CJC_H2_CHEM_P3 Prelim

CATHOLIC JUNIOR COLLEGE Preliminary Examination 2009 Higher 2 CHEMISTRY 9746/03 Paper 3 Free Response 15 September 2009 2 hours Candidates answer on separate paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS CAREFULLY Write your name and class in the spaces on the writing paper. Write in dark blue or black pen on both sides of the writing paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use correction fluid. Answer any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all the answer scripts securely together. This document consists of 8 printed pages and no blank pages. CJC Chemistry Department © CJC 2009

 CJC 2009 9746/03/prelim/09 2 Answer any four questions 1 Fireworks are an integral part of the Olympics opening ceremony. One of the most spectacular displays in recent history took place at the 2006 T orino Winter Olympics, where the largest ever Olympic rings were lit up in fireworks. (a) The colours of fireworks are due to various colour -producing chemicals present, of which compounds of the Group II metals are particularly important. Another component of fireworks is the oxidiser, w hich decomposes upon heating to provide oxygen for burning of the fireworks fuel. One such oxidiser in common use is potassium nitrate, KNO 3, which gives potassium nitrite, KNO 2, and oxygen gas when heated. (i) Write a balanced equation for the decompositi on of KNO 3. (ii) From your answer in (a)(i), explain, includi ng any relevant equations, why potassium nitrate is a better choice as an oxidiser as compared to magnesium nitrate. [3] (b) When a 2.75 g mixture of potassium nitrate and a G roup II carbonate from a red firework was heated thoroughly, enough oxygen was produced t o oxidise 0.0141 mol of aluminium, which is used as fuel in fireworks. In addition, wh en the gases produced were cooled to room temperature and pressure, it was found that 10 0 cm 3 of carbon dioxide was produced. Given that complete decomposition of the nitrate and carbonate took place, find the identity of the Group II metal used. [4] (c) Phosphorus and sulfur have various applications in pyrotechnic materials, including fireworks. (i) Predict, with reasons, how the radius of P 3– compares with that of S 2–. (ii) Both phosphorus and sulfur exhibit two diff erent oxidation states in their oxides and chlorides. Explain why phosphorus can form chlorides of two different oxidation states, while nitrogen can only form one chloride, NC l 3. [4] (d) Another fuel also used in fireworks is gallic acid . A closely related compound which is used as a base material