2021 CJC H2 CHEM Prelim P2 Mark Scheme (with comments)

9729/02 CJC JC2 Preliminary Examination 2021 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/02 Paper 2 Structured Questions August 2021 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. 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. At the end of 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 16 printed pages. Catholic Junior College JC2 Preliminary Examination Higher 2 MARK SCHEME AND WORKED SOLUTIONS

2 9729/02/CJC JC2 Preliminary Examination 2021 1 (a) Calcium is a Group 2 element. Table below shows the radius of the calcium atom and the radius of the calcium ion. Element Radius / pm Ca 197 Ca2+ 100 Explain the difference in size between calcium atom and calcium ion. ………...…....………….…………………………………… …………………………. ……………...…....………….……………………………………………………… [1] (b) Describe and explain the trend in ionic radii down Group 2. ……………...…....………….………………………………………………………… ……………...…....………….………………………………………………………… ...………….………………………………………………………………...…....…… ………………………………………………………………………………………. [2] (c) A student investigates the thermal decomposition of the carbonates of Group 2 elements. He separately heats the carbonates of magnesium, calcium and barium and records the total volume of carbon dioxide collected every 10 seconds. In each experiment, he uses the same amount, in moles, of each carbonate and uses the hottest flame of a Bunsen burner. The graph of his experimental results is given below. Ca2+ has one quantum shell less than that of Ca atom. Hence the outer electrons are more strongly attracted by its nucleus. [1] Ionic radii increase down group 2 due to increasing number of electron shells and hence also increasing shielding effect. [1] These increases outweigh the increase in nuclear charge. Hence, weaker electrostatic attraction of the valence electrons by the nucleus.[1] Most students only explain that Ca is bigger than Ca 2+ due to higher number of quantum shells. They need to be made aware that the attractive force of nucleus on the valence electrons plays a part in affecting the size of atom. A number of students mistaken Principal Quantum Shell as Subshell. Most students failed to compare the effect of Increasing Nuclear Charge and that of Increasing Shiel