2021 RI H3 Chem Prelims P1 Questions

1 © Raffles Institution 2021 9813/01/S/21 [Turn over RAFFLES INSTITUTION 2021 YEAR 6 PRELIMINARY EXAMINATION Higher 3 CANDIDATE NAME CLASS 2 1 S 0 CENTRE NUMBER S 3 0 1 6 INDEX NUMBER CHEMISTRY 9813/01 Paper 1 28 September 2021 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: Data Booklet Insert READ THESE INSTRUCTIONS FIRST Write your name, class and index number on all the work you hand in. 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. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer two questions. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 44 printed pages and 1 insert. Question Marks Section A (answer all) 1 / 20 2 / 20 3 / 20 Section B (answer two) 4 / 20 5 / 20 6 / 20 Total / 100

2 © Raffles Institution 2021 9813/01/S/21 Section A Answer all questions in this section. The information provided in the insert is taken from several published scientific articles. Other published articles may not agree with all of this information. You should read the whole insert before you start to answer any questions and use the information it contains to answer the questions. 1 (a) An example of a feldspar (aluminium based silicates) weathering reaction may be represented by the overall equation: 2KAlSi3O8 + 2CO2 + 11H2O  Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ + 2HCO3− ------ (1) As part of reaction (1), the silicate anions are involved. For example: SiO44− + H+  HSiO43− and so on till H3SiO4− + H+  H4SiO4 CO2 reacts with water according to the following equations CO2(g) + aq  CO2(aq) ------ (2) CO2(aq) + H2O(l)  H2CO3(aq) ------ (3) H2CO3(aq)  H+(aq) + HCO3−(aq) ------ (4) HCO3−(aq)  H+(aq) + CO32−(aq) ------ (5) (i) The pKa1 values of H2CO3 and H4SiO4 are 6.35 and 9.84 respectively. By considering the structures of their conjugate bases , suggest two reasons for the relative p Ka1 values of H2CO3 and H4SiO4. [2] (ii) With reference to equations (2) to (5), comment on the implication of the pKa1 values in (a)(i) on the equilibrium between silicate and CO2, and hence the thermodynamic feasibility of silicate weathering in CO2 removal. [2] (iii) SiO2 has the following characteristics: melting point / °C solubility in water at 25 °C / mmol dm−3 hardness chemical reactivity 1713 2 very hard substance reacts very slowly with bases Explain the physical properties of SiO2. [2] ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ………………………………………………………………………………………………..

3 © Raffles Institution 2021 9813/01/S/21 [Turn over ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ………………………………………………………………………………………………..

4 © Raffles Institution 2021 9813/01/S/21 (iv) Silicic acid, H4SiO4, is usually used synonymously with silica, SiO 2. They exist in equilibrium in oceans. Write an equation showing the formation of silica from silicic acid. Identify the type of reaction and d escribe the mechanism for the reaction using curly arrows for electron transfer. [3] (v) By considering the information in (a)(iii) and (a)(iv), suggest one positive and one negative impact on the effectiveness of reaction (1) in the removal of CO2. [2] ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ………………………………………………………………………………………………..

5 © Raffles Institution 2021 9813/01/S/21 [Turn over (b) Fig. 1 contains three graphs plotted to show pH, calcite saturation ( Ω), and relative calcification levels as a function of atmospheric pCO 2 and increasing alkalinity of ocean waters. The colour gradient chart beside each graph indicates the colours that correspond to a range of values for each parameter. Each graph has three arrows representing the following effects of ocean acidification from atmospheric CO2: • with no OAE, • mitigation using air equilibrated OAE, • mitigation using non equilibrated OAE. For example, in graph A in Fig. 1 , the vertical arrow showing no OAE tells us that for a given alkalinity of ocean water, as pCO2 in the atmosphere increases, the acidity increases unmitigated, as the arrow goes upwards from the yellow region (pH ~ 8.6) to the blue region (pH ~ 8). (i) With reference to graph A in Fig. 1, explain the arrows showing the changes of pH with non-equilibrated OAE and air-equilibrated OAE. [2] (ii) By using the graphs in Fig. 1, explain how changing pH levels affect the relative calcification of marine organisms. [1] (iii) When implementing Ocean Alkalinity Enhancement (OAE) as a method to remove carbon dioxide, we need to consider the impact of OAE on the marine ecosystem, including the various organisms (calcifying and non-calcifying) and food producers. Comment on two ways how OAE would affect the marine ecosystem. [2] ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ………………………………………………………………………………………………