2023 EJC H2 Chemistry Prelims Paper 2 (solution with comments)

© EJC 9729/02/J2PE/23 [Turn over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2023 General Certificate of Education Advanced Level Higher 2 Paper 2 Suggested Solutions with Marker’s comments 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] Comments: • Generally, well done except for a small group of students who were careless in (iii) • Question analysis and references(where applicable) This is a simple application question that requires students to pick out information from the question and construct relevant equations. • Common mistakes: - For (ii), not calculating percentage increase. - For (iii), Using → instead of ⇌. - Misinterpreting a decrease in pH as increase in OH‒ in the system instead of H+. - Writing HCO – 3 + H + → H2CO3. This is incorrect as the buffer system comprises of CO 2– 3 , the conjugate base and HCO – 3(aq), the acid. To resist a drop in pH, the conjugate base will remove acid (H+) to maintain relatively constant pH. 25.9% 8.1 8.2 8.2 10 10percentage increase 100% 10 −− − −=  = CO 2– 3 (aq) + H+(aq) → HCO – 3(aq) CO2(g) → CO2(aq) CO2(aq) + H2O(l) → H2CO3(aq) H2CO3(aq)  2H+(aq) + CO 2– 3 (aq) or H2CO3(aq)  H+(aq) + HCO – 3(aq) HCO – 3(aq)  H+(aq) + CO 2– 3 (aq)

2 © EJC 9729/02/J2PE/23 (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 . ................................ ..........