2023_DHS_H2_Chem_Prelim_Paper_3_Suggested_Solutions

© DHS 2023 9729/03 [Turn over Suggested Solutions DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY Paper 3 Free Response Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 21 September 2023 2 hours READ THESE INSTRUCTIONS FIRST Write your centre number, index number, name and class 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. 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 one question. A Data Booklet is provided. 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. For Examiner’s Use Section A 1 20 2 20 3 20 Section B 4 / 5 20 Total 80 This document consists of 25 printed pages and 1 blank page.

2 © DHS 2023 9729/03 Section A Answer all the questions in this section. 1 (a) Group 2 salts, such as carbonates, decompose when heated. Table 1.1 shows information of some Group 2 salts that undergo thermal decomposition. Table 1.1 Group 2 salt thermal decomposition temperature / °C MgCO3 350 CaCO3 825 (i) Explain the difference in thermal decomposition temperature of MgCO3 and CaCO3. [2] The Ca 2+ cation has a lower charge density and hence lower polarising power compared to the Mg2+ cation. The electron cloud of CO 3 2– anion is distorted/polarised by Ca 2+ to a lesser extent compared to Mg 2+. Hence the covalent bonds in CO 3 2– are weakened to a lesser extent by Ca 2+ and higher energy is required for the thermal decomposition of CaCO3. (ii) Hence predict the thermal decomposition temperature for SrCO3. [1] Any temperature higher than 825°C (iii) Write an equation for the thermal decomposition of CaCO 3. Include state symbols in your answer. [1] CaCO3 (s) → CaO (s) + CO2 (g) (iv) State and explain the sign of S for the thermal decomposition of CaCO 3. Hence explain how spontaneity of the reaction changes as temperature increases. [2] S > 0 / is positive since there is an increase in number of gaseous particles from 0 to 1. G = H – TS. Hence G becomes more negative as temperature increases and spontaneity increases. (v) Outline a suitable experiment to verify that the thermal decomposition of a sample of Mg(OH)2 is complete. No details regarding use of specific glassware are required. [2] 1. Transfer the sample to a crucible. 2. Weigh the sample using an electronic mass balance. 3. Heat the sample for a few minutes.