燃烧热的测定

1. 燃烧热的测定 Determination of Combustion Heats 退出

2. Contents Purposes and Demands Principle Apparatus and Reagent Procedure Data Records and Processing Questions Attentions Demonstrated

3. Purposes and Demands • 1. Determine combustion heats of sucrose by oxygen bomb calorimeter. • 2.Understand the definition of combustion heats and the difference between combustion heats in isopiestic pressure and constant volume.

4. 3. Acquire the function of main portions of the calorimeter and to use the oxygen bomb calorimeter. • 4. Proofread the change of temperature by Renault illustration methods.

5. Principle • Combustion heat means that the heat which is released when 1 molar substance completely burns.

6. It is defined that isometric combustion heat（Qv）which is determined in constant volume and isotonic combustion heat（Qp）in isopiestic pressure.

7. Qv is equal to the enthalpy change ΔH while Qp is equal to the energy change ΔU in chemical reaction. Given that reactant or resultant is ideal gas, we obtain the equation: 1) Qp ＝ Qv＋ΔnRT

8. In this experiment oxygen bomb calorimeter is used to determine combustion heats of sucrose. • Sample burns completely in the bomb and released energy that makes temperature of calorimeter and water rounding the bomb increases.

9. The equation of the relationship between temperature change and combustion heat is: 2) Qv ＝ WⅹΔT - QLⅹ(mL-mL’) QL in the equation standsfor the combustion heat of fuse wire(iron, QL＝6.694kJ﹒g-1). mL is the mass of fuse wire before reaction and mL’ is the mass after reaction.

10. With a standard substance such as benzoic acid(C7H6O2, Ov 298K）=-3226.7KJ)determine the value of W, and then determine combustion heats of any another substance by the equation two.

11. Oxygen bomb is a container made up of special stainless steel. Make sure that substance can be melt completely and the bomb is filled full of high-pressure oxygen gas. We expect that the heat transfer just to calorimeter and water surrounding, but it is impossible.

12. It’s inevitable that part of heat would dissipate to circumstance and calorimeter temperature is smaller than the true one. So the temperature change could not be determined directly, it must be proofread by Renault illustration methods.

13. Apparatus and Reagent • Oxygen bomb calorimeter • Bean machine • Multimeter • Digital Beckmann thermometer • Thermometer（0℃－100℃） • Fuse wire • Volumetric flask (1000ml) • Oxygen cylinder • Sucrose (A.R.) • Benzoic acid (A.R.)

14. Procedure Determination of calorimeter constant • 1. Rub-up the model of bean machine with a fabric. Press pellets of the substance of 1g benzoic acid. Shave the pellets with a knife or spatula if it is dirtied in the face then knock it on a clean glassboard (why?). Weight it accurately by analytical balance.

15. 2.Screw down the bomb cap with your hands, never use a wrench, and fix it on a special hob.

16. 3. Cut 8cm of the iron wire and weigh it accurately. Make the middle part of iron wire to 5-6 rings part of which touch the pellet. Tie the terminals with ignition leads and check the currency with a multimeter.

17. 4. Install the pellet and wire in the bomb. Carefully assemble the bomb and tighten the waist nut with a wrench. Attach the bomb to the oxygen-filling apparatus. Open the supply valve, fill the bomb gas to 1Mpa.

18. Then deflate gas until the pressure is 0.5Mpa and aerate the bomb again. Repeat the complete procedure twice (why?). Keep the pressure at 1Mpa for half a minute for the third aeration. With multimeter check if there is any electric current, if no, release the pressure and check about the bomb.

19. 5. Fill the outer-jacket with water at 25℃. With measuring container fill the pail accurately with 3L water that temperature is lower than out-jacket’s to about 1℃. Stir the agitator blade, and regulate it, and make sure that the agitator blade does not touch the wall.

20. Use the multimeter to check electric current again and connect ignition leads with two poles. Install the Beckmann thermometer, and start the agitator.

21. 6. When temperature change at a very liner rate begin time-temperature readings every 0.5min. As soon as you get the tenth numbers start igniting. It means that ignition was over when indicator light shine and soon die.

22. Recordthe temperature until the temperature brings down. Then take ten more readings for the final stage. At the beginning and final stage temperature data must be precise at 0.002℃. Do remember turn off the ignition switch, and avoid ignite spontaneously next time.

23. 7. Finally, stop agitating, disassemble the apparatus, release bomb pressure slowly. If the substance burnt completely, remove and weight any unburned iron wire accurately by analytical balance. Pour out the water and wipe dry all bomb parts. Get ready for next experiment.

24. 8. Press a pellet of sucrose for 0.6g. Repeat the complete procedure. Notice that water filled in the pail is the same all through.

25. Sketch-maps of Equipments

26. Data Records and Processing • 1. Figure out Δt and proofread it by Renault illustration methods. Calculate the calorimeter constant. • 2.Calculate the standard molar combustion ΔcHm and contrast to the literature value.

27. Questions • 1. In the experiment, which parts are systemor environment? Is there heat change between them? What influence of heat change to the result and how to proofread? • 2. What attention must be paid to when we use the oxygen cylinder and the reducing valve?

28. Attentions • 1. Press the pellets with moderate strength and make sure that the substance will completely burn.

29. 2. As adding water to the pail, pay attention not to wet the poles. • 3. When you have to do the experiment again you must release bomb pressure entirely, or elsethe bomb can’t be opened.