Calculations
Trial 1
Moles of Ethanol = m/mr
Moles of Ethanol = 1.85/46.08 = 0.0401 mol
% uncertainty in mass = 0.02/1.85*100 = 1.1%
% uncertainty in molar mass = 0%
% uncertainty in moles = 1.1+0 = 1.1%
% uncertainty to absolute uncertainty of moles = 1.1*0.0401/100 = 0.0004
∴ Absolute uncertainty of moles = 0.0401 ±0.0004 mol
Q = m*c* ΔT
Q = 200.0*4.18*40.0
Q = 33440J = 33.44 KJ
% uncertainty in volume =0.1/200.0*100 = 0.050%
% uncertainty in temperature = 1/40.0*100 = 2.50%
% uncertainty in Q = 0.050 + 2.50 = 2.55%
% uncertainty to absolute uncertainty of Q = 2.55*(33.44)/100 = 0.852
∴ Absolute uncertainty of Q = 33.44 ±0.85 KJ
ΔH = Q/n
ΔH = 33.44/0.0401
ΔH = 833.92 KJ
% uncertainty for ΔH = 2.55 + 1.10 = 3.65%
% uncertainty to absolute uncertainty of ΔH = 3.65*833.92/100 = 30.43 KJ/mol
Absolute uncertainty of ΔH = -833.92 ±30.43 KJ/mol
Trial 2
Moles of Ethanol = 1.90/46.08
Moles of Ethanol = 0.0412 ±0.0004 mol
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. Due to this, the method used is not very dependable and effective.
The systematic error was caused due to the loss of heat in the air. Additionally, the type of metal used is the can is unknown and there was paper and plastic on the can which absorbed different amounts of heat. This is probably what led the result to be so much smaller. The experiment was not conducted under STP which is another cause for the large uncertainty.
Evaluation
- The major cause for the systematic error was the fact that the can absorbed heat. The can was made of an alloy of different metals and contained some paper and traces of plastic on it. Due to this, it is hard to determine the amount of heat absorbed. Additionally there was heat lost to air from the top. To improve this, I could have used a calorimeter so as to reduce the heat loss and so that I could calculate the amount of heat lost to the surroundings.
- 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.
- 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.
- 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.
by
harshalpatil (student)
