MI_H2_CHEM_P3_Answer Prelim

Class Adm No Candidate Name:____________________________________ [Turn over 2015 Preliminary Examination II Pre-university 3 H2 CHEMISTRY 9647 / 03 Paper 3 Free Response 22nd Sept 2015 2 hours Additional Materials: Data Booklet Writing Paper Cover Page Graph Paper READ THESE INSTRUCTIONS FIRST Write your name, class and index number on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or 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 your work securely together. Begin each question on a fresh sheet of paper.

2 1 The formation of a protective shield of ozone in the stratosphere shields life on Earth. However, the amount of ozone in the stratosphere decreases due to the release of chlorofluorocarbons (CFCs) from aerosol propellants and refrigerants. The mechanism that leads to ozone destruction is thought to involve three steps. I. There is an initial homolytic breaking of a CFC molecule to give the chlorine radical. II. This is followed by the chain reaction between chlorine radical and O 3 to give ClO· and a gas. III. The third step involves the reaction between ClO· and O3 to give Cl· and a gas. Due to ozone destruction, CFCs have now been replaced by alkanes in many applications. (a) Dichlorodifluoromethane is one of the CFCs present in refrigerants. (i) Use the information given above to draw out the full mechanism that leads to ozone destruction. You are advised to use structural formulae for all species, so that it is clear which bonds are broken and which are formed. Indicate any unpaired electrons by a dot (·). [3] (ii) State the type of reaction that is occurring. [1] Free radical substitution (iii) Write the overall equation for steps II and III to show the depletion of ozone. [1] 2O3 → 3O2 (iv) Explain how one chlorine radical can lead to the destruction of a large number of ozone molecules. [1] Each step in the reaction pr oduces a new highly reactive free radical which can continue in a chain reaction. (v) Suggest one potential hazard of using alkanes instead of CFCs. [1] Alkanes are flammable.

3 (b) Draw a ‘dot -and-cross’ diagram to show the bonding in O 3 molecule and use VSEPR theory to predict the shape and bond angle.