2019 TJC H2 Chem Prelim P3 ANS

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] When aqueous ammonia is added dropwise to blue Cu2+(aq) solution , a blue precipitate of Cu(OH)2 is formed. Ammonia acts as a base. NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH–(aq) [Cu(H2O)6]2+(aq) + 2OH–(aq) ⇌ [Cu(OH)2(H2O)4](s) + 2H2O(l)  (1) Blue ppt OR Cu2+(aq) + 2OH–(aq) ⇌ Cu(OH)2(s) In excess ammonia, blue precipitate Cu(OH)2(s) dissolves to give a dark blue solution due to the formation of the complex ion, [Cu(NH3)4(H2O)2]2+(aq). Ammonia acts as a ligand (or ligand exchange has taken place). [Cu(H2O)6]2+(aq) + 4NH3(aq) ⇌ [Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l)  (2) Dark blue solution Formation of the complex shifts the equilibrium in equation (1) to the left, thus, the blue precipitate dissolves. (ii) Suggest why ethylenediamine can function as a chelating ligand. [1]  Ethylenediamine molecule has two lone pair of electrons on the nitrogen atoms to form two dative bonds with the Cu2+ ion. (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] Cu(H2O)6]2+ + 2en  [Cu(en)2(H2O)2]2+ + 4H2O [Cu(H2O)6]2+(aq) + 4NH3(aq)  [Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l)  Ethylenediamine (a biden