2021 DHS H2 Prelim Paper 2_Solutions

© DHS 2021 9729/02 [Turn over Name: Centre/Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 15 September 2021 2 hours READ THESE INSTRUCTIONS FIRST Write your centre number, index number, name and class at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 12 2 12 3 13 4 14 5 24 Total 75 This document consists of 30 printed pages.

2 © DHS 2021 9729/02 Answer all the questions in the spaces provided. 1 (a) Water chemistry plays an important role in aquariums. For disinfection purposes, chlorine, Cl2, and chloroamine, NH2Cl, are introduced into our tap water to destroy bacteria and viruses. However, these two chlorine–containing compounds are extremely harmful to fishes as they cause gill burns and blood poisoning. The typical concentration of some chemical species found in our tap water are listed in Table 1.1. Table 1.1 chemical species concentration / mol dm-3 Cl2 3.24 ´ 10-5 NH2Cl 3.88 ´ 10-5 PO43- 1.26 ´ 10-7 Fe (as Fe2+ & Fe3+) 1.08 ´ 10-7 To prepare an aquarium tank, a water conditioner containing sodium thiosulfate, Na2S2O3, is added to the tank to remove both the aqueous chlorine and chloroamine present in tap water. Sodium thiosulfate reacts with chlorine and chloroamine respectively as shown in the reactions below. 4Cl2 + S2O32- + 5H2O ® 8Cl- + 2SO42- + 10H+ NH2Cl + 2S2O32− + H2O ® NH3 + Cl− + S4O62− + OH− (i) Calculate the volume of 0.010 mol dm-3 sodium thiosulfate solution that would be required to prepare a tank containing 360 dm3 of water. [3] moles of Cl2 present = 3.24 x 10–5 x 360 = 1.167 x 10–2 mol moles of NH2Cl present = 3.88 x 10–5 x 360 = 1.397 x 10–2 mol moles of S2O32– to react with Cl2 = ¼ x 1.167 x 10–2 = 0.002916 mol moles of S2O32– to react with NH2Cl = 2 x 1.397 x 10–2 = 0.02794 mol Volume of S2O32– required = !.!!#$%&'!.!#($)!.!%! = 3.09 dm3 (ii) State the full electronic configuration of iron in Fe3(PO4)2. [1] 1s22s22p63s23p63d6

3 © DHS 2021 9729/02 [Turn over (b) The direct reaction of hydrogen with chlorine gives hydrogen chloride. However, hydrogen chloride is produced industrially by treatment of halide salts with sulfuric acid. On the other hand, hydrogen iodide, HI, is produced by the reac