NYJC 2021 H2 Chemistry 9729 P2

[Turn Over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS TUTOR’S NAME CHEMISTRY 9729/02 Paper 2 Structured Questions 01 September 2021 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class 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. 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 1 /14 2 /16 3 /20 4 /25 Total /75 This document consists of 20 printed pages..

2 H2 Chemistry 9729/02 NYJC J2/2021 PRELIM. Answer all the questions in the spaces provided. 1 This is a question on the salts of magnesium. (a) Struvite, MgNH4PO4·6H2O, is a fertiliser mineral that can be obtained from wastewaters. It is a salt that contains two cations and one anion. (i) Draw a dot-and-cross diagram of MgNH4PO4. [2] (ii) Explain why MgNH4PO4 is only slightly soluble in water. …..………………………………………………………………………………………… ..…………………………………………………………………………………………… ….…………………………………………………………………………………………. …………………………………………………………………………………………. [1] (iii) Thermal decomposition of MgNH4PO4 produces an alkaline gas, water and a white powder of magnesium pyrophosphate, Mg2P2O7. Write an equation with state symbols for this reaction. …………………………………………………………………………………………. [1] (b) The value of the solubility products and melting points of some magnesium salts at 298 K are shown in Table 1.1. Table 1.1 salt value of solubility product melting point / oC magnesium chloride, MgCl2 714 magnesium hydroxide, Mg(OH)2 1.8 × 1011 350 magnesium phosphate, Mg3(PO4)2 4.0 × 1025 1184 In this question, give each of your numerical answers to two significant figures. Magnesium phosphate is widely used in medication to support the relaxation of muscles. This compound is used to prevent the cramping of muscles and vitamin E deficiency. (i) With an appropriate equation, define the lattice energy of magnesium phosphate. …..………………………………………………………………………………………… ..…………………………………………………………………………………………… …………………………………………………………………………………………. [2]

3 H2 Chemistry 9729/02 NYJC J2/2021 PRELIM. [Turn Over (ii) Suggest an explanation, in terms of structure and bonding, for the difference in melting points between magnesium chloride and magnesium phosphate. …..………………………………………………………………………………………… ..…………………………………………………………………………………………… …..………………………………………………………………………………………… ..…………………………………………………………………………………………… ….…………………………………………………………………………………………. …………………………………………………………………………………………. [2] (iii) Write an expression for the solubility product of magnesium phosphate, stating its units. …………………………………………………………………………………………. [1] (iv) Determine the [Mg2+(aq)] of a saturated solution of magnesium phosphate. [2] (v) Solid magnesium chloride was slowly added to a solution containing 0.15 mol dm3 of sodium hydroxide and 0.32 mol dm3 of sodium phosphate at 298 K. Calculate the concentration of hydroxide ions in the solution when the first trace of magnesium phosphate appears. [3] [Total: 14]

4 H2 Chemistry 9729/02 NYJC J2/2021 PRELIM. 2 Nitrogen dioxide, NO2, is a significant air pollutant. It is a precursor for a number of harmful secondary air pollutants such as ozone and particulate matter, and plays a role in the oxidation of atmospheric sulfur dioxide. PM refers to particulate matter in the atmosphere that are so small they can only be detected using an electron microscope. Most of these particles form in the atmosphere as a result of pollution emitted from power plants, industries and automobiles. PM10 is particulate matter 10 Pm or less in diameter, PM2.5 is particulate matter 2.5 Pm or less in diameter. Due to their small size, they can penetrate the deepest parts of the lungs as well as access the gas exchange regions of the lung via diffusion. To safeguard public health, concentration of NO2, PM10 and PM2.5 around Singapore are closely monitored. Table 2.1 average reading over last 4 years reading during Circuit Breaker NO2 (Pg m-3) 33.1 15.1 PM10 (Pg m-3) 29.8 23 PM2.5 (Pg m-3) 14.4 10.1 Apr 07 – May 11, 2019 Apr 07 – May 11, 2020 Fig. 2.1 shows mean atmospheric concentrations of NO2 from 7 April to 11 May 2019 (top) and 2020 (bottom) imaged using data obtained from the ESA Copernicus Sentinel 5P satellite. Table 2.1 shows change of air quality parameters in Singapore during the Circuit Breaker period of 2020, a stay-at-home order and cordon sanitaire implemented by the Government of Singapore in response to the COVID-19 pandemic in the country on 7 April 2020. During this period, a ll non-essential workplaces were closed and all schools transitioned to home-based learning. Fig. 2.1 NO2 tropospheric column (Pmol m-2)

5 H2 Chemistry 9729/02 NYJC J2/2021 PRELIM. [Turn Over (a) (i) Describe and explain, with the aid of suitable equations, the role of NO2 in the oxidation of atmospheric sulfur dioxide. …..………………………………………………………………………………………… ..…………………………………………………………………………………………… ….…………………………………………………………………………………………. …………………………………………………………………………………………. [2] (ii) Using your understanding of the source of NO2, suggest a reason for the trend shown in Table 2.1. …..………………………………………………………………………………………… …………………………………………………………………………………………. [1] (iii) Suggest why the measured PM10 and PM2.5 did not change as significantly as NO2. …..………………………………………………………………………………………… …………………………………………………………………………………………. [1] (b) Haze events have been occurring in China megacities in recent years. These hazes mainly consist of aqueous sulfates, SO42, produced as a result of SO2 and NO2 reacting under particularly adverse meteorological conditions. One possible reaction is shown by the following unbalanced equation, where NO2 undergoes reduction to form NO2. NO2 + HSO3 + H2O o H + NO2 + SO42 Complete the chemical equation for the reaction. ……NO2 + ……HSO3 + ……H2O o ……H + ……NO2 + ……SO42 [1]

6 H2 Chemistry 9729/02 NYJC J2/2021 PRELIM. (c) NO2 can be formed from NO, as shown in equation 1. equation 1 2NO(g) + O2(g) o 2NO2(g) Three experiments were carried out to determine order of reaction with respect to NO and O2, and hence the rate constant for this reaction. Experiment 1 was carried out with initial concentrations of NO and O2 at 0.10 mol dm-3 and 0.0050 mol dm-3 respectively. Fig. 2.2 shows the concentration of O2 recorded against time for this experiment. Fig. 2.2 The experiment was repeated to find the initial rate of reaction at different initial concentrations of NO and O2. The results of these experiments are shown in Table 2.2. Table 2.2 Experiment Initial [NO] / mol dm3 Initial [O2] / mol dm3 Initial rate of formation of NO2 / mol dm3 s1 2 0.001 0.002 5.55 u 1010 3 0.002 0.003 3.33 u 109 (i) Define the terms order of reaction and rate constant. …..………………………………………………………………………………………… …..………………………………