TYPE OF BOND ENERGY REQUIRED TO BREAK THE BOND(j)
C-H 410
C-O 360
O-H 510
O=O 496
C=O 740
C-C 350
To separate C-H bond you need to apply 410 joules of energy. There are five such bonds in ethanol so you multiply 410 by five to get 2050 joules. You do these calculations for all the other types of bonds that make up ethanol, add them all together and you get 3270 joules. All of the other alcohols can be broken up in this way. Below is a table showing the energy required to break up the bonds in each alcohol.
Type of alcohol Energy required to break the bonds in the alcohol (j)
Methanol 2100
Ethanol 3270
Propanol 3740
Butanol 4560
Pentanol 5380
As you can see a longer molecule takes more energy to break it´s bonds, in this case Pentanol. Compared to a smaller molecule, methanol which requires less energy to do so. I can come to predict that the longer the molecular structure in the alcohol the more energy it will take to remove the bonds. So when I come to predicting results I can safely say that pentanol will evolve more energy than methanol simply because it has more bonds to break.
Type of alcohol Before exp 1 After exp 1 Before exp 2 After exp 2 Before exp 3 After exp 3 Average temperature difference(ºC) Average increase in weight (g)
Methanol Temp(ºC) 23 73 23 73 23 73 50
Weight(g) 153.12 152.12 162.84 159.56 156.35 154.09 -2.18
Ethanol Temp(ºC) 26 76 23 73 21 71 50
Weight(g) 169.78 166.76 169.21 163.56 176.00 172.91 -3.92
Propanol Temp(ºC) 23 73 24 74 23 73 50
Weight(g) 191.56 189.21 168.11 157.00 189.01 187.32 -5.05
Butanol Temp(ºC) 23 73 21 71 24 74 50
Weight(g) 159.76 156.71 98.12 84.32 187.56 181.58 -7.61
Pentanol Temp(ºC) 24 74 22 72 23 73 50
Weight(g) 164.85 157.23 167.94 155.38 161.83 153.19 -9.61
Analysis
As you can see in the obtaining section I have recorded all the information from the results that I need to complete the calculations…
· Energy evolved
· Energy per gram
· Energy per mole
First of all I will calculate the amount of energy evolved. I will achieve this by using this simple formula…
Energy evolved = Mass x Rise in temperature x SHC
Energy evolved = 100g x 50ºC x 4.2
As you can see all of the alcohols will have the same amount energy evolved because all the numbers that are filled in the formula remain constant for each alcohol and the same numbers are applied for each individual alcohol. Below is a table showing the amount of energy evolved in each case…
Type of alcohol Energy evolved in (j) Energy evolved in (kj)
Methanol 21 000 21
Ethanol 21 000 21
Propanol 21 000 21
Butanol 21 000 21
Pentanol 21 000 21
To find out how much energy is produced per gram we use the formula…
Energy per gram of fuel = Energy evolved x Mass of fuel burnt
Energy per gram of fuel = 21kj x ?
Below is a table showing how much energy is produced per gram when burning the alcohols in question…
Type of alcohol Energy per gram (kj)
Methanol 9.63
Ethanol 5.36
Propanol 4.16
Butanol 2.76
Pentanol 2.19
As you can see the energy per gram decreases as the length of the molecule increases. This is because more fuel is burnt so there is more of it to be filled with the energy. This is shown in graph 1.
To find out how much energy is produced per mole you have to use this formula..
Energy per mole = Energy per gram x Formula mass
Here is a list comprising all the formula masses of the chosen alcohols…
· Methanol 32g
· Ethanol 46g
· Propanol 60g
· Butanol 74g
· Pentanol 88g
Below is a table showing the energy produced per mole…
Type of alcohol Energy per mole(kj/mol)
Methanol 308.16
Ethanol 245.56
Propanol 249.60
Butanol 206.46
Pentanol 192.72
Again the results decrease as the molecule length increases suggesting that during the experiment more energy was last in the longer alcohols, this is shown in graph 2.
As you can see, graph 1 has a nice smooth curve suggesting that the results are reliable. But graph 2 has an anomalous result (x), this does not allow the curve to follow a smooth pattern. I think this is because the energy per gram in ethanol and propanol was only slightly different and when multiplied with the formula mass, propanol comes out larger.
My prediction was not clear enough because to vary the amount of heat evolved, time would have to come into it. But I did predict that the energy per gram would decrease as the molecular length increases. I think this because the alcohols with more carbon atoms in like pentanol burnt more fuel so there would be less energy per gram because more fuel has been burnt. The reason why more fuel has been burnt is because of the large number of carbon atoms and large molecular length, hence the surface area is large allowing more energy to be released.
EVALUATION
I believe my results were very inaccurate. Below is a list of reasons why it was an inaccurate experiment.
1. Energy given off through sound and light.
2. Heat conducted and convected away through the air.
3. Radiation of heat out into the atmosphere.
4. The fact that the beaker gets hot.
5. The rubber clamp transferred heat way.
6. Heat may be take away through guts of wind.
7. The fact that at higher temperatures, heat is lost faster to the air and out of the beaker, due to the bigger heat difference, making the higher temperatures more inaccurate, and making a shallower gradient on the graph.
8. By incomplete combustion
9. The amount of energy you give the alcohol originally.
10. The availability of alcohol for the wick to burn, if not enough then the wick would burn not the alcohol which would give an inaccurate result.
11. Evaporation of water so there will be less water to heat, making the water hotter.
12. The size of the wick.
13. Not all of the water was the same temperature.
14. The flame size changed due to the type of alcohol, hence it was a different distance away from the beaker each time.
Numbers 1 – 10 would decrease the reading and Numbers 11 – 14 would increase the reading.
The equipment that I used in this experiment was very inaccurate because heat is a bad way of transferring energy without any loss of it. Any molecule will conduct heat, radiation happens and can be reduced but not completely halts. I feel that the most limiting factor of the experiment is the convection of air and to a lesser extent, of water. Also during the experiment, some of the water will have evaporated, thus the water mass/temp reading will be altered.
Looking at the overall experiment and procedure used to find the results obtained, I thought that it was a good enough way to make me investigate the combustion of different alcohols. The results I got were near enough of what I would have expected from the alcohols used, which I mentioned in my conclusion. The only set of results I was unhappy with were the temperature gained by the alcohol propanol. This is because propanol looks to be an anomalous result. Propanol didn't follow the pattern in the sequence of the carbon atoms in the hydrocarbon chain as it’s produced the least amount of heat, which was wrong, as methanol should have produced the least amount of heat energy as it had the least amount of carbon atoms. The Propanol alcohol should have produce a heat energy between the alcohols Ethanol and Butanol, which should have been about 26,000 joules, as Ethanol was 25,000 and Butanol was 27,000, so propanol should have been in between the two figures as Butonal had 4 carbon atoms and ethanol had 2, where as Propanol has 3 making the results in between the two. The odd result of the Propanol alcohol could have been an anomalous result for a number of reasons. It could have been because the tip of the fame might have not been touching the bottom of the copper calumeter flask, making the alcohol produce less heat to the water in the calumeter flask. It could have also been because of the flame as it might have been too small, which might have also gave a low temperature reading to heat the 150ml of water. The procedure of the experiment was good enough of finding out the energy gained from the indivdual alcohols, but changes could have been made to make the reliability of the results more accurate. This could have been done by using a better and more accurate thermometer, such as an electric thermometer, which have gave me a much more pin point set of results.
If I were to do this experiment again I would make a number of improvements or changes, one of my changes would have to be to use a Bunsen burner which would heat each alcohol and I could record its temperature gain every minute. This could be done by placing about 100ml of alcohol in a glass flask which would be heated with a Bunsen flame recording the results and comparing them to each other. I could also then measure the amount lost after recording the alcohol temperature after about 8 minutes, these results will be really high but we'll have a greater difference between each alcohol.
Looking at the experiment I did I thought we could have extended the results we achieved by using different flame lengths, this would have told us how much the alcohol uses its energy when the flame is increased. I could have also changed the amount of water in the copper calumeter, or even change the metal of the calumeter to change the conductivity.
I feel that this experiment could have been improved by using a wider range of alcohols such as hexanol and heptanol. This would give a better graph reading and a wider range of results to support a firm conclusion. On the other hand, if I had started below room temperature, so that the amount of energy gained, from room temperature might equal the energy lost at temperatures higher than room temperature. Next time reducing heat lost would be my main priority. Improving insulation techniques would be a valuable asset in obtaining the most reliable data I could.
Another error is that of incomplete combustion. Complete combustion occurs if there are lots of oxygen atoms available when the fuel burns, then you get carbon dioxide (carbons atoms bond with two oxygen atoms).
If there is a limited supply of oxygen then you get carbon monoxide (each carbon atom can only bond with one oxygen atom). This is when incomplete combustion has occurred. This is so because the carbon monoxide could react some more to make carbon dioxide. If the oxygen supply is very limited then you get some atoms of carbon released before they can bond with any oxygen atoms. This is what we call soot. Since heat is given out when bonds form, less energy is given out by incomplete combustion. So this is why it affects the outcome of the experiment. To overcome this problem, I would have to make sure a sufficient supply of oxygen was involved in the reaction.
