NJC 2021 Prelims Paper 2 QP

NJC/H2 Chem Preliminary Examination/02/2021 1 [Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on Question Paper. Additional Materials: Data Booklet 9729/02 24 August 2021 2 hours READ THE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use paper clips, highlighters, glue or correction fluid. Answers all questions. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 /8 2 /12 3 /8 4 /11 5 /17 6 /19 Paper 2 Total /75 Marks Weightings Paper 1 /30 15% Paper 2 /75 30% Overall Percentage Paper 3 /80 35% Paper 4 /55 20% Grade This document consists of 23 printed pages and 1 blank page.

NJC/H2 Chem Preliminary Examination/02/2021 2 Answer all the questions in the spaces provided. 1 (a) Iodine can undergo a reaction with aqueous potassium hydroxide to form iodate and iodide ions as shown in the following equation. 3I2(s) + 6KOH(aq) o KIO3(aq) + 5KI(aq) + 3H2O(l) This reaction has a potential application in times of nuclear catastrophe when radioactive and volatile iodine-131 is produced. Spraying alkalis into the nuclear reactors can convert the volatile iodine into non-volatile iodate and iodide ions, thus minimizing the damaging radioactive effects. (i) Name the type of reaction for the above equation. «««««««««««««««««««««««««««««««« [1] (ii) Write the two balanced half-equations for the above reaction. «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« [2]

NJC/H2 Chem Preliminary Examination/02/2021 3 [Turn over (b) The reaction of iodide and peroxodisulfate ions is very slow and can be catalysed by using a homogeneous catalyst. S2O82– + 2I– o 2SO42– + I2 (i) Explain why the rate of the above reaction is slow. «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« [1] (ii) With the aid of the Boltzmann distribution, explain how addition of a homogenous catalyst helps to increase the rate of a reaction. «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« [2]

NJC/H2 Chem Preliminary Examination/02/2021 4 (iii) By considering relevant Eꝋ values from the Data Booklet, explain how Fe3+(aq) can act as a homogenous catalyst in this reaction. «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« [2] [Total : 8]

NJC/H2 Chem Preliminary Examination/02/2021 5 [Turn over 2 (a) The Period 3 elements vary in their physical properties. (i) On the axes below, sketch the melting point and electrical conductivity trends for the stated elements. [2] (ii) Sulfur is an element in Period 3 of the Periodic Table. The graph below shows the second ionisation energies of eight elements with consecutive proton number. Which of the elements A to H represents sulfur? Explain your answer. «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« [2]

NJC/H2 Chem Preliminary Examination/02/2021 6 (b) A 0.400 g solid sample of a mineral, XY(CO3)2 (where X and Y are Group 2 elements) was heated strongly to give a mixture of oxides of X and Y and carbon dioxide. The solid mixture has a total mass of 0.275 g. The solid mixture was added to excess water and stirred. The suspension was filtered, and the oxide of X was obtained as a residue. The dried residue weighed 0.057 g. (i) Write a balanced equation for the decomposition of XY(CO3)2 «««««««««««««««««««««««««««««««« [1] (ii) Calculate the mass of carbon dioxide produced. [1] (iii) Hence, or otherwise, identify the metals, X and Y, showing your working clearly. [3]

NJC/H2 Chem Preliminary Examination/02/2021 7 [Turn over (c) Aluminium is commonly extracted from its oxide, Al2O3. (i) Al2O3 dissolves in hot aqueous solution of sodium hydroxide. Write an ionic equation to explain the reaction. «««««««««««««««««««««««««««««««« [1] (ii) Al2O3 is dissolved in molten cryolite. The mixture is electrolysed using graphite electrodes. The cell operates at a very high current of 50 000 A. Calculate the time needed to obtain 1 kg of pure aluminium. [2] [Total : 12]

NJC/H2 Chem Preliminary Examination/02/2021 8 3 A student wanted to investigate the effect of pH on the electrode potential, E, of an iron(II)/ iron half-cell. She started the experiment with 1 mol dm−3 Fe2+ with Fe electrode at pH 0. NaOH(aq) was added dropwise to the solution and the following data was collected. (i) Using the information provided, show that the green Fe(OH) 2 precipitate is produced at approximately pH 6.9. (Solubility product, Ksp, of Fe(OH)2 = 6.0 × 10–15 mol3 dm–9 at 25 qC) [2]

NJC/H2 Chem Preliminary Examination/02/2021 9 [Turn over (ii) With the use of the Data Booklet, explain the shape of the graph as fully as you can. «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« [3] (iii) Explain why Fe(OH)2 is green. «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« «««««««««««««««««««««««««««««««« [3] [Total : 8]

NJC/H2 Chem Preliminary Examination/02/2021 10 4 (a) Nitroglycerin, C3H5(NO3)3, is a flammable oil commonly used to manufacture dynamite. The atomisation of nitroglycerin is represented by the equation: C3H5(NO3)3(l) o 3C(g) + 5H(g) + 3N(g) + 9O(g) Table 4.1 Standard enthalpy change of formation of nitroglycerin(l) / kJ mol‒1 −364 Standard enthalpy change of atomisation of carbon (graphite) / kJ mol‒1 +715 Standard enthalpy change of formation of H2O(g) / kJ mol‒1 −242 Standard enthalpy change of formation of CO2(g) / kJ mol‒1 −394 (i) Using data from Table 4.1 and relevant values from the Data Booklet, draw an energy cycle and use it to calculate the standard enthalpy change of atomisation of nitroglycerin. [3]