The energy released by a fuel, such as alcohol, depends on two things. Firstly the number of bonds to be broken and made and secondly the type of bonds involved. These are exothermic reactions.
Table 3 Bond energies
Combustion of alcohols
Methanol
CH3OH + 1.5O2 CO2 + 2H2O
Ethanol
C2H5OH + 3O2 2CO2 + 3H2O
Propanol
C3H7OH + 4.5O2 3CO2 + 4H2O
Butanol
C4H9OH + 6O2 4CO2 + 5H2O
Pentanol
C5H11OH + 7.5O2 5CO2 + 6H2O
Hexanol
C6H13OH + 9O2 6CO2 + 7H2O
Table 4 Theoretical energy released when alcohols burnt
Calculated from bond energies
Table 4 shows that the theoretical energy released from alcohol combustion increases as the size of the alcohol molecule increases. This is because the bigger molecules have more bonds to be broken and more bonds are made in the products of the reaction (water and carbon dioxide). The extra bonds that are broken in the alcohol with more carbon in it need more energy but the products release even more energy as they formed therefore the overall energy released by the reaction (combustion) goes up. The results of the experiment showed the same pattern but the results were much lower due to inefficient energy transfer to the water (Graph 3). Table 5 shows that a fairly similar percentage of energy was transferred to the water from the combustion of each alcohol but the percentage went down slightly as the size of the alcohol molecule increased.
Calculation of measured energy as a percentage of the theoretical energy released:
(Measured energy/Theoretical energy released) x 100
For example: Methanol
213.8/658 = 0.325
0.325 x 100 = 32.5%
Table 5
Graph 4 shows clearly that the number of carbon atoms in the alcohol is proportional to the theoretical energy produced during its combustion, as the points are all virtually on a straight line. This increase is regular because as an alcohol gains one carbon atom it also gains 2 hydrogen atoms and needs 1.5 more molecules of oxygen to burn. This provides one more molecule of carbon dioxide and one more molecule of water in the products releasing the additional energy seen.
The prediction that “As the alcohol gets bigger the amount of energy produced gets bigger” is supported by the results of the experiment. Graph 2 shows that the measured energy transfer was proportional to the number of carbon atoms in the alcohol but the points were scattered about the line of best fit because the energy transfer was not very efficient and varied slightly between alcohols.
Evaluating
Compared to the theoretical results, the results obtained were quite low. This means that much of the energy released by the burning alcohol did not reach the water.
There were no obviously anomalous results. However this may have been because there were five sets of results and the averages were used which would tend to make anomalous results less apparent. The results of the experiment are not totally reliable but they do at least show the right trend.
The measurement had been made as fair a test as possible by:
- Using the same can for each alcohol burnt.
- Taking fresh water for each alcohol burnt.
- Stirring the water with the thermometer before each temperature reading, distributing the heat evenly.
There were many sources of error in the experiment.
- The amount of water put into the can may not have been measured completely accurately because of the way the level of the water is seen in the measuring cylinder.
The water could be weighed into the can using a digital balance.
- The height above the spirit burner may not have been consistent with each alcohol.
The height of can above the spirit burner could have been measured before each alcohol was burnt.
- The spirit burner did not have a lid on it during the weighing time as the lid made the burner too heavy for the scales, so the alcohol could evaporate.
Better scales could have been used so that the lid could have been kept on during weighing.
- The wick varied in length.
The wick could have been adjusted to the same length before each alcohol was burnt.
- Heat from the flame could have escaped out to the sides of the spirit burner.
Draught excluders could have been put up around the spirit burner and can.
- The can was made of copper which meant that it absorbed and transferred heat well to the water but it also lost heat to the air.
The can could have been insulated at the sides. The can already had a lid.
Although the results are low they do show a pattern of increasing energy transfer to water with increasing carbon content of the alcohol, suggesting some reliability. If the results were completely unreliable they would not show any pattern. The trend in a decreasing percentage energy transfer to the water compared to the theoretical with increasing size of the alcohol molecule may have been due to the alcohol burning faster causing increased heat loss and decreased heat transfer.
The results were good enough to come to a firm conclusion but further evidence could be provided with further reading from this experiment and from another experiments. The liquid in the can could have been changed to something else with a different heat capacity.
