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 SO3 [4]

4 © ACJC2020 9729/02/Prelim/2020 [Turn over (ii) State one source of SO2 pollution. ………………………………………………………………………………….[1] (iii) State an environmental consequence of SO2. ………………………………………………………………………………….[1] (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 A lCl3 and state the C l−Al−Cl bond angle. dot-and-cross diagram of AlCl3 [1] bond angle: ………………………………. [1] (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]

5 © ACJC2020 9729/02/Prelim/2020 [Turn over (iii) With a small amount of water, BeC l2 hydrolyses in the same manner as AlCl3. Write an equation for the hydrolysis of BeCl2. ………………………………………………………………………………….[1] [Total: 14]

6 © ACJC2020 9729/02/Prelim/2020 [Turn over 2 This question concerns the chemistry of the halogens and hydrogen halides. (a) Fluorine, F2, and chlorine, Cl2, are halogens that exist as gases at room temperature and pressure. (i) With reference to their structure s and bonding, e xplain why C l2 has a higher boiling point than F2. ………………………………………………………………………………...…. ………………………………………………………………………………...…. ………………………………………………………………………………...…. ………………………………………………………………………………...…. ………………………………………………………………………………...…. …………………………………………………………………………………[2] (ii) Use data from the Data Booklet to compare the oxidising strengths of F2 and Cl2. ………………………………………………………………………………...…. ………………………………………………………………………………...…. ………………………………………………………………………………...…. ………………………………………………………………………………...…. …………………………………………………………………………………[2] (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 densities 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]

7 © ACJC2020 9729/02/Prelim/2020 [Turn over (ii) Complete Fig. 2.1 by stating the colours observed in the test tube. Fig. 2.1 [1] (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 pressures 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. ………………………………………………………………………………...………... ………………………………………………………………………………...………... ……………………………………………………………………………………..…[1] [Total: 7]

8 © ACJC2020 9729/02/Prelim/2020 [Turn over 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] (ii) Write the Kp expression for this reaction. [1] (iii) State the type of catalyst used in this reaction. …………………………………………………………………………………[1] (b) 0.05 mol of carbon dioxide and 0.05 mol of hydrogen were introduced in a 1 dm3 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]

9 © ACJC2020 9729/02/Prelim/2020 [Turn over (ii) Determine the equilibrium partial pressures, in kPa, of:  carbon dioxide  hydrogen  water [2] (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]

10 © 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 (ii) The temperature is then increased from 353 °C to 400 °C. Sketch in Fig. 3.1 how the following would change at the higher temperature.  time at which equilibrium is reached  forward and backward rates of the Sabatier reaction [2] (iii) State and explain how the Kc value for the Sabatier reaction changes when the temperature is increased from 353 °C to 400°C. ………………………………………………………………………………...…. ………………………………………………………………………………...…. ………………………………………………………………………………...…. …………………………………………………………………………………[2] (d) Suggest a reagent that can be used to remove carbon dioxid e from the equilibrium mixture. ………………………………………………………………………………..………[1] time / s t rate / mol dm−3 s−1

11 © ACJC2020 9729/02/Prelim/2020 [Turn over (e) With the aid of a labelled Boltzmann distribution, e xplain how the addition of ruthenium metal increases the rate of the reaction. ………………………………………………………………………………...….…..... ………………………………………………………………………………...….…..... ………………………………………………………………………………...….…..... ………………………………………………………………………………...….…..... ………………………………………………………………………………..………[3] [Total: 17]

12 © ACJC2020 9729/02/Prelim/2020 [Turn over 4 In potentiometric titrations, the electrode potential, E, is measured against the volume of a solution added from the burette. To determine the concentration of acidified FeCl2 solution , 25.0 cm3 of this solution was placed in a beaker and titrated against 0.02 mol dm−3 acidified K2Cr2O7 from a burette. It was found that 20.80 cm3 of K2Cr2O7 was needed to reach the end–point. The electrode potential, E, was measured against the standard hydrogen electrode . All me