The energy released when different alcohols are burned
Chemistry coursework
The energy released when different alcohols are burned
Experimental results for the combustion of alcohols
Methanol
Ethanol
Propanol
Butanol
Mass of spirit lamp & fuel at start
75.89
88.33
79.67
66.54
Mass of spirit lamp & fuel at end
74.8
87.32
78.71
65.6
Mass of alcohol burned
.09
.01
0.96
0.94
Mass of water heated up
00
00
00
00
Temperature of water at the start
25
23
25
24
Temperature of water at the end
50
48
50
49
Temperature rise
25
25
25
25
The experimental results for each alcohol
Using our results, we could calculate how much heat was produced in the burning of each alcohol. The reaction is exothermic, as more energy is needed to make bonds
The results
The alcohol
The energy released in the combustion of that alcohol
(KJmol )
Methanol
423.85
Ethanol
478.22
Propanal
656.25
Butanol
826.60
The theoretical results for each alcohol
In this next stage of the investigation I am going to find the theoretical results, to compare these with the experimental results
The alcohol
The energy released in the combustion of that alcohol (KJmol )
Methanol
502
Ethanol
654
Propanol
500
Butanol
999
The results of both theoretical and experimental calculations
The alcohol
Molar mass
The energy produced (Experimental values)
The energy released (the theoretical values)
Methanol
44
423.85
502
Ethanol
46
478.22
654
Propanol
60
656.25
500
Butanol
74
826.60
999
Analysis
If we analyse these results, we can clearly see that as the molar mass increased, so did the energy produced in the combustion of each alcohol. As the alcohol compound ...
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500
Butanol
999
The results of both theoretical and experimental calculations
The alcohol
Molar mass
The energy produced (Experimental values)
The energy released (the theoretical values)
Methanol
44
423.85
502
Ethanol
46
478.22
654
Propanol
60
656.25
500
Butanol
74
826.60
999
Analysis
If we analyse these results, we can clearly see that as the molar mass increased, so did the energy produced in the combustion of each alcohol. As the alcohol compound increased in size, the molar mass also increased. As the number of atoms in each compound increased, so did the number of bonds connecting them. This meant that as the molar mass increased, the amount of energy to break initial bonds, also increased. This then had a 'knock on' effect, which meant that as the molar mass increased, more energy was required to then form these new bonds. The energy required forming new bonds increased, as the molar mass increased because there were more bonds to form. As the number of bonds to form increased, so did the energy required in forming them, as the reaction is exothermic. This can be seen also by the results from the mass of alcohol burnt for each one. As the molar mass for each alcohol increased, less of it was burned. This was because the higher the mass of the alcohol, the faster bonds could be broken, as there were fewer bonds within the substance to break. It took less activation energy to break initial bonds, as the molar mass decreased.
If we look at the graph we can also see that there was a considerable difference between the experimental values and he theoretical values. This was because a lot of heat energy supplied by the spirit lamp, was released into the atmosphere, instead of heating up the alcohols. This led to less mass being burned, which was had the effect of decreasing the energy released. The theoretical calculations didn't consideration that heat could be wasted and released into the environment.
This agrees with my earlier prediction, and as I can observe, both the set of theoretical and experimental results prove the theory in my prediction to be correct. The results of my calculations show that there is a linear relationship between the molar mass and the energy released. As the graph has a positive gradient, my results show that as the molar mass increases, so does the energy released from the combustion of each alcohol. Using my results I could predict the results for pentanol by extrapolating my line of best fit. But first I would need to work out the mass of 1 mole of pentanol.
Evaluation
As we can see from looking at the results, and the graph which represents this data, the trend is clear. These results clearly, and accurately prove that as the molar mass increased in a alcohol, the energy released when they are burnt will also increase. The anomalous result I have circled on my graph was probably caused by a slight miss calculation of the mass of alcohol burned. I feel the method I used was effective, as the experiment was fairly simple to set up, it didn't have to many safety concerns, and it provided me with reliable results, which proved the scientific theory I learned in class right.
However, there are changes I would make to the experiment if I were to do it again. I would have started the experiment with equal masses for the sprit lamp and the fuel at the start, as a higher mass can take longer to begin burning. I would have also started the experiment with each alcohol at the same temperature, as more heat causes bonds to have more energy. Using a water bath could have done this. I would have also collected the results of more alcohols, to more strongly support my findings.
I would improve my investigation, by not only looking at the exothermic reaction of alcohols burning, but at other exothermic reactions. I would get more substances to test, which would enable me to get an overall picture of how molar mass effects the amount of energy produced. This would prove, without any doubts, the scientific theory I have stated in my prediction and analysis. For example, I could experiment with neutralisation reactions, to see if those exothermic reactions had the same trends. For example, I could react ammonia with sulphuric acid, nitric acid and hydrochloric acid, which all have different molar masses, to find out if the results would be the same, and support the theory put forward in this chemistry Sc1.
The resources I used
To aid my investigation I used the following resources for reference:
* CGP chemistry revision guide
* MEG study guide for triple award science
* Encarta 99 (exothermic reactions)
* Help sheets provided by MR Griffiths
Brett Marsden
Chemistry