2021 CJC H2 CHEM Prelim P2 QP

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 19 printed pages and 1 blank page. For Examiner’s Use Paper 1 30 Paper 2 Q1 /14 Q2 /8 Q3 /12 Q4 /9 Q5 /15 Q6 /17 75 Paper 3 80 Paper 4 55 OVERALL (100%) Grade Catholic Junior College JC2 Preliminary Examination Higher 2

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 ea ch carbonate and uses the hottest flame of a Bunsen burner. The graph of his experimental results is given below. t/s

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 carbonates 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] (e) Compare and explain the difference betwe en the lattice energy of barium carbonate and magnesium carbonate. ……………...…....………….………………………………………………………… ………...…....………….……………………………………………………………… .…...…....………….…………………………………………………………………... …....………….…………………………………………………………………...….... ………….……………………………………………………………………………… ……………………………….……………………………………………………… .[2]

4 9729/02/CJC JC2 Preliminary Examination 2021 (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] (g) Lithium nitrate is the only Group 1 nitrate that decomposes on heating, in the same way as Group 2 nitrates. Suggest a reason for why its behaviour is similar to Group 2 nitrates, and give an equation for its decomposition. ……………...…....………….………………………………………………………… ……………...…....………….………………………………………………………… ……...…....………….……………………………………………………….......…… ……...…....………….………………………………………………………….…….. ……...…....………….………………………………………………………...……… ……..…………………………………………………………………………………..[2] [Total: 14]

5 9729/02/CJC JC2 Preliminary Examination 2021 [Turn over 2 (a) (i) Calculate the average oxidation number of sulfur in sodium thiosulfate, Na2S2O3. [1] (ii) Draw the structure of a thiosulfate anion, S 2O32-, with a single central S atom, and clearly label the actual oxidation number of each S atom. [2] (b) Thiosulfate anions undergo disproportionation as shown. S2O32– + H2O → SO42– + HS– + H+ By considering the change in oxidation states in all the sulfur -containing species, deduce the number of moles of electrons involved in this reaction. [2]

6 9729/02/CJC JC2 Preliminary Examination 2021 (c) The values of standard reduction potentials are given for the following redox systems. S4O62– / S2O32– E  = +0.09 V MnO4– / Mn2+ E  = +1.52 V (i) Among the species above, identify the strongest oxidising agent and the strongest reducing agent. Strongest oxidising agent: ……………...…....………………………………..[1] Strongest reducing agent: ..…….……...…....………………………………...[1] (ii) With reference to the Data Booklet, write a balanced ionic equation for the most spontaneous reaction involving the four species above. ……………...…....……………………………………………………………[1] [Total: 8]

7 9729/02/CJC JC2 Preliminary Examination 2021 [Turn over 3 (a) Use of the Data Booklet is relevant to this question. The reaction between peroxodisulfate( VII) ions, S2O82−, and iodide ions, I−, is represented by the following equation: 2I−(aq) + S2O82−(aq)  I2(aq) + 2SO42−(aq) This reaction is catalysed by adding a suitable transition metal species which acts as a homogeneous catalyst for the reaction. (i) Suggest why the above reaction requires the addition of a catalyst. ................................................................................................................... ............................................................................................................... [1] (ii) Explain why the transition metal can be used as a homogeneous catalyst. ................................................................................................................... ................................................................................................................... ............................................................................................................... [1] (iii) By considering suitable E  values from the Data Booklet, suggest a catalyst for the reaction between S2O82− and I− ions, and write equations to show how the homogeneous catalysis occurs. ................................................................................................................... ...............................................................