2024 RI Prelim H2 Chem Paper 2 QP Final

© Raffles Institution 2024 9729/02/S/24 [Turn Over RAFFLES INSTITUTION 2024 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS INDEX NUMBER CHEMISTRY 9729/02 Paper 2 Structured Questions 10 September 2024 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not open this question booklet until you are told to do so. Write your name, class and index number in the spaces 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. A Data Booklet is provided. Do not write anything in it. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 / 15 2 / 19 3 / 22 4 / 19 Total / 75 This document consists of 24 printed pages.

© Raffles Institution 2024 9729/02/S/24 [Turn Over 2 Answer all questions in the space provided. 1 Boron and its compounds have diverse applications across industries. (a) (i) Write an equation to represent the fourth ionisation energy of boron. ……………………………………………………………………………………..…......... [1] (ii) Explain why the fourth ionisation energy of boron is much higher than that of carbon. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..…......... [2] (b) Borane, BH3, is a useful compound of boron. Borane undergoes complete combustion in excess oxygen to form boron trioxide, B2O3, as shown in equation 1. equation 1 BH3(g) + 3 2 (g) 1 2 (g) + 3 2 (g) (i) Draw a dot-and-cross diagram of the BH3 molecule. [1] (ii) State the shape and bond angle in BH3. ……………………………………………………………………………………..…......... [2] ∆Hr O BB OOO O O HH

© Raffles Institution 2024 9729/02/S/24 [Turn Over 3 (iii) Table 1.1 shows some bond energy data. Table 1.1 bond bond energy / kJ mol–1 B–B 293 B–H 330 B–O 536 B=O 837 Use the data in Table 1.1, together with data from the Data Booklet, to calculate the enthalpy change, ∆Hr, for the reaction in equation 1. [2] (iv) Boron trioxide, B2O3, is a relatively inert glass-like solid at 298 K. The standard enthalpy change of combustion of borane, BH 3(g), is more exothermic than ∆Hr. Explain why. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..…......... [1]

© Raffles Institution 2024 9729/02/S/24 [Turn Over 4 (c) At certain temperatures, BH3 dimerises into diborane, B2H6. Similarly, AlCl3 also dimerises to form Al2Cl6. (i) Explain why AlCl3 can undergo dimerisation. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..…......... [1] (ii) By considering the shape around the Al atom, state the change in hybridisation of the Al atom when AlCl3 dimerises to Al2Cl6. ……………………………………………………………………………………..…......... [1] Fig. 1.1 shows the structure of B2H6. B B Ha Ha Ha Ha Hb Hb Fig. 1.1 (iii) The central hydrogen atoms, H b, bond to both boron atoms. Two electrons are shared across the three atoms in B−Hb−B. Explain why the B−Hb bond is longer than B−Ha bond. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..…......... [1]

© Raffles Institution 2024 9729/02/S/24 [Turn Over 5 (d) Borazine, B3N3H6, is similar in structure to benzene and consists of a six -membered ring with alternating boron and nitrogen atoms. (i) The borazine structure can be represented by two different resonance structures, X and Y, as shown in Fig. 1.2. The arrow indicates that the actual structure of borazine is somewhere in between these two structures. Complete Fig. 1.2 by adding curly arrows to structure X. N B N B N B H H H H H H N B N B N B H H H H H H X Y Fig. 1.2 [1] (ii) Explain why borazine is less resonance stabilised than benzene. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..…......... [1] (iii) Borazine undergoes addition reactions due to the less resonance stabilised structure. X accounts for the major products formed from addition reactions of borazine. When one mole of X reacts completely with three moles of H Cl, compound Z is formed as the only product. Draw the structure of Z. [1] [Total: 15]

© Raffles Institution 2024 9729/02/S/24 [Turn Over 6 2 Water chemistry plays a critical role in the well-being of fishes in an aquarium . Fish waste releases ammonia into water which is toxic to fish. During nitrification, ammonia is broken down by beneficial bacteria into less toxic nitrate ions as shown in Fig. 2.1. ammonia NH3 nitrite NO2 – nitrate NO3 – beneficial bacteria A beneficial bacteria B Fig. 2.1 (a) (i) Under acidic conditions, the equation for nitrification is shown. NH4+ + 2O2 → 2H+ + NO3− + H2O Use relevant standard redox potentials from the Data Booklet to show that this reaction is spontaneous under standard conditions. [1] (ii) Despite nitrification being spontaneous under the conditions in an aquarium, the presence of beneficial bacteria is necessary for any appreciable breakdown of NH 4+ to be observed. Suggest the role of beneficial bacteria in the nitrification process. ……………………………………………………………………………………..…......... [1]

© Raffles Institution 2024 9729/02/S/24 [Turn Over 7 (b) Setting up a new aquarium involves establishing colonies of beneficial bacteria in the tank before fish are added. Ammonia is added daily and the concentrations of different water parameters are monitored. Fig. 2.2 shows a graph of some water parameters over 35 days. The presence of the respective products in Fig. 2.1 indicates that the beneficial bacteria has been established. Fish can only be safely added after the concentration of NO2− increases then decreases to 0 ppm as NO2− is also toxic to fish. The total ammonia nitrogen, TAN, measured is the sum of the concentrations of NH 3 and NH4+. Fig. 2.2 (i) State the day number that first indicates that beneficial bacteria A is established. ……………………………………………………………………………………..…......... [1] (ii) State the day number that is first safe for the addition of fish. ……………………………………………………………………………………..…......... [1] (iii) On day 35, 25% of the tank water is replaced with water that is free of NO3−. Calculate the new concentration of NO3−, in ppm. [1] 0 5 10 15 20 25 30 35 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 TAN NO2− NO3− concentration / ppm day number

© Raffles Institution 2024 9729/02/S/24 [Turn Over 8 (iv) At pH 7.4, ammonia released by fish waste exists as a mixture of NH3 a