SAJC Prelim P2_MS

Turn Over NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION Chemistry (9729) Paper 2 Structured Questions 11 September 2017 2 hours Additional Materials: Data Booklet READ THESE INSTRUCTIONS: 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 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: Question 1 2 3 4 5 6 Total Marks 17 5 10 16 6 21 75 This document consists of XX printed pages (including this page).

2 Answer all the questions 1 Sulfur is a common element on Earth that forms many important chemical compounds. One of these compounds is sodium thiosulfate, Na2S2O3, an ionic compound used to treat several medical conditions, such as cyanide poisoning and fungal growths. (a) (i) Draw a dot -and-cross diagram for sodium thiosulfate, Na 2S2O3. State the shape and bond angle in the thiosulfate ion. [2] shape around sulfur – tetrahedral bond angle - 109° 1 mark for correct dot-cross diagram 1 mark for both shape and bond angle Double bond between S-S atoms is accepted Additional electrons can be shown as either dot or cross. Triangle/square/circle/any other shape cannot accept. (ii) Below are the melting points of sodium thiosulfate and sulfur. Compound Melting point / °C Sodium thiosulfate 49 Sulfur, S8 115 Explain why sulfur has a higher melting point than sodium thiosulfate. [3] Sodium thiosulfate has a giant ionic lattice structure with electrostatic forces of attraction between Na+ and S2O32- ions. [1] Sulfur/S8 is a simple non-polar covalent molecules with instantaneous-dipole- induced-dipole (id-id) interactions between its molecules.[1]

3 Turn Over The large number of electrons in each sulfur molecule leads to strong id-id interactions, which require more energy to overcome compared to the ionic bonds in sodium thiosulfate, hence it has a higher melting point than sodium thiosulfate[1] (b) Another important sulfur compound is sulfuric acid, H 2SO4. Before the Contact Process was discovered, concentrated sulfuric acid for industrial purposes was produced by the following method. The mineral pyrite, FeS2, was first heated in air and oxidised to solid Fe 2(SO4)3 and sulfur dioxide gas. Fe2(SO4)3 decomposes at 480 °C to form iron(III) oxide and sulfur trioxide gas. The sulfur trioxide gas could be mixed with any volume of water to produce sulfuric acid of the desired concentration. However, this process was expensive an