2023 EJC H2 Chem Prelims Paper 2 QP

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2023 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 2 2 – INDEX NUMBER CHEMISTRY Paper 2 Structured Questions 9729/02 13 September 2023 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, civics group, index number in the spaces 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. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Paper 2 1 / 12 2 / 16 3 / 16 4 / 16 5 / 15 Total / 75 This document consists of 27 printed pages and 1 blank page.

2 © EJC 9729/02/J2PE/23 1 (a) In the past two decades, scientists have observed a steady decrease in the pH of the oceans. This phenomenon is known as ocean acidification. (i) Ocean acidification occurs when carbon dioxide dissolves and reacts with water in the ocean to form carbonic acid, H2CO3. Write equations to illustrate how carbon dioxide decreases the pH of seawater. ........................................................................................................................... ........................................................................................................................... ....................................................................................................................... [2] (ii) In the past two centuries, the average pH of seawater has dropped from 8.2 to 8.1. Calculate the percentage increase in the concentration of H +(aq) ions in sea water. [1] (iii) The significant change in pH of seawater is unexpected as the oceans have a natural buffering system involving CO 2– 3 (aq) and HCO – 3 (aq). Write an equation to show how the CO 2– 3 (aq)/HCO – 3 (aq) buffer system helps resist the decrease in the pH of seawater when carbon dioxide dissolves. ....................................................................................................................... [1] (b) The main cause of ocean acidification is the rising carbon dioxide levels in the atmosphere, which is a result of the increase in the use of fossil fuels. To combat this, alternative fuels such as methanol are being explored. (i) Define the term standard enthalpy change of combustion, cH∆  . ........................................................................................................................... ........................................................................................................................... ....................................................................................................................... [1]

3 © EJC 9729/02/J2PE/23 [Turn over (ii) When 1.86 g of methanol , CH 3OH, was combusted in a bomb calorimeter, a temperature rise of 5.1 K was measured. In a separate experiment, 1.00 g of benzoic acid, C 6H5CO2H, produced a temperature rise of 3.2 K when combusted in the same bomb calorimeter under identical conditions. Given that the enthalpy change of combustion of benzoic acid is −3226 kJ mol−1, calculate the enthalpy change of combustion of methanol. [3]

4 © EJC 9729/02/J2PE/23 (c) Methanol can be synthesi sed through the reaction between carbon dioxide and hydrogen, as shown in equilibrium 1 below. equilibrium 1 CO2(g) + 3H2(g)  CH3OH(l) + H2O(g) 1H∆  = −49.5 kJ mol−1 At the same time, a side reaction, as shown in equilibrium 2, can occur. equilibrium 2 CO2(g) + H2(g)  CO(g) + H2O(g) 2H∆  = +41.2 kJ mol−1 (i) Explain why the synthesis of methanol is usually conducted at low temperature and high pressure. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ....................................................................................................................... [2] (ii) CO(g) is sometimes added to the reaction mixture at constant volume after synthesis has occurred to a certain extent. Suggest one advantage of doing so. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ....................................................................................................................... [2] [Total: 12]

5 © EJC 9729/02/J2PE/23 [Turn over Question 2 starts on the next page

6 © EJC 9729/02/J2PE/23 2 Copper is a key metal needed in the production of electrical wirings that powers the modern world. (a) Copper is first extracted from mineral ores such as cuprite (Cu2O) and tenorite (CuO). (i) State the oxidation number and the electronic configuration of copper in Cu and CuO in Table 2.1. Table 2.1 copper species oxidation number electronic configuration Cu CuO [2] (ii) Explain why copper exists in different oxidation states such as in Cu2O and CuO. ........................................................................................................................... ....................................................................................................................... [1] (b) Samples of tenorite (CuO) was analysed for its isotopic composition of copper by means of mass spectrometry. The results of the analysis are given in the table below. Table 2.2 mass number 16 63 65 % abundance 49 15 36 Using Table 2.2, calculate the Ar value for copper. [1]

7 © EJC 9729/02/J2PE/23 [Turn over (c) After extraction from one of its mineral ores, the crude copper metal containing iron and silver metal impurities is further purified via electrolysis. (i) Draw a labelled diagram of the electroly tic cell used to purify a piece of crude copper metal. Include details of the cathode, anode and electrolyte. [1] (ii) Using relevant data from the Data Booklet, explain what happens at the anode and the cathode during the purification process. Anode: ...............................................................................................................