RI H2 Chem 2013 Prelim P2 Soln

1 © Raffles Institution 2013 9647/02/S/13 Raffles Institution 2013 H2 Chemistry Preliminary Examinations Paper 2 (Suggested Solutions) 1 (a) C6H12(l) + 9O2(g)  6CO2(g) + 6H2O(l) (b)(i) Assuming no heat loss, heat evolved from combustion = heat gained by calorimeter set-up – [cis–hex–3–ene] × amt of cis–hex–3–ene = C × temperature rise – (–3733)(0.20 / 84.0) = C(5.0) C = 1.7776 = 1.78 kJ K–1 (3 s.f.) (b)(ii) Assuming no heat loss, heat evolved from combustion = heat gained by calorimeter set-up – [trans–hex–3–ene] × amt of trans–hex–3–ene = C × temperature rise – [trans–hex–3–ene] × (0.22 / 84.0) = (1.7776)(5.4) [trans–hex–3–ene] = –3665 = –3670 kJ mol–1 (3 s.f.) (b)(iii) = [cis–hex–3–ene] – [trans–hex–3–ene] = –3733 – (–3665) = –68.0 kJ mol–1 (c) Experimental set-up 1. Fill up one spirit lamp with cis–hex–3–ene (to about 80% capacity), taking care to avoid getting any cis–hex–3–ene on the outside surface of the lamp. 2. Set up the apparatus as shown above, such that the beaker is about 2-3 cm from the tip of the wick. cap beaker spirit lamp containing alkene to be burnt 203

2 © Raffles Institution 2013 9647/02/S/13 Calibration of calorimeter set-up 3. Using a 250 cm3 beaker, measure out 200 cm3 of water. 4. Using an electronic weighing balance, w eigh the mass of the spirit lamp filled with cis–hex–3–ene. Record this mass. 5. Using the thermometer, read the temperature of the water and record it. 6. Use the lighter to l ight the lamp and place it under the beaker of water , centering the flame under the bottom of the beaker and ensuring the flame is at an approximately constant distance below it. 7. Using the thermometer, carefully and gently stir the water. 8. When the temperature has risen to about 5 °C, blow out (extinguish) the flame. 9. Continue to stir the water, recording the highest temperature reached. 10. While the lamp is cooling, clean the soot off from the bottom of the beaker. 11. Re-weigh the spirit lamp when it has cooled to room temperature. Record this mass. 12. Repeat Steps 3 to 11, using the same lamp after cleaning it. (For a more accurate estimate of the heat capacity.) Determining mass and temperature changes using trans–hex–3–ene 13. Repeat Step 1 by filling the other spirit lamp with trans–hex–3–ene. 14. Repeat Steps 3 to 11, using this new spirit lamp, recording the relevant masses and temperatures. 15. For a more accurate estimate of the data, Step 14 can be repeated. The value for the standard enthalpy change of combustion of trans–hex–3–ene can then be found by similar calculations as shown in (b). (d) Hexenes are inflammable liquids and are hence a fire hazard. Any of the following: Containers of the hexenes should be covered and stored away, when not in use; proper disposal o