Determination of the Combustion Heats

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Purposes and Demands PrincipleApparatus and ReagentProcedureData Records and Processing QuestionsAttentionsDemonstrated

Contents

Purposes and Demands 1.Determine the combustion heats of sucro

se by the oxygen bomb calorimeter.

2.Understand the definition of the combustion heats and its difference between isopiestic pressure and constant volume.

3. Acquire the function of main portions of the calorimeter and how to use the oxygen bomb calorimeter.

4. Proofread the change of temperature by Renault illustration methods.

Principle The combustion heat means that the

heat which is released when 1 molar substance completely burns.

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

Qv is equal to the enthalpy change ΔH while Qp is equal to the energy change ΔU in chemical reaction.

Given that the reaction gas is ideal, we obtain

the equation: 1) Qp ＝ Qv ＋ ΔnRT

In this experiment,the oxygen bomb calorimeter is used to determine the combustion heats of sucrose.

The energy released when the sample burns completely in the bomb makes the temperature of calorimeter and water rounding the bomb increase.

The equation of the relationship between the temperature change and the combustion heat is:

2) Qv ＝ WⅹΔT - QLⅹ(mL-mL’)

QL in the equation stands for 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.

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

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

It’s inevitable that the part of heat would dissipate to the circumstance and the 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.

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.)

Procedure

Determination of the 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 glass board (why?). Weight it accurately by the analytical balance.

2. Screw off the bomb lid with your hands, and fix it on a special hob.

3. Cut 8cm of the iron wire and weigh it accurately. Make 5-6 loops touch the pellet in the middle part of iron wire. Tie two ends of the iron wire to the ignition terminals and check the currency with a multimeter.

4. Install the pellet and wire in the bomb. Carefully assemble the bomb and

tighten the bomb lid with your hands.

Attach the bomb to the oxygen-filling apparatus. Open the supply valve, fill with the bomb gas to 1Mpa.

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. Check with the multimeter whether there is any electric current or not, release the pressure and check about the bomb.

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

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

6. Record the temperature value every 0.5min when the temperature changes at a very liner rate. Switch on the ignition as soon as you get the tenth numbers. When the indicator light shines and soon dies ,it means that the ignition is over.

Record the temperature until the temperature brings down. Then record more than ten datum for the final stage. At the beginning and final stage ,the temperature data must be precise at 0.002℃. Turn off the ignition switch, and avoid igniting spontaneously next time.

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

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

Sketch-maps of Equipments

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 in contrast to the literature value.

Questions

1. In the experiment, which parts are system or environment? Is there any heat change between them? How does the heat change influence to the result and how do we proofread Δt by Renault illustration methods?

2. What attention must be paid to when we use the oxygen cylinder and the reducing valve?

Attentions

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

2. When you fill the pail with water, pay attention not to wet the poles.

3. When you have to do the experiment again, you must release the bomb pressure entirely, or else the bomb can’t be opened.