RI H2 Chem 2013 Prelim P2 QP

© Raffles Institution 2013 9647/02/S/13 [Turn over RAFFLES INSTITUTION 2013 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS INDEX NUMBER CHEMISTRY 9647/02 Paper 2 Structured Questions 18 September 2013 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet. 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 in the spaces provided. 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 all questions. A Data Booklet is provided. Do not write anything on it. 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. For Examiner’s Use 1 / 12 2 / 13 3 / 11 4 / 10 5 / 18 6 / 8 Total / 72 This document consists of 22 printed pages. 181

2 © Raffles Institution 2013 9647/02/S/13 [Turn over 1 Planning (P) The standard enthalpy change of isomerisation, , measures the enthalpy change when 1 mole of a cis–alkene isomerises to 1 mole of the corresponding trans–alkene under standard conditions: However, this enthalpy change cannot be measured directly by experiment. A student decided to determine the standard enthalpy change of isomerisation of cis–hex–3–ene to trans–hex–3–ene using their respective standard enthalpy changes of combustion, . Both alkenes are volatile liquids at 298 K. From his literature review, the student found the of cis–hex–3–ene to be –3733 kJ mol–1. However, the of trans–hex–3–ene was not available. For Examiner’s Use (a) Write a balanced equation, with state symbols, to describe the standard enthalpy change of combustion, , of cis–hex–3–ene, C6H12. ………………………………………………………………………………………... [1] (b) To determine the standard enthalpy change of combustion of trans–hex–3–ene, the student decided to conduct a flame calorimetric experiment. There are two stages to this experiment: Stage I Calibration of calorimeter set-up (i.e. container and water). In the experiment, the calorimeter set-up must first be calibrated by determining its heat capacity , C, which is the amount of heat required to raise its temperature by 1 K. Stage II Determination of the standard enthalpy change of combustion of trans–hex–3–ene using the calibrated calorimeter set-up. cis–hex–3–ene trans–hex–3–ene (l) (l) 182

3 © Raffles Institution 2013 9647/02/S/13 [Turn over The student carried out the flame calorimetric experiment using a copper can. Part of his results is shown below. For cis–hex–3–ene: mass of water = 250 g change in temperature of water = 5.0 °C change in mass of spirit lamp with cis–hex–3–ene = 0.20 g For trans–hex–3–ene: mass of water = 250 g change in temperature of water = 5.4 °C change in mass of spirit lamp with trans–hex–3–ene = 0.22 g For Examiner’s Use (i) Given that [cis–hex–3–ene] = –3733 kJ mol –1, calculate the heat capacity, C, of the calorimeter set -up based on the above experimental results. Show the units of C clearly. heat capacity, C = ...................................... (ii) Hence estimate a value for the standard enthalpy change of combustion of trans–hex–3–ene, [trans–hex–3–ene]. [trans–hex–3–ene] = ...................................... (iii) Using results from (b)(i) and (b)(ii), calculate the standard enthalpy change of isomerisation, , of cis–hex–3–ene to trans–hex–3–ene. [cis–hex–3–ene] = ...................................... [3] 183

4 © Raffles Institution 2013 9647/02/S/13 [Turn over (c) Using information from (b), write a plan which will allow you to estimate a value for the standard enthalpy change of combustion of trans–hex–3–ene in your school laboratory. You are not given a copper can. However, you may assume that you are provided with the following:  cis–hex–3–ene ( = –3733 kJ mol–1)  trans–hex–3–ene  250 cm3 beaker  two spirit lamps with a 5 cm-wick each  deionised water  a lighter  thermometer  apparatus normally found in a school or college laboratory Your plan should contain the following:  a diagram of the experimental set-up  appropriate quantities of chemicals and solutions  all essential experimental details For Examiner’s Use ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ 184

5 © Raffles Institution 2013 9647/02/S/13 [Turn over ………………………………………………………………………………………........ For Examiner’s Use ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………... [7] (d) Identify one potential safety hazard in this experiment and state how you would minimise this risk. ………………………………………………………………………………………........ ………………………………………………………………………………………........ ………………………………………………………………………………………... [1] [Total: 12] 185

6 © Raffles Institution 2013 9647/02/S/13 [Turn over 2 Hydrazine, N2H4, is a colourless flammable liquid with an ammonia-like odour. It boils at 114 °C, and gaseous hydrazine can decompose to form hydrogen and nitrogen gases. For Examiner’s Use (a) Draw the dot-and-cross diagram of N2H4. ………………………………………………………………………………………... [1] (b) State the hybridisation of the N atom and suggest what the H –N–H bond angle is in N2H4. hybr d at n …………… b nd angle …………… [2] A graph of pV nRT against p for N2(g) is given below. (c) On the axes above, sketch the corresponding graphs for H2(g) and N2H4(g). ………………………………………………………………………………………... [2] 1.0 N2 pV nRT p ideal gas 186

7 © Raffles Institution 2013 9647/02/S/13 [Turn over In a closed reaction vessel of 10 dm3 maintained at a temperature of 150 °C, gaseous hydrazine decomposes into nitrogen and hydrogen . The system reaches equilibrium with a total pressure of 1 atm. (Take 1 atm = 101 kPa.) N2H4(g) ƒ N2(g) + 2H2(g) The average Mr of the equilibrium gas mixture in the 10 dm3 vessel is found to be 20. For Examiner’s Use (d) Calculate the mass of the gaseous mixture inside the reaction vessel at the given temperature and pressure. mass of gaseous mixture = ...............................