NJC Prelims Paper 2_Answers Final

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. Disproportionation Note : I is simultaneously oxidized from O.S. = 0 in I2 to O.S. = +5 in IO3− and reduced from O.S. = 0 in I2 to O.S. = −1 in I− [1] (ii) Write the two balanced half-equations for the above reaction. [O] I2 + 12OH– o 2IO3– + 6 H2O + 10e– (construct by following the steps in balancing half equation) [R] I2 + 2e– o 2I– (obtained from Data Booklet) State symbols not required [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– (aq) + 2I– (aq) o 2SO42– (aq) + I2 (aq) (i) Explain why the rate of the above reaction is slow. The rate of the reaction is very slow because the activation energy is very high due to the collision of two ions of the same charges. [1] (ii) With the aid of the Boltzmann distribution, explain how addition of a homogenous catalyst helps to increase the rate of a reaction. A catalyst provides an alternative reaction pathway with lower activation energy (Ea2). The fraction of particles with K.E. ≥ Ea increases as shown in the Boltzmann distribution. The frequency of effective collisio