2025 ASRJC Prelim H1Chem P2 MS

ASRJC JC2 Prelim 2025 8873/H1 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2025 JC 2 Preliminary Paper NAME:______________________________ ( ) CLASS: 25 / ___ CHEMISTRY 8873/02 Higher 1 27 August 2025 2 hours Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and register number in the spaces provided at the top of this page. Write in dark blue or black pen. You may use a pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. Section A 1 /12 Section B 7/8 / 20 2 /9 Paper 1 / 30 3 /13 Paper 2 / 80 4 /12 Percentage Overall 5 /10 6 /4 Grade This document consists of __ printed pages and ___blank pages.

2 ASRJC JC2 Prelim 2025 8873/H1 Section A Answer all the questions in this section in the spaces provided. 1 (a) Two Period 3 elements react with an excess of oxygen at room pressure. (i) Complete Table 1.1. Table 1.1 Period 3 element Physical state of oxide at room temperature and pressure Approximate pH of solution made when oxide is added to water Na solid 13-14 S gas 1-2 [2] any two correct [1] (ii) The solutions made from oxides of sodium and sulfur of Table 1.1 are mixed together. Name the type of reaction that occurs. ……………………………….………………………………………………….[1] Acid-base or neutralisation [1] (b) Aluminium oxide, Al2O3 is amphoteric. (i) Explain what is meant by amphoteric. ……………………………………………………………………………………... …………………………………………………………………………………..[1] Reacts with both acid and bases / shows both acidic and basic behaviour. [1] (ii) Aluminium oxide is added to separate samples of dilute sulfuric acid and aqueous sodium hydroxide. Write two equations to describe the reactions that occur. ……………………………………………………………………………………... ……………………………………………………………………………………... …………………………………………………………………………………..[2] Al2O3(s) + 3H2SO4 (aq) → Al2(SO4)3(aq) + 3H2O(l) [1]

3 ASRJC JC2 Prelim 2025 8873/H1 [Turn over Al2O3(s) +2NaOH(aq) + 3H2O(l) → 2Na[Al(OH)4] (aq) [1] Ignored state symbols although we encouraged we written equations. Maximum 1 credit for correct ionic equations. (c) Chorine gas reacts with oxygen to form chlorine dioxide. When use d in small quantities, chlorine dioxide can disinfect water. (i) Complete the electronic configurations of Cl. Cl: 1s2 …………………………………… [1] Cl: 1s2 2s2 2p6 3s2 3p5 (ii) State the total number of outer shell electrons in one molecule of ClO2 and explain why this is an unusual number. ……………………………………………………………………………………... …………………………………………………………………………………..[2] 6+6+7 = 19 electrons. [1] It has odd number of electrons. [1] (iii) ClO2 contains two double bonds in the molecule. Draw a ‘dot-and-cross’ diagram for the molecule ClO2. [1] (iv) Deduce whether ClO2 is a polar molecule. Explain your answer. ……………………………………………………………………………………... ……………………………………………………………………………………... …………………………………………………………………………………..[2] Polar [1] It has polar C=O bonds and the shape of the molecule is bent. Dipole moments are not cancelled /accept similar idea. [1] [Total:12]

4 ASRJC JC2 Prelim 2025 8873/H1 2 Atmospheric carbon dioxide can dissolve in seawater. The three inorganic carbon-containing species present in the seawater are: • dissolved CO2(g) • CO32─(aq) • HCO3─(aq). Increasing levels of CO 2(g) in the atmosphere can change the proportions of these inorganic carbon-containing species present in sea water. This can lead to a series of chemical reactions resulting in the increase of concentration of hydrogen ions. This is ocean acidification. This process also causes carbonate ions to be less abundant. Equilibrium 1 CO2 + H2O HCO3─ + H+ K1 Equilibrium 2 HCO3─ CO32─ + H+ K2 When changes to the amount of dissolved carbon dioxide in the ocean occur, the pH of seawater remains approximately constant between pH 7.5 and 8.5. This is due to CO32─(aq) and HCO3─(aq) acting as a buffer. (a) Write the expression for the equilibrium constant, Kc, for equilibrium 2. [1] 𝐾𝑐 = [𝐶𝑂32−][𝐻+] [𝐻𝐶𝑂3 −] [1] (b) Approximately 200 years ago, the average pH of sea water taken from the ocean surface was 8.2. At present, this is 8.1. Calculate the hydrogen ion concentrations of the sea water taken from the ocean surface at 8.2 and 8.1. Hence, calculate the percentage increase in hydrogen ion concentration of the sea water taken from the ocean surface at present compared with 200 years ago. [3] pH = -log [H+] [H+] = 10─8.1 = 7.943 x10-9 mol dm─3 [H+] = 10─8.2 = 6.309 x10-9 mol dm─3 both [1] Amount increase =7.943 x10-9 ─ 6.309 x10-9 = 1.634 x10-9 [1] Percentage increase = (1.634 x10-9 / 6.309 x10-9) x 100 = 25.9 % [1]

5 ASRJC JC2 Prelim 2025 8873/H1 [Turn over (c) Using equilibrium 1 and 2, explain how the increase in dissolved CO2(g) leads to a lower abundance of CO32─(aq). …………………………………………………………………………………...………… …………………………………………………………………………………...………… …………………………………………………………………………………………...... ……………………………………………………………….…………………………[2] When CO2(g) dissolved in sea water, position of equilibrium in reaction 1 shifts right increasing concentration of H+. [1] This leads to increase in concentration of H+ and position of equilibrium in reaction 2 shifts left. This causes concentration of CO32─ to decrease. [1] (d) Suggest an impact on the ecology in the sea due to the decrease in concentration of carbonate ions. ……………………………………………………………………………………………. ……………………………….…………………………………………………………[1] It leads to difficulty for building and maintain shells for calcifying organisms such as clams, oysters and corals etc. [1] (e) Kc of equilibrium 2 for a sample of sea water collected increases when temperature is increased. State the nature of equilibrium 2 as either endothermic or exothermic. Explain your answer. …………………………………………………………………………………...………… ……………………………………………………………………………………………... …………………………………………………………………………………..………… ……………………………….………………………………………………………….[2] Endothermic. [1] When temperature increases, position of equilibrium shift right producing higher concentrations of products to remove (excess) heat . This result s in increase in the value Kc. [1] [Total: 9]

6 ASRJC JC2 Prelim 2025 8873/H1 3 (a) NaBiO3 is an oxidising agent with similar properties to KMnO4. Fig 3.1 shows an example of the use of NaBiO3 as an oxidising agent. Fig. 3.1 (i) Explain why the reaction in Fig 3.1 is considered an oxidation reaction. …………………………………………………………………………………… …………………………………………………………………………………[1] Removal of H atoms from X. [1] Or Oxidation number of carbons increa