MI H2 CHEM P3 Prelims

Class Adm No Candidate Name:____________________________________ This question paper consists of 11 printed pages and 1 blank page.    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 C o v e r P a g e 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 Answer any four questions. 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 C lO· 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 Due to ozone destruction, CFCs have now been replaced by alkanes in many applications.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] (iii) Write the overall equation for steps II and III to show the depletion of ozone. [1] (iv) Explain how one chlorine radical can lead to the destruction of a large number of ozone molecules. [1] (v) Suggest one potential hazard of using alkanes instead of CFCs. [1] (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. [3]

3 [Turn over (c) In an experiment to determine the enthalpy change of pentane, ∆Hc, a quantity of the fuel was burned underneath a copper can containing 200 g of water. It was found that the temperature of the water rose by 37 °C after 0.7 g of pentane had been burned. (i) Calculate the ∆H c of pentane from the data given. Ignore the heat capacity of the copper can, and use the value of 4.18 J g -1 K -1 for the specific heat capacity of water. [3] (ii) Use the following data to construct an energy cycle to calculate a value for the ∆Hc of pentane. Enthalpy change of combustion of C(s) -394 kJ mol -1 Enthalpy change of formation of H2O(l) -286 kJ mol-1 Enthalpy change of formation of C5H12(l) -173.5 kJ mol-1 [3] (iii) Compare the values you calculated in c(i) and (ii) and suggest a reason for the discrepancy. [1] (iv) Use the following data to calculate the entropy change of the system when pentane undergoes combustion. Compound Entropy / J mol –1 K–1 C5H12(l) 263.5 O2(g) 205.0 H2O(l) 69.9 CO2(g) 213.6 Hence, suggest how the spontaneity of the reaction changes with increasing temperature. [3] [Total: 20]

4 2 (a) In 1887, the German chemist Siegmund Gabr iel discovered the Gabriel synthesis, which transforms primary alkyl halides to primary amines as shown below.   The reaction is thought to proceed through the following stages. (i) State the type of reaction that takes place in stages I and II. [2] (ii) Explain why aryl amines cannot be prepared using this method. [1] (iii) Both chloroethane and bromoethane can undergo Gabriel synthesis to form the respective primary amines. Describ e and explain how the reactivities of the two reactions differ. [2] (b) The Gabriel synthesis can be used in the following synthesis. (i) State the reagents and conditions for step III. [1] (ii) Identify the structure of compound B. [1] (iii) Suggest a simple test-tube reaction by which compounds A and C can be distinguished from each other. The reagents and conditions must be different from those used in step III. You should state all reagents and conditions for each test, and describe the expected observations. [3]

5 [Turn over (c) When a sample of an oxide of phosphorus, P xOy, was vapourised in a suitable apparatus at 178 °C and 36 kPa, the density of the gas was found to be 2.13 g dm -3. (i) Calculate the molar mass of the gas and hence suggest the identity of PxOy. [3] (ii) Explain why the molar mass obtained in (c)(i) is different from the theoretical value. [1] (iii) Using your answer from (c)(i), write an equation for the reaction between water and P xOy. Suggest the approximate pH of the resulting solution formed and describe the colour observed when Universal Indicator was added. [3] (iv) At room temperature and pressure, P xOy exists as liquid but SiO 2 exists as solid. Explain. [3] [Total: 20]

6 3 Tyrosine is a non-essential amino acid whic h acts as a building block for several important brain chemicals known as neurotransmitters including epinephrine, norepinephrine, and dopamine. It is involved in the structure of almost every protein in the body. The three pK a values associated with Tyrosine are 2.2, 9.1 and 10.1. (a) What is meant by the term non-essential amino acid? [1] (b) Use the p Ka values given to draw the structures of the major species present in solutions of Tyrosine at the following pH values. (i) pH 1.2 (ii) pH 5.7 (iii) pH 9.5 (iv) pH 11.5 [4] (c) The following structure represents part of the formula of a protein molecule containing Tyrosine. (i) What reagents and conditions could you use to hydrolyse this protein into its amino acids? [1] (ii) Draw the structures of the two amino acid residues (other than Tyr