Methanol 715 kJmole-1
Ethanol 1371 kJmole-1
Propan-1-ol 2010 kJmole-1
Butan-1-ol 2673 kJmole-1
They were measured under the conditions of a temperature of 289 Kelvin and a pressure of 1 atmosphere.
By comparing these to my result I can work out the percentage error/ accuracy of my results AlcoholAverage Enthalpy of Combustion (my results) Percentage of Exact Enthalpy of Combustion (bomb calorimeter)
Methanol278.5 kJmole-138.9%
Ethanol539.2 kJmole-139.3%
Propan-1-ol995.6 kJmole-149.5%
Butan-1-ol1214.6 kJmole-145.5%
As you can see from the table above the accuracy of my results from my experiments were not very good. Methanol was the least accurate at 38.9% of what it should have been and Propon-1-ol at 49.5% was the most accurate (note the increase in accuracy and then the drop of butan-1-ol, which I highlighted above).
The results I have got have been consistently in accurate, the gap between the most accurate and the least accurate being only 10% they are all out by along way, the most accurate is only 50% of what it should be. This gives me the impression that my results are not just wrong because of human error i.e.
*Reading of the thermometer (I can only read to accuracy of nearest degree)
*Measuring of the weight of the Alcohol
*Measuring of water to be heated
* Impurities in the water (may change the specific heat capacity of the water)
* The enthalpy’s of combustion that I am comparing my results to were measured under different conditions so this means they would be different any way
By looking at the set up of my experiment it is quiet clear why the accuracy of the results are not very good.
A lot of heat produced in the experiment was
aloud to escape before it had even entered the apparatus and even heat that got into the water could escape back out of the calorimeter, as the good conducting copper which let the heat in could just as easily let it out again. I decided to see if I could increase the accuracy of my results, by stopping the heat escaping once inside the apparatus so I added insulation to the calorimeter and repeated the experiment again.
I redid the experiments now using the insulation material, attaching it to the sides of the calorimeter. The results that I got from the experiment this time were an improvement on the previous attempt, the results are displayed in the table below they have already been converted to the form kJmole-1 and averaged Alcohol Average Enthalpy of combustionPercentage increase of my results on previous attempt
Methanol302.1 kJmole-1 9%
Ethanol588.8 kJmole-1 10%
Propan-1-ol1086.2 kJmole-1 9%
Butan-1-ol1320.3 kJmole-1 8% My results show a clear increase of around 9% although this does vary from alcohol to alcohol. My results are still not very accurate, but that is because a lot of the heat that is produced does not even enter the apparatus, it is aloud to escape immediately.
This is an area where the accuracy of my experiment could have been improved a lot, by not allowing any heat escape, but to do this I would have to use a bomb calorimeter, which was not available to me. The graph below compares the enthalpy of combustion from the bomb calorimeter to the results that I got from my experiments.
From the graph above it is possible to see why I have been pleased with the results that I have got from my experiments. The only thing that could have been improved would be to have been to be able to measure all the heat energy that had been released by the combustion of the alcohol’s, increasing the accuracy which was not possible with the equipment available to me. I was able to improve my experiment after I initially completed it but I don’t think I could get much more accurate results. I did though manage to meet the aims of the investigation by finding how the number of carbon atoms within the alcohol affects the enthalpy of combustion. I did have an idea on how to further increase the accuracy of my results but I did not have time to put in to practice. I thought that I could make something that directed more of the heat produced towards the apparatus. A sketch of it is shown below.
This would keep more of the heat produced during combustion close to the calorimeter so more is absorbed. Lining the reflector with silver/ shiny surface would also mean a lot more of the heat is kept in the apparatus so that I am able to measure it.
There are other aspects of the enthalpy of combustion of alcohols that I could have also investigated. Firstly I could have looked into whether the position of the OH group within the molecule effects the enthalpy change and also whether branching within the molecule also has any effect on the enthalpy of combustion. Sadly I didn’t get time to do this.