2019 TJC H2 Chem Prelim P3 QP

1 9729 / TJC Prelim / 2019 [Turn Over PRELIMINARY EXAMINATIONS HIGHER 2 CHEMISTRY 9729/03 Paper 3 Free Response 16 September 2019 2 hours Candidates answer on separate booklet. Additional Materials: 12-Page Answer Booklet Data Booklet READ THESE INSTRUCTIONS FIRST Write your Civics Group, Centre number, index number and name on all the work you hand in. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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. This document consists of 14 printed pages and 2 blank pages.

2 9729 / TJC Prelim / 2019 [Turn Over Section A Answer all the questions from this section. 1 The term chelates, originates from the Greek word chele for “claw”. It refers to compounds containing ligands bonded to a central metal atom or ion at two or more points. Ligand exchange occurs when ammonia and ethylenediamine (en), H2NCH2CH2NH2, are added separately to copper sulfate solution. (a) (i) Explain all the changes observed when aqueous ammonia is added dropwise to Cu2+(aq) till in excess. Suggest the formulae of all relevant copper compounds formed and write equations for all reactions. [4] (ii) Suggest why ethylenediamine can function as a chelating ligand. [1] (iii) When ethylenediamine is added to Cu2+(aq), the following occurs. [Cu(H2O)6]2+ + 2en  [Cu(en)2(H2O)2]2+ + 4H2O Predict whether ethylenediamine or ammonia gives a more positive entropy change when added to Cu2+(aq). [2] (iv) Suggest the coordination number and shape of [Cu(en)2(H2O)2]2+. [1] (v) The complex ion [Cu(en)2(H2O)2]2+ can exist as three stereoisomers, A, B and C. Isomers A and B rotate plane-polarised light but C does not. The structure of A is represented below. where represents Isomer A ethylenediamine (en) Draw the structure of C. [1] (b) Photochromic glass used for sunglasses contains AgCl crystals which are added while the glass is in molten state. The glass darkens when exposed to bright light and the following reaction is involved. Ag+ + Cl ⇌ Ag + Cl (i) Suggest why the glass darkens when exposed to bright light. [1] The glass darkens significantly within about a minute of exposure to bright light but takes a longer time to clear when there is less light. Small amount of CuCl crystals are often added to the glass to speed up the reverse process. No elemental copper is involved in the process. Ag + Cl  Ag+ + Cl

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