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 Due to ozone destruction, CFCs have now been replaced by alkanes in many applications.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. [3] Shape: bent Bond angle: <120° (c) In an experiment to determine the enthalpy change of pentane, ∆ Hc, a quantity of the fuel was burned undern eath a copper can containing 200 g of water. It was found that the temp erature of the water rose by 37 °C after 0.7 g of pentane had been burned. (i) Calculate the ∆Hc 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] Amount of pentane = 0.7 / (5(12.0) + 12) = 0.009722 mol Q = 200 (4.18) (37) = 30932 J ∆Hc = - [(30932 / 0.009722)] = -3180 kJ mol-1 (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] ∆Hc (C5H12(l)) = -(-173.5) + 5(-394) + 6(-286) = -3510 kJ mol-1 (iii) Compare the value s you calculated in c(i) and (ii) and suggest a reason for the discrepancy. [1] Heat may be lost to the surroundings during the calorimetric experiment; thus the experimental value is less exothermic than the theoretical value.

4 (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] ∆S = [5(213.6) + 6(69.9)] – [263.5 + 8(205)] = -416 J mol-1 K-1 As G = H −TS, When T increases, G becomes less negative (because both H and S is negative)  reaction becomes less spontaneous. [Total: 20]

5 2 (a) In 1887, the German chemist Siegmund Gabriel 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] Stage I: acid base reaction OR neutralisation Stage II: nucleophilic substitution (ii) Explain why aryl amines cannot be prepared using this method. [1] This is because the p orbitals of the halogen atom overlap with the  electron cloud of the benzene ring, strengthening the C -X bond, making the C -X bond in aryl halide too strong to be broken. (iii) Both chloroethane and bromoethane can undergo Gabriel synthesis to form the respective primary amines. Describe and explain how the reactivities of the two reactions differ. [2] Reactivity increases from C 2H5Cl to C 2H5Br as the C -X bond strength decreases down the group. The C-Br bond is weaker than C -Cl bond and thus, easier to break. (b) The Gabriel synthesis can be used in the following synthesis. (i) State the reagents and conditions for step III. [1] Br2(l) in CCl4, r.t.

6 (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] To a test tube containing the unknowns, add in Br2(aq) Compound A: orange solution decolourises to a colourless solution Compound C: orange colour remains. OR To a test tube containing the unknowns, add in KMnO4, H2SO4(aq), heat. Compound A: purple colour decolourises to a colourless solution Compound C: purple colour remains. OR To a test tube containing the unknowns, add in KMnO4, NaOH(aq), cold. Compound A: purple colour decolourises to give a brown precipitate Compound C: purple colour remains. (c) When a sample of an oxide of phosphorus, P xOy, was vapourise d 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] 36 x 1000 × M =