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Determination of Heat of Combustion of Ethanol

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Introduction

Harshal Patil Determination of Heat of Combustion of Ethanol ________________ Aim In this experiment, I heated water to 40ºC using a with a spirit burner containing ethanol. Then using the mass of ethanol lost while heating the water, I determined the heat of combustion of ethanol. C2H5OH(l) + 3O2(g) ï 2CO2(g) + 3H2O(l) Data Trial 1 Trial 2 Initial Temperature Water (°C) ±0.5 13.0 16.0 Final Temperature Water (°C) ±0.5 53.0 56.0 Volume Water (cm3) ±0.1 200.0 200.0 Initial Mass Spirit Burner (g) ±0.01 184.28 182.13 Final Mass Spirit Burner (g) ±0.01 182.43 180.23 Mass of Ethanol used (g) ±0.02 1.85 1.90 Observations The temperature of the can rises rapidly since it is a good insulator and ash is collected at the bottom of the can turning it black from the outside. The temperature of the water inside rises slowly and after a while there are traces of visible water vapor emerging from the can. ...read more.

Middle

Q = 200.0*4.18*40.0 Q = 33.44 ±0.85 KJ ΔH = Q/n ΔH = 33.44/0.0412 ΔH = -811.65 ±28.44 KJ/mol Average = (833.92+811.65)/2 =-822.79 ±14.89 ΔH of Ethanol = -822.79 ±29.43 KJ/mol Conclusion The aim of the experiment was to find the heat of combustion of ethanol. I found the heat of combustion of ethanol -822.79 ±29.43 KJ/mol. As seen from my percentage error, I can safely say that my experiment was not very successful. The actual value for the heat of combustion of ethanol is -1367 KJ/mol which is quite far away from my experimentally determined value. The percentage error is 39.8% which is much larger than the percentage uncertainty which is 3.65%. There is also a vast difference between the two trials which tells us that the results aren’t very dependable and there is a large uncertainty. All this signals to the fact that there was not only random error but also a significant amount of systematic error. ...read more.

Conclusion

2. Another cause for this error was the flame. The size of the flame changed slightly and the flame moved about a lot due to air movement. Even though I tried to keep the bottom of the can just touching the tip of the flame, it was not possible. Also there was a large amount of energy lost directly from the flame to heat the air surrounding it. This was all caused because the experiment was conducted in a lab full of students and with open doors. I could have done the experiment in a closed box or an empty room with closed doors and windows to reduce these errors. 3. I also observed some soot forming at the bottom of the can which was caused due to incomplete combustion of the ethanol. This means that the energy released during incomplete combustion was lower where as in my experiment I assumed there was only complete combustion. 4. Finally the theoretical value I used is for STP which was not the case in my experiment. Due to this fact the calculations used to determine the heat of combustion of ethanol are not accurate. ...read more.

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