The general formula for combustion of an alcohol is as so:
Alcohol + oxygen carbon dioxide + water + energy
We can predict how much energy might be given out by using our knowledge of molecules, bonds and their strengths. This simple bond diagram shows how ethanol might react with oxygen when burnt in air:
Let us first take the reaction of methanol with air:
CH3OH + 11/2O2 CO2 + 2H2O
We can predict how much energy would be given out or taken as heat in this reaction by using the theoretical energy values of each bond:
Bonds broken: C-H 3×435 =1305
C-O 1×358 =358
O-H 1×464 =464
O=O 11/2×497 =745.5
Total 2872.5
Bonds made: C=O 2×803 =1606
O-H 4×464 =1856
Total 3462
Energy from bonds made – energy used in breaking bonds = latent energy
3462 - 2872.5 = 589.5 kJ mol-1
We can see that a positive answer is given proving that this is an exothermic reaction. We can also see that burning one molecule of methanol with oxygen gives out 589.5 kJ mol-1.
I can use the same method to show how the other alcohols will react.
Ethanol:
CH3CH2OH + 3O2 2CO2 + 3H2O
Bonds broken: C-H 5×435 =2175
C-O 1×358 =358
O-H 1×464 =464
C-C 1×347 =347
O=O 3×497 =1491
Total 4835
Bonds made: C=O 4×803 =3212
O-H 6×464 =2784
Total 5996
5596 – 4835 = 1161 kJ mol-1
This means one molecule of ethanol burned with oxygen gives out 1161 kJ mol-1.
Propanol:
CH3CH2CH2OH + 41/2O2 3CO2 + 4H2O
Bonds broken: C-H 7×435 =3045
C-O 1×358 =358
O-H 1×464 =464
C-C 1×347 =347
O=O 41/2×497 =2236.5
Total 6450.5
Bonds made: C=O 6×803 =4818
O-H 8×464 =3712
Total 8530
8530 – 6450.5 = 2079.5 kJ mol-1
This means that one molecule of propanol burned with oxygen gives out 2079.5 kJ mol-1.
Butanol:
CH3CH2CH2CH2OH + 6O2 4CO2 + 5H2O
Bonds broken: C-H 9×435 =3915
C-O 1×358 =358
O-H 1×464 =464
C-C 1×347 =347
O=O 6×497 =2982
Total 8066
Bonds made: C=O 8×803 =6424
O-H 10×464 =4640
Total 11064
11064 – 8066 = 2998 kJ mol-1
This means that one molecule of butanol burned with oxygen gives out 2998 kJ mol-1.
Pent-1-ol
CH3CH2CH2CH2CH2OH + 71/2O2 5CO2 + 6H2O
Bonds broken: C-H 9×435 =3915
C-O 1×358 =358
O-H 1×464 =464
C-C 1×347 =347
O=O 71/2×497 =3727.5
Total 8811.5
Bonds made: C=O 10×803 =8030
O-H 12×464 =5568
Total 13598
13598 – 8811.5 = 4786.5 kJ mol-1
This means that one molecule of butanol burned with oxygen gives out 4786.5 kJ mol-1.
We can compare these theoretical values for what energy should be given out when each alcohol is burned in a plentiful supply of air:
Methanol 1 carbon atom 589.5 kJ mol-1
Ethanol 2 carbon atoms 1161 kJ mol-1
Propanol 3 carbon atoms 2079.5 kJ mol-1
Butanol 4 carbon atoms 2998 kJ mol-1
Pentan-1-ol 5 carbon atoms 4786.5 kJmol-1
As we can see, methanol with only one carbon atom has the least energy output from combustion. Ethanol has 2 carbon atoms in one molecule, and gives nearly twice as much energy as methanol. Propanol has 3 carbon atoms, and gives nearly twice as much still as ethanol. Butanol gives nearly one thousand kJ mol-1 more than propanol and has 4 carbon atoms. Butanol gives out the most energy during the combustion of one of its molecules. From these calculations using theoretical values for bond energies from my chemistry book, (“GCSE Chemistry” by B. Earl & L. D. R. Wilford), I am able to predict which alcohol will be the most productive in terms of energy.
The following graphs show what I predict will be the outcome of these experiments:
As we can see, I think the mass difference between before and after weights shall decrease as we use alcohols with more carbon atoms in one molecule. As I have said, this is because the bonds produce more energy when there are more of then, more energy than the bonds take to break.
Results:
Using the formula below, I can roughly see how much energy was needed to raise the temperature of the water by burning different alcohols.
Heat energy = mass of water (50g) × specific heat capacity (4.2kJ) × temperature change (40°C) = 8400kJ
I can now see the various masses of alcohol which give out 8400kJ
Now I can work out how many moles of each alcohol was used during the experiment.
Methanol: 1.31/32 = 0.040938
Ethanol: 1.1/46 = 0.023913
Propanol: 0.69/60 = 0.0115
Butanol: 0.85/74 = 0.011486
Pentan-1-ol: 0.70/88 = 0.007955
Using these values, I can calculate the enthalpy combustion of the alcohols (how much energy is released by each mole of each alcohol), and henceforth, compare the energy values from the experiment with the theoretical values:
Methanol: (8400/0.040938)/1000 = 205.188 kJ mol-1
Ethanol: (8400/0.023913)/1000 = 351.273 kJ mol-1
Propanol: (8400/0.0115)/1000 = 730.435 kJ mol-1
Butanol: (8400/0.011486)/1000 = 731.325 kJ mol-1
Pentan-1-ol: (8400/0.007955)/1000 = 1055.94 kJ mol-1
These are the theoretical values:
Methanol 1 carbon atom 589.5 kJ mol-1
Ethanol 2 carbon atoms 1161 kJ mol-1
Propanol 3 carbon atoms 2079.5 kJ mol-1
Butanol 4 carbon atoms 2998 kJ mol-1
Pentan-1-ol 5 carbon atoms 4786.5 kJmol-1
The following table shows the efficiency of each experiment we did:
Analysis:
My hypothesis seems to be correct – alcohols with more carbon atoms give out more energy. This is because though energy is lost through breaking these bonds; more energy is made when the new bonds for carbon dioxide and water are made. If an alcohol has more carbon atoms, it will also have more hydrogen atoms as well.
We can see my hypothesis is correct, as the masses of alcohol needed to produce the energy required to raise 50g of water by 40°C (8400kJ) decrease as the alcohol structure has more carbon atoms. Methanol burnt 1.31g of methanol to heat up the water 40°C more than its original temperature. Ethanol burnt 1.1g of ethanol to heat up the water 40°C. Propanol burnt 0.69g of propanol to heat up the water 40°C. Butanol required 0.85g burnt to heat up the water 40°C, and Pentan-1-ol required 0.7g to heat the water 40°C. The following graph shows this more clearly:
As we can see, propanol does not fit the otherwise near-perfect line. I think this is because somebody must have filled the propanol alcohol burner with pentan-1-ol, probably by accident. I do not think that an error this drastic could have been caused otherwise by our errors in the experiment or inefficiencies, as all the results we obtained for propanol were constant, and the result obtained was not caused by one freak result. Also, we can see that for at least the other four alcohols, for each of which three experiments were performed, the results were as expected in relation to each other. This leads me to believe that the alcohol burner for propanol was in fact filled with pentan-1-ol.
The efficiencies I have calculated may seem low, but the limitations in our abilities made this unavoidable without great effort. This did not affect what I was able to conclude from our results, as it still proved that the bigger the alcohol, the larger the energy produced.
The graph above shows how the values we obtained from the experiment show that as the size of the alcohol increases, the energy produced increases. The propanol result is not as high as the pentan-1-ol result even though nearly the same mass was burnt because I have calculated the propanol using the propanol RAM.
Evaluation:
There were many problems and inefficiencies in these experiments, which would have lead to the results not being exactly as the theoretical values were. However, apart from the results for propanol, I feel that the results we obtained were very good in relation to each other, and illustrated the point that alcohols with larger structures produce more energy when burnt with oxygen.
Inefficiencies were mainly in the form of heat loss. This was reduced by placing heat mats around the alcohol burner, so that the main direction of heat was to the base of the metal beaker with the water in it, but this did not fully eliminate heat loss around the alcohol burner. Heat loss into the surroundings would also have occurred from the metal beaker and the water heating the air around it. All of these inefficiencies made it take longer to raise the temperature by 40°C, meaning the burner would have had to burn longer, meaning more alcohol would have been used. These inefficiencies would have meant only a fraction of the heat energy produced is used to heat the water. The combustion would be incomplete, and so soot would have been produced, further lowering the amount of heat energy heating the water.
Making the difference between start and finish temperatures larger than would otherwise be due to these inefficiencies. We also could not fill up the beaker exactly with 50g of water each time, as we are not accurate enough, but this would have minorly affected the results. The temperature measured was not extremely accurate as the water was not stirred so there would be an uneven distribution of temperature of the water.
The only clear anomalous results were for propanol. As I have already said, I think this was due to pentan-1-ol being in the burner rather than technical errors and inefficiencies. I think so because all the other experiments produced decisive results which related well to each other. Also, the average difference for propanol and pentan-1-ol was very similar – the difference between these two results was a mere hundredth of a gram, showing them to be almost exactly the same in terms of heating up 50g of water.
I have worked out the efficiencies of our experiments, and on average they were about 30% efficient. This was unavoidable due to the limitations in our abilities and equipment. The experiment could have been made more efficient using a bomb calorimeter. Calorimetry is the science of measuring a quantity of thermal energy in the process of heat transfer. A calorimeter is an instrument used to measure the amount of thermal energy; one widely used type consists of an insulated container of water, a stirring device, and a thermometer. A heat source is placed in the calorimeter, the water is stirred until equilibrium is reached, and the rise of temperature is noted by reading the thermometer. Because the heat capacity of the calorimeter is known, (or can be measured by using a standard heat source) the amount of energy given out can be easily calculated. When the heat source is a chemical reaction, such as the burning of a fuel, the reacting substances are placed in a heavy steel vessel called a bomb. The bomb is placed within the calorimeter, and the reaction is started by ignition with an electric spark. This would greatly increase efficiency as all heat would be used to heat the water.
To further extend this work on alcohols, we could try using more alcohols, to obtain more results.
