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∆  . ........................................................................................................................... ............................................................................................