NJC Prelim P3_Question_Paper

NJC SH2 Preliminary Examination 9729/03/17 [Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on separate paper. Additional Materials: Data Booklet Answer Paper 9729/03 Monday 11 Sep 2017 2 hours READ THESE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use paper clips, highlighters, glue or correction fluid/tape. Section A Answers all questions. Section B Answers one question. 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. This document consists of 11 printed pages and 1 blank page.

2 NJC SH2 Preliminary Examination 9729/03/17 Section A Answer all the questions in this section. 1 (a) (i) Write an equation for the thermal decomposition of barium carbonate. [1] (ii) Describe and explain the trend observed in the ease of thermal decomposition of the carbonates of the Group 2 elements. [2] (b) Monuments made of marble or limestone, such as the Taj Mahal in India and the Mayan temples in Mexico, are eroded by acid rain containing sulfuric acid. The carbonate stone is converted into sulfate by acid rain. CaCO3(s) + H2SO4(aq)  CaSO4(s) + H2O(l) + CO2(g) The life of such monuments is now being extended by treating them with a mixture of urea, (NH2)2CO, and barium hydroxide solutions. reaction 1: After soaking in the pores of the ca rbonate stone containing water, urea reacts gradually to give ammonia and carbon dioxide. reaction 2: Carbon dioxide produced then reacts with barium hydroxide to form barium carbonate. (i) State the shape and bond angle around the C atom of (NH2)2CO. [2] (ii) Write balanced equations for reactions 1 and 2. [2] (c) When carbon dioxide gas dissolves in water, carbonic acid, H2CO3, is formed. CO2(g) CO2(aq) CO2(aq) + H2O(l) H2CO3(aq) In a school laboratory, a student was instructed to titrate 25.0 cm 3 of 0.100 mol dm –3 carbonic acid with 0.125 mol dm −3 NaOH using phenolphthalein indicator. (Ka1 of H2CO3 = 4.3 × 10−7 mol dm−3) (i) Calculate the pH of 0.100 mol dm–3 H2CO3. [1] (ii) Calculate the volume of NaOH needed to completely react with 25.0 cm 3 of