NYJC 2020 Prelim 9729 P3 Answers

H2 Chemistry 9729/03 NYJC J2/20 PX [Turn over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS TUTOR’S NAME 1 9 CENTRE NUMBER S INDEX NUMBER CHEMISTRY 9729/03 Paper 3 Free Response 16 September 2020 2 hours Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, 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. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 /23 2 /19 3 /18 4 or 5 /20 Total /80 This document consists of 32 printed pages and 0 blank page.

2 H2 Chemistry 9729/03 NYJC J2/20 PX For Examiner's Use Section A Answer all questions in this section. 1 Lithium metal and its compounds have many uses, ranging from nuclear che mistry, batteries, air purification and pharmaceuticals. (a) A button cell is a small single cell battery shaped like a button, with sizes typically 5 to 25 mm. Lithium -iodine button cells are widely used to power small portable electronics devices such as watches and calculators. (i) Using appropriate data from the Data Booklet, calculate Eʅ cell of a lithium - iodine button cell, and write the equations that take place at the cathode and anode. [3] From Data Booklet, Eʅ /V (1) Li+ + e ∏ Li – 3.04 (2) I2 + 2e ∏ 2I– +0.54 Eʅ cell = 0.54 – (– 3.04) = + 3.58 V [1] with (+) sign Cathode: ½ I2 + e → I– OR I2 + 2e → 2I– [1] with single arrow Anode: Li → Li+ + e [1] with single arrow (ii) State the polarities of the cathode and anode. [1] Cathode : positive Anode : negative [1] (iii) The actual voltage of a lithium-iodine button cell is +2.80 V. Suggest why this is different from the value you have obtained from (i). [1] In the part (i ) calculations, we have assumed that the concentrations of the ions are at 1 mol dm -3, and conditions are at 25 OC and 1 bar but the working conditions of the button cell are not at standard conditions. [1] (iv) After a current has been drawn for some time, the voltage of the button cell