These values were calculated using the equation function on Microsoft Excel. Methanol Balanced chemical equation: 2CH3OH Total Energy In 20976 Total Energy Out 27236 Energy total (kJ): 20976 – 27236 = – 6260 Energy total per mole of alcohol: – 6260/2 = – 3130 Every result given is a minus figure, showing that this reaction is exothermic as energy is released in the reaction. The results of our own test are therefore minus figures in relation to the energy of the reaction, as the reaction is losing energy. The balanced formulas for methanol, propanol and pentanol all have 2 of the alcohol to balance the equation.
Therefore to find the value per mole of the alcohol it is necessary to divide by 2. An example of how I calculated these values is shown over the page, using methanol as an example. Methanol Balanced chemical equation: 2CH3OH + 3O2 i?? 2CO2 + 4H2O The diagram shows the reaction as described by the formula. At 1 energy is taken in during the reaction in order to break all the bonds. By calculating the number of each bond from these diagrams, we may then find out the total energy taken in using the bond energy values that we know already.
– O bonds broken. 2 X 464 = 928J ==> 3 O = O bonds broken. 3 X 498 = 1494J This gives a total of 5616J taken in as bonds are broken. At 2 energy is given out in during the reaction in order to create new bonds. By calculating the number of each bond from these diagrams, we may then find out the total energy given out using the bond energy values that we know already. There are: ==> 8 H – O bonds broken. 8 X 928 = 3172J C = 0 bonds broken. 4 X 805 = 3220J This gives a total of 6932 given out as bonds are made.
The total energy given out is taken away from the total energy taken in to give the total energy exchange of the reaction. This gives – 1316J. To make this figure a representation of the alcohol being burnt, and allow comparison to our own results we must find this value per mole of alcohol. In our calculation two moles of alcohol are used, and thus the figure is divided by two. -1316/2= – 658J. This may be seen on an energy level diagram, which shows the changes of energy during the reaction. To assist summarizing the accuracy and reliability of my results, the theoretical results may be compared to my own results on a graph. CONCLUSION:
In analyzing my findings it is important to restate the hypothesis: I believe that the increased complexity of a molecule and the energy released by it are proportional. When looking at the graphs we can see that my results support this hypothesis. This can be seen by the line of best fit, as it passes through the origin in a positive direction, denoting proportionality between energy released and the number of carbon atoms. The reason for this (as explained early with the bond energy calculations) is that each type of bond requires a certain amount of energy to break it, and the same amount of energy is released when this bond is formed.
Thus as the molecule increases in complexity, the greater the number of bonds it will have, and the greater the amount of energy will be released when it is combusted. For example methanol has only one carbon atom, and according to the bond energy values produces only 658J of energy when combusted, whereas the more complex pentanol, with five carbon atoms produces 3130J of energy. As the reaction is releasing energy, we say the reaction is exothermic. The reason that a combustion reaction is exothermic relies on the principle that as bonds are broken energy is taken in and as bonds are made energy is given out.
The reaction is exothermic because the amount of energy taken in by the reaction to break the bonds is less than the energy released when new bond are made. Thus more energy is released than is taken in, and the surroundings gain heat energy. Although the results are quite scattered and do not show perfect correlation, we are able to spot the trend that the increase of carbon atoms and energy released are proportional, thus proving the hypothesis correct. The accuracy of my hypothesis may be seen by the similarity of the actual graphs and my hypothetical graph.
EVALUATION: As the graphs show, the results of my experiment are not entirely accurate. They fluctuate in places, there are anomalies, and there is a significant difference between the theoretical results and the actual results. The theoretical results suggest that all the energy produced in the reaction is transferred to the water. In reality this is not the case as energy is transferred to other things other than just the water. Our results are lower than the theoretical results, thus not all the energy has been transferred to the water in the can.
The results also fluctuate, suggesting that energy is not being transferred entirely to the can. Possible ways that this energy could have been otherwise transferred are as follows: ==> As light energy in the flame ==> Heating up the aluminium can ==> Heating up the surrounding air Although fluctuations did occur, there was only one fluctuation big enough to be counted as an anomaly. This anomaly was a value higher than the other results for that alcohol, suggesting an increased efficiency in the method. Other procedural problems were noticed.
One major problem was the occurrence of incomplete combustion. This was noticed as a black layer of carbon was noticed to develop on the base of the can, thus the burner is losing mass that is not being used to heat the water. As a result my initial calculations are slightly inaccurate. It also meant that heat energy transfer from the flame to the water is less efficient, as it must heat the added carbon in the process of heating the water. The dependent variables were controlled during the experiment by the following methods:
==> The distance from the base of the can and the wick was kept constant by measuring this distance to a set value of 2cm each time. ==> The mass of water heated was controlled by measuring the amount of water in a measuring cylinder to a set value of 250ml each time. ==> The type of can used was kept constant by using the same can each time. ==> The starting temperature of the equipment was kept constant by allowing the equipment to cool to a temperature of 20i?? C each time. ==> The stirring of the water was kept constant by stirring the water frequently with medium vigor for each experiment.
These were controlled quite well, as we see only minor fluctuations in the results. However the difference between the theoretical results and these results suggests that our method was not greatly efficient. The main areas that were ineffective in providing accurate results were: ==> Although draft excluders minimized cold drafts entering the experiment area, it did not stop heat energy being transferred to the surroundings. ==> For the water to be heated it was necessary for the can to be heated first, thus energy is “wasted” in heating the can.
==> The methods of measuring may not always have been totally accurate. For example it was noticed on a few occasions, that although the burner had been blown out, the temperature continued to rise. Therefore it has taken time for the energy transferred to the water to be measured on the thermometer, and thus a smaller temperature recording of 20i?? C has been used in a calculation with a mass loss that is relative to a greater temperature. If able to repeat this investigation with better equipment it would be possible to improve the method so as to provide more accurate results.
Some improvements might be: ==> Electronic measuring equipment used to increase accuracy by eliminating human error. ==> The energy taken to heat the can accounted for accurately in the calculations. ==> The same type of burners used each time, so that the type of flame produced by each alcohol is similar. ==> Somehow controlling the heat conditions around the experiment so that the surrounding temperature is higher, and thus less heat is transferred to the surroundings. ==> Then use of a bomb calorimeter measures the energy transfer to a greater accuracy.
==> Supply oxygen directly to prevent incomplete combustion. Further investigation could include continuing to test different alcohols in the series to see if this trend continues. Also to look at the combustion of other fuels could aid investigating the relationship between complexity and energy produced. EVALUATE…. Incomplete combus Taking off blackness Wax on base Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Electricity and Magnetism section.