2021 DHS H2 Prelim Paper 2 Questions

© 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 19 printed pages and 1 blank page.

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] (ii) State the full electronic configuration of iron in Fe3(PO4)2. ……………………………………………………………………………………....[1]

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 reaction of iodine with hydrogen sulfide or with hydrazine. (i) Explain briefly why it is not industrially viable to produce HCl through the direct reaction of hydrogen with chlorine. ……………………………………………………………………………………........ ……………………………………………………………………………………....[1] (ii) Write a balanced equation showing the formation of HI from iodine and hydrogen sulfide. Hence, with reference to the Data Booklet, show that the reaction is spontaneous. S + 2H+ + 2e- ⇌ H2S Eo = +0.14 V ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………....[2] (iii) Explain why the pKa values decrease from HCl to HI. ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………....[2] (c) In an experiment, a sample of chloride ions was vapourised and passed through an electric field. Analysis revealed that a beam of 37Cl− gives an angle of deflection of −1.0°. (i) State the angle of deflection for a beam of deuterium nuclei in the same experimental set–up. (D is deuterium,H12) ……………………………………………………………………………………....[1]

4 © DHS 2021 9729/02 (ii) Under identical conditions, a beam of particles, Z, each having 32 times the mass of a proton, was deflected by an angle of −2.3°. Given that the proton number of Z is 16, deduce the identity of Z. [2] [Total: 12]

5 © DHS 2021 9729/02 [Turn over 2 The electrochemical oxidation of benzylic alcohols to the corresponding aldehydes via a two-phase electrolysis system was reported in 2007. An example is shown in Fig. 2.1. Fig. 2.1 (a) State the reagents and conditions for the conversion of benzylic alcohols to the corresponding aldehydes in a school laboratory. ……………………………………………………………………………………………....[1] (b) The two-phase electrolysis system comprises the aqueous NaBr and organic CHCl3 layers. (i) Draw simple diagrams to illustrate how a water molecule can be attached to a sodium cation, and to a bromide anion in the aqueous layer. Label each diagram to show the type of interaction involved. [2] Na+ cation Br- anion (ii) Predict and explain which layer the benzylic alcohols would predominantly dissolve in. ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………....[2] 1 OH R H O R R = H or alkyl Pt electrodes, aqueous NaBr/CHC l3 solvent layers room temperature

6 © DHS 2021 9729/02 (c) (i) Describe the change in oxidation state of the benzylic carbon, labelled 1 in Fig. 2.1, when the alcohol is oxidised to the corresponding aldehyde. ……………………………………………………………………………………........ ……………………………………………………………………………………....[1] (ii) In experiment 1 where R = H in Fig. 2.1, one mole of benzylic alcohol is oxidised to give 96 % yield of aldehyde. Using your answer in (c)(i), determine the charge, in Faraday, required to yield the aldehyde. [1] (iii) Current efficiency describes the efficiency with which charge is transferred in an electrolysis system and is given by the equation below. current efficiency = charge requiredcharge passed ´ 100% In experiment 1, 5.5 F of charge is passed per mole of benzylic alcohol. Using your answer in (c)(ii), calculate the current efficiency of experiment 1. [1]

7 © DHS 2021 9729/02 [Turn over (iv) Some data obtained from experiments 2 and 3 are shown in Table 2.1. Table 2.1 Experiment No. R group in Fig. 2.1 Current efficiency / % 2 -CH3 42.5 3 -C(CH3)3 64.0 Using the data provided in Table 2.1, deduce the relationship between the electronic effect of the R group on the benzene ring and the current efficiency of the electrochemical process. ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………....[1] (v) Hence, suggest how you would expect the current efficiency when R = -OCH3 to compare with that when R = -CH3. Explain your reasoning. ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………........ ……………………………………………………………………………………....[1] (d) In the two-phase electrolysis system, two platinum electrodes are placed in the upper aqueous NaBr layer and the bromide ions are oxidised at the anode. State the polarity of the anode and quote relevant values from the Data Booklet to explain the preferential production of bromine at the electrode. ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………....[2] [Total: 12]

8 © DHS 2021 9729/02 3 (a) The cis-trans isomerisation of but-2-ene in the gas phase is promoted by transition metal catalysts. (i) Complete the energy diagram below by drawing arrows to show how the enthalpy changes in Table 3.1 can be used to determine DH, the enthalpy change of conversion of cis-but-2-ene t