I predict that the amount of heat energy produced by burning the fuel (we are using ethanol) will be proportional to the mass of ethanol burned. I have based this prediction on the following scientific knowledge (as suggested in a secondary source – ‘Chemistry: A Practical Approach’ by A.L Barker and K.A Knapp):
Within the reactant molecules of a chemical reaction, there are many tiny atoms which are held together by very strong forces. These forces which link atoms in molecules together are called bonds.
All chemical reactions consist of bonds in the reactant molecules being broken, and new bonds being formed. The chemical reaction that I am investigating is that of ethanol burning in oxygen to produce carbon dioxide and water, and this idea of bonds applies here too. ‘It is impossible to measure the total energy stored up in a particular substance, but we can measure the change in it which occurs during a chemical reaction.
The symbol used for such a change is H where (delta) means ‘change of’ and H is the ‘heat content’ or enthalpy of the system.
‘ An endothermic reaction is one which takes in energy from the surroundings, usually in the form of heat, and because of this, H is positive because the system gains energy from the surroundings. Energy must be supplied to break existing bonds, so bond breaking is an endothermic process, whereby energy is gained from the surroundings. In an endothermic reaction, the energy required to break old bonds is greater than the energy released when new bonds are formed.
Read also surface area and heat loss experiment
In contrast, an exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat, therefore H is negative for an exothermic change because the system loses energy to the surroundings. Energy is released when new bonds are formed, so bond formation is an exothermic process, whereby energy is given out. In an exothermic reaction, the energy released in bond formation is greater than the energy used in breaking old bonds. ‘When we talk about the enthalpy change which occurs during a reaction, we are referring to the total quantity of heat which must be transferred between the products and the surroundings in order for the products to end up at the original temperature of the reactants.
For a definition, we can say that the enthalpy of reaction ( H) is the heat change which occurs when the number of moles of reactants indicated by the equation react together. These changes in enthalpy may be represented on energy level diagrams’. In this experiment, bonds in ethanol and oxygen molecules will be broken, and new bonds will be formed when the atoms combine again, forming carbon dioxide and water molecules (because alcohols combust to give CO2 and H2O). This has been expressed in the following equation:
Ethanol + Oxygen Carbon Dioxide + Water
C2H6O + 3O2 2CO2 + 3H2O
Due to the fact that ethanol is a pure substance, the number of molecules must be proportional to mass burned (thus also the heat given out), and hence heat produced must be proportional to the mass of ethanol burned. This process of bond making and bond breaking can also be shown in an energy level diagram, as explained above. I have demonstrated on the energy level diagram on the next page what I expect to occur in this experiment.
I shall observe the following procedures to ensure that this experiment is a fair test, and thus in this way obtain reliable results:
I shall observe the following safety procedures to ensure that this experiment is not hazardous:
Using the method as described above, I have conducted my preliminary experiments. My first experiment was to decide what would be the ideal amount of tap water to use in the can, so as to perform an accurate yet safe final investigation, bearing in mind the time restrictions. I used a spread of volumes of water (between 50 – 150cm3), and for each, I obtained results for a selection of masses of ethanol (1g, 3g, and 5g). I chose these masses because they would provide me with a wide range of results of the maximum temperature reached with these volumes of water. I have demonstrated these results in the table below:
*Although the temperature was continuing to rise at this point, the scale of the thermometer did not surpass 110?C, and thus I was unable to record any further.
These results show me that when the amount of water in the can is 50.0cm3, the temperature of the water rises very rapidly, even when the mass of ethanol is small. We can see that when using 5g of ethanol, the water temperature rises too high even to be calculated with the equipment I have, and for this reason, it would be unwise to choose this volume of water to conduct my experiment with.
At the same time, these results show that when the amount of water in the can is 150.0 cm3, the temperature of the water rises slowly, with the highest temperature reached being only 78?C, as opposed to the 110+?C obtained when the volume of water is 50.0cm3. For this reason, this data may not offer me the variety I would need to deduce a trend in the results, and thus I will not use this volume of water in my experiment.
The results show that although 50.0cm3 is too small a volume of water, and
150.0 cm3 too high, because of the rate at which the temperature increases in both, 100.0 cm3, as would be expected, gives results which are roughly mid-way between those obtained by the other volumes. Although the temperature does not soar as dramatically as with 50.0cm3, the results offer more of a range than with 150.0cm3, hence this is the volume of water I shall use.
My second preliminary experiment was to obtain a general idea of what trend I should expect from my final results. Having decided above to use 100.0 cm3 of water in the can, I thus conducted my experiment, using three spread-out masses of ethanol (1g, 2g and 3g). I have demonstrated these results in the table below:
Having obtained these results, I plotted a graph to demonstrate them, with the mass of ethanol on the x-axis and the change in temperature on the y-axis. (On the next page).
The graph of my preliminary results (P.T.O) demonstrates a straight line of best fit, which goes through the origin. In this way, this graph substantiates my prediction, in that it proves that the amount of heat energy produced by burning ethanol is proportional to the mass of ethanol burned. However, this straight line through the origin also suggests a relationship of direct proportionality between the two factors being investigated, and thus I shall extend my initial prediction to allow for this new observation:
Enhanced Hypothesis
I predict that the amount of heat energy produced by burning the ethanol will be directly proportional to the mass of ethanol burned.
Having conducted my preliminary experiments, I can draw certain conclusions, based on the results obtained. I will use 100.0cm3 of water in the experiment, because this is an adequate volume, as it offers a wide range of results, yet does not exceed the scale on the thermometer, allowing me to be able to plot all of the actual results. I have also seen that 5.0g of ethanol takes approximately 8 minutes to burn, and thus I will not experiment with any larger quantities of ethanol, as would be very time consuming, because the larger the mass of the fuel, the longer time taken for it to burn.
Instead, I shall use a range of masses of ethanol between 0.5g and 5.0g, at 0.5g intervals, so a large enough number of results will be collected for me to then plot them on a graph and determine the trend, if any, shown. Also, from my preliminary results, it appears that there will be a relationship of direct proportionality between the amount of heat energy produced and mass of ethanol burned; therefore I now know what trend to expect from my final results.
Mass of Ethanol (g)
Initial Mass Crucible (g)
Final Mass Crucible (g)
Mass of Ethanol burnt (g)
Initial Temperature of water (?C)
Final Temperature of water (?C)
Change in Temperature (?C)
Heat produced (J) m x 4.2 x t
0.50
14.26
14.26
0.50
21.0
27.0
6.0
2520
1.00
14.27
14.27
1.00
22.1
34.2
12.1
5082
1.50
14.26
14.26
1.50
21.0
37.1
16.1
6762
2.00
14.25
14.26
1.99
21.0
54.1
33.1
13902
2.50
14.26
14.26
2.50
24.0
73.0
49.0
20580
3.00
14.26
14.26
3.00
24.5
78.5
54.0
22680
3.50
14.26
14.26
3.50
22.2
84.0
61.8
25956
4.00
14.27
14.27
4.00
24.0
89.0
65.0
27300
4.50
14.25
14.25
4.50
24.0
92.0
68.0
28560
5.00
14.26
14.27
4.99
24.3
99.4
75.1
31542
Second Repeated Results:
Mass of Ethanol (g)
Initial Mass Crucible (g)
Final Mass Crucible (g)
Mass of Ethanol burnt (g)
Initial Temperature of water (?C)
Final Temperature of water (?C)
Change in Temperature (?C)
Heat produced (J) m x 4.2 x t
0.50
14.26
14.26
0.50
21.0
24.2
3.2
1344
1.00
14.26
14.27
0.99
23.1
38.0
14.9
6258
1.50
14.27
14.27
1.50
21.0
39.0
18.0
7560
2.00
14.26
14.26
2.00
23.5
57.1
33.6
14112
2.50
14.25
14.25
2.50
22.0
61.0
39.0
16380
3.00
14.25
14.26
2.99
23.2
65.6
42.4
17808
3.50
14.26
14.26
3.50
22.0
81.0
59.0
24780
4.00
14.26
14.26
4.00
26.0
83.1
57.1
23982
4.50
14.27
14.27
4.50
24.1
87.0
62.9
26418
5.00
14.26
14.26
5.00
24.0
98.2
74.2
31164
Average Results:
Mass of Ethanol (g)
Initial Mass Crucible (g)
Final Mass Crucible (g)
Mass of Ethanol burnt (g)
Initial Temperature of water (?C)
Final Temperature of water (?C)
Change in Temperature (?C)
Heat produced (J) m x 4.2 x t
0.50
14.260
14.260
0.500
21.00
27.40
6.40
2688.0
1.00
14.263
14.270
0.997
22.07
37.43
15.36
6451.2
1.50
14.260
14.260
1.500
21.83
44.70
22.87
9605.4
2.00
14.260
14.263
1.997
22.83
58.23
35.40
14868.0
2.50
14.257
14.257
2.500
24.00
67.67
43.60
18312.0
3.00
14.253
14.257
2.997
22.40
70.10
47.70
20034.0
3.50
14.260
14.260
3.500
21.73
77.67
55.94
23494.8
4.00
14.263
14.267
3.997
23.73
84.07
60.34
25342.8
4.50
14.260
14.260
4.500
23.37
85.00
61.63
25884.6
5.00
14.257
14.263
4.997
24.10
92.87
68.77
31403.4
SKILL AREA A: ANALYSING EVIDENCE
A quick glance at my results tables allows me to establish that in general, as the mass of ethanol increased, the change in temperature also increased, thus supporting my prediction to an extent. In order to find out whether these results entirely support my initial prediction, that the amount of heat energy produced by burning the ethanol would be directly proportional to the mass of ethanol burned, I decided to plot the average results onto a graph, with mass of ethanol on the x-axis and change in temperature on the y-axis (PTO).
The graph supports a line of best fit which goes through the origin; however, this line is not straight, as expected, but instead begins to curve gradually as the mass of ethanol increases. This immediately suggests that perhaps my prediction was incorrect: the change in temperature is not directly proportional to the mass of ethanol burned, for had this been the case, then my line of best fit would have been straight. Thus, either the scientific knowledge which I stated and quoted in my plan, upon which I based this prediction, was incorrect in stating that ‘heat produced ? mass of ethanol burned’; or alternatively, my results were imprecise. To determine whether my results were indeed flawed, I have drawn error bars onto the points on my graph. Although some are negligible, most are significantly large enough to demonstrate obvious inaccuracies in the obtaining of the results, the largest being the third point on the graph (i.e. where the mass of ethanol was 1.5g). I shall discuss these possible flaws in my evaluation.
However, my graph clearly demonstrates that change of temperature and mass of ethanol burned are proportional to each other, because as one of these factors increased, so too did the other, at the same rate for a certain period – between 0.0-3.0g of ethanol. This is because as the ethanol burned, bonds in the ethanol, as well as oxygen molecules, were broken. This resulted in the loss of energy from the surroundings into the system. Yet simultaneously, energy was emitted from the system into the surroundings, as the atoms joined again, forming new bonds in carbon dioxide and water molecules.
I can, however, explain why my graph did not show a straight line as predicted, but instead a curve. This is because, as explained in ‘Chemistry – Higher Level’ by Richard Parsons, ‘When a substance is boiling, all the heat energy supplied is used for breaking bonds rather than raising the temperature’, hence when the substance reaches its boiling point, the graph should begin to level out and flatten into a curve. My results show that as the mass of ethanol increased, the final temperature of the water edged increasingly closer to its boiling point (100?C), with the highest temperature reached being 99.4?C, in the first repeated results, when the mass of the ethanol was 5.0g. This means that when the mass of ethanol in my experiment began to get significantly high (at about 3.0g), energy began to be wasted into breaking bonds in the water to boil, and for this reason, it was at this point in my graph that the line began to curve.
This also explains why my preliminary results show a straight line of best fit and thus do support my prediction, as opposed to my final results: in my preliminary investigation, I only experimented with 0.0-3.0g of ethanol, with the highest temperature reached being only 63?C, and thus the fuel did not begin to boil, as in my final experiment. Hence for this reason, the heat energy derived from burning the ethanol at this point was not all being used for breaking bonds, but also for raising the temperature, and so in this preliminary experiment I found the relationship I had expected: heat produced ? mass of ethanol burned. Judging from my final results, we can assume that had I increased the mass of ethanol in my preliminary results beyond 3.0g, this graph would too have demonstrated a curve of best fit.
SKILL AREA E: EVALUATING EVIDENCE
As mentioned previously, I can see from the several anomalous results in my graph that there must have been flaws in my experiment, thus in this way I resulted in some erroneous data.
The reasons for these errors could be:
* Measuring the distance between the base of the can and the crucible – in collecting the data, I was slightly pushed for time and hence would not have been very careful in measuring this distance.
* Using a measuring cylinder to measure out the 100cm3 of water – it was not entirely accurate, so I should have used either a burette, or if not then a pipette to be more precise.
* Drawing my graph by hand – I should have used a computer to reduce the chance of human error.
Having concluded thus, I could extend this experiment further now.
To do this I would experiment with different alcohols such as Butanol and Methanol.
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