2019 ACJC Prelim H2 Chem P2 ANS

1 © ACJC2019 9729/02/Prelim Exam/2019 [Turn over Index No. Name Form Class Tutorial Class Subject Tutor ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY 9729/02 Higher 2 Paper 2 Structured Questions 26 August 2019 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, index number, form class, tutorial class and subject tutor’s name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. 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. 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 Question no. Marks 1 / 7 2 / 21 3 / 15 4 / 7 5 / 11 6 / 14 TOTAL / 75 This document consists of 21 printed pages and 1 blank page. 9729/02/Prelim/19 ANGLO-CHINESE JUNIOR COLLEGE © ACJC 2019 Department of Chemistry [Turn over

2 © ACJC2019 9729/02/Prelim Exam/2019 [Turn over 1 (a) The element aluminium and its compounds have some properties characteristic of metals, and some of non -metals. Aluminium hydroxide, for example, is known to be amphoteric. (i) Explain the meaning of the word “amphoteric”. [1] The ability to react with both acids and bases. Aluminium sulfate and calcium oxide are sometimes added to water supplies to co - precipitate suspended solids and bacteria. A small amount of aluminium -containing ions remains in solution and its presence in drinking water may contribute to the mental illness known as Alzheimer’s disease. (ii) Write a balanced equation for the reaction that occurs when aluminium sulfate and calcium oxide are added to water, given that aluminium hydroxide is one of the products formed. [1] Al2(SO4)3 + 3CaO + 3H2O  2Al(OH)3 + 3CaSO4 (iii) By considering the nature of calcium oxide, explain why adding too much of it would increase the risk of contracting Alzheimer’s disease. Write an equation to illustrate how “aluminium-containing ions remains in (drinking water)” as a result of adding too much calcium oxide. [2] CaO dissolves slightly to form an alkaline solution. OH- thus formed reacts with Al(OH)3 to form a soluble complex. Al(OH)3 + OH-  Al(OH)4- 1 (b) Beryllium oxide (BeO) is amphoteric, just like Al(OH)3. (i) Beryllium oxide reacts with sodium hydroxide according to the equation, 2NaOH + BeO Na2BeO2 + H2O Given the position of beryllium in the Periodic Table, explain how this reaction illustrates the amphoteric nature of beryllium oxide. [2] BeO: Gp 2 ( metallic) oxide, expected to form basic oxide and to react with acid. but here it is reacting with an alkali, illustrating its acidic nature.

3 © ACJC2019 9729/02/Prelim Exam/2019 [Turn over (ii) To further illustrate its amphoteric nature, at 500 oC, BeO reacts with Na2O to form compound F as the sole product . The molar masses of all three compounds are tabulated below. compound molar mass / g mol–1 BeO 25 Na2O 62 F 149 Write a balanced equation of the above reaction. [1] BeO + 2Na2O  Na4BeO3 [Total: 7] 2 (a) In the laboratory, there are three bottles labelled X, Y and Z. Each bottle contains one of the following reagents: KI(aq), Cl2(aq), and NaBr(aq) Three tests were carried out using the reagents in the bottles. The results are summarised in the table below: test procedure observations 1 mix reagent in bottle X with reagent in bottle Z no change in colour 2 mix reagent in bottle Y with reagent in bottle Z mixture turns brown 3 mix reagent in bottle Y with reagent in bottle X mixture turns reddish- brown (i) By comparing relevant standard reduction potential values from the Data Booklet, explain how it can be deduced that Y is aqueous chlorine. There is no need for calculations. [1] Chlorine has the most positive standard reduction potential (compared to that of bromine and iodine) hence it is able to oxidise both iodide and bromide.

4 © ACJC2019 9729/02/Prelim Exam/2019 [Turn over (ii) Tests 2 and 3 were executed to determine the reagents in bottles X and Z. Hexane was added to the resulting reaction mixture after the tests were conducted. The bottles were then shaken and allowed to stand. State the observations that will indicate whether th e bottles contained KI(aq) or NaBr(aq) initially. [2] The bottle with orange-red organic layer contains NaBr (aq) initially. The bottle with violet / purple organic layer contains KI (aq) initially. 2 (b) Sulfur dichloride, SCl2, is a cherry-red liquid at room temperature and pressure. SCl2 is formed from S8 and Cl2. (i) Explain why S8 exists as a solid while Cl2 exists as a gas at room temperature. [2] Both exist as non-polar simple covalent molecules. The size of the electron cloud of sulfur is so much larger hence the idid interactions amongst sulfur molecules are stronger than the idid amongst chlorine molecules. Therefore the melting point of sulfur is significantly higher than that of chlorine. (ii) The formation of SCl2 from S8 and Cl2 takes place in two steps. The first step involves disulfur dichloride, S2Cl2, as an intermediate. S8 + 4Cl2  4S2Cl2 Write the equation of the second step. [1] S2Cl2 + Cl2  2SCl2 Some chemists speculate that the intermediate is not disulfur dichloride but K. K shares the same elemental mass percentages as sulfur dichloride and has a molar mass of 206.2 g mol-1. (iii) State the molecular formula of K. [1] S2Cl4 (iv) All the chlorine atoms in K are terminal. There are only two central atoms in K. The bond angles about each central atom are different. State the shape around each central atom and the respective bond angles. [2] See-saw (900 and/or 120o). (880 and/or 1180) and bent (1050 or 104.50)

5 © ACJC2019 9729/02/Prelim Exam/2019 [Turn over 2 (c) Sulfuryl chloride, SO2Cl2, is commonly confused with thionyl chloride, SOCl2. The properties of these two sulfur oxychlorides are quite different. SO2Cl2 is a source of chlorine while SOCl2 is a source of chloride ions for various organic reactions. When heated, sulfuryl chloride decomposes endothermically as follows: SO2Cl2 (g) SO2 (g) + Cl2 (g) In an experiment, 1.00 mol of SO2Cl2 vapour was heated in a closed 4.00 dm3 flask at 500 K until equilibrium was established. The flask was then rapidly cooled to liquefy SO2Cl2. After removing gaseous SO 2 and Cl2, excess water was then carefully added to the liquid SO2Cl2, causing the following reaction to occur. SO2Cl2 (l) + 2H2O (l) → H2SO4 (aq) + 2HCl (aq) The resulting solution was made up to 250 cm3 in a standard graduated fla