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 Shielding Effect down the group . They need to be made aware that the Increase in Shielding Effect Outweighs Increase in Nuclear Charge. A number of students mistaken Nuclear Charge as the Net Positive Charge of Cation. Most students failed to describe the effect of Increase in Shielding Effect on the Electrostatic Forces of Attraction between Nucleus and Electrons.

3 9729/02/CJC JC2 Preliminary Examination 2021 [Turn over (i) Write an equation to represent the thermal decom position of barium carbonate, indicating clearly the state symbols. ………………………………………………………………………………… [1] (ii) Identify the least thermally stable carbonate and explain how the graph supports your answer. ……………...…....………….…………………………………………………… ……………...…....………….…………………………………………………… …………...…....………….……………………………………………………… ………...…....………….………………………………………………………… ……………………………………………………………………………………[2] (d) The student wanted to perform the same investigation on the carbonat es of the elements in Group 13 but found that A l2(CO3)3 does not exist at room temperature. Suggest a reason for this. ……………...…....………….………………………………………………………… ……………...…....………….………………………………………………………… ……………...…....………….…………………………………… ……………………. ……………...…....………….…………………………………… ……………………. …………………………………………………………………………………………..[2] t/s BaCO3 (s)  BaO (s) + CO2 (g) [1] MgCO3 is the least thermally stable. [1] The gradient of the graph for MgCO3 is steepest. [1] Or Given the same amount of carbonates, MgCO3 produce the most CO2 upon thermal decomposition. [1] With its small size and high charge, Al3+ cation has a very high charge density [1]. Thus, Al3+ is able to polarise the electron cloud of the large carbonate anion (or CO32) and hence the C-O bond is weakened to such a large extent [1] such that Al2(CO3)3 decomposes readily to give Al2O3 and CO2 even at room temperature. Almost all students are able to provide correct answer. Almost all students are able to provide correct answer. Very few students misinterpret “Least Thermally Stable” compound as one that gives off least amount of gaseous products.

4 9729/02/CJC JC2 Preliminary Examination 2021 (e) Compare and explain the difference between the lattice energy of barium carbonate and magnesium carbonate. ……………...…....………….………………………………………………………… ………...…....………….……………………………………………………………… .…...…....………….…………………………………………………………………... …....………….…………………………………………………………………...….... ………….……………………………………………………………………………… ……………………………….……………………………………………………… .[2] (f) The first ionisation energies of Group 2 elements are given below: Group 2 Elements 1st I.E/ kJ mol-1 Be 900 Mg 736 Ca 590 Sr 548 Explain why the first ionisation energies decrease in magnitude down Group 2. ……………...…....………….………………………………………………………… ……………...…....………….………………………………………………………… ……………...…....………….…………………………………………………………….…… ……………...…....………….………………………………………………………………….. ……………...…....………….…………………………………… ……………………. ……………...…....………….…………………………………… ……………………. ……………...…....………….…………………………………… ……………………. ……………...…....………….…………………………………… ……………………. …………………………………………………………………………………………..[2] Down Group 2,  Nuclear charge increases  The atomic radius increases as outermost electron is in a higher principal quantum shell. Thus, the outermost electron is further from the nucleus. [1] [1] +- +- qqL.E α r +r Both the cations and anions are doubly charged/ have the same charge. Ionic radius: Ba2+ > Mg2+ [1] Thus, the lattice energy of Ba CO3 is less exothermic/ smaller in magnitude[1] than that of MgCO3.  The number of inner shells of electrons increases, thus the outermost electron is better shielded by the inner shells of electrons  Distance from nucleus and better shielding outweighs greater nuclear charge.  Therefore, the outermost electron becomes less strongly attracted by the positive nucleus and so, less energy is required to remove the electron.∴ 1st IE decreases A number of students are able to relate the non-existence of Al2(CO3)3 to the fact that it is very Thermally Unstable at room temperature , hence it is due to Large Extent of Polarisation of Anion Electron Cloud by Al3+ with Very High Charge Density. A number of students have the misconception that non-existence of Al2(CO3)3 is due to very low lattice energy of the ionic compound. They failed to understand that low lattice energy only results in the compound having low melting point. However the compound still exists but in molten state. Most students are able to relate Lattice Energy of the compounds to the expression: +- +- qqL.E α r +r Most students are able to compare the ionic size of Ba2+ and Mg2+. Most students failed to use the terms “More Exothermic” or “More Endothermic” to compare lattice energies of compounds.

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