An experiment to investigate the factors that determine the amount of energy released when alcohols are burned.
Chemistry SC1
An experiment to investigate the factors that determine the amount of energy released when alcohols are burned
By Khalid Attia
Planning
Statement of Problem
In this investigation, I will be investigating all of the factors that determine the amount of energy released when any given alcohol is burnt.
Input Variables
In this investigation, there are many different variables that will affect the amount of energy produced when alcohols are burnt. These variables are called the Input Variables and they are the following:
) Number of Carbon atoms present (type of alcohol)
2) Concentration of alcohol (purity)
3) Amount of alcohol (volume or mass)
4) Oxygen supply (for combustion)
Each of these variables will affect the amount of energy produced when an
alcohol is burnt in similar ways but some variables have a larger affect than others, therefore choosing the right one is essential.
If the number of carbon atoms in the alcohol is increased, its molecular structure will be larger. This can be seen from this general equation - CnH2n+1OH. When combustion is complete, carbon dioxide and water are formed, therefore if there are more carbon atoms, there will be more carbon dioxide and water produced, this will require a large amount of energy to make the bonds in these compounds. Therefore if more carbon atoms are present in the alcohol, more energy will be released. I can predict that the two will be directly proportional.
The purity of alcohol is a variable that will have the same effect on the energy produced as the number of carbon atoms present in the alcohol. A pure alcohol will have many more alcohol molecules in it than a less pure one. Having more alcohol molecules means having more carbon atoms, and as I've explained previously, having more carbon atoms in an alcohol will produce more energy. I can again predict that purity of alcohol and the energy produced by the alcohol will be directly proportional.
The amount of alcohol (i.e. volume or mass), will, again, have a similar effect on the energy produced by a burning alcohol, as the previous two variables. Increasing the volume or mass means increasing the amount of alcohol molecules available for complete combustion, therefore produce more energy; therefore again, I can predict that the two will be directly proportional.
Oxygen supply is a very complex variable to work with although it will definitely have an effect on the energy produced. The more oxygen there is available for the combustion to occur the more carbon dioxide and water will be produced, therefore more energy will be produced for the bonds within the products. Therefore the more the oxygen supply, the better the combustion, therefore more energy will be produced. Although there is a point where the supply of oxygen will be enough for the complete combustion to occur and therefore any more oxygen will make no difference whatsoever. The two will therefore be directly proportional up until the point where the graph will level off.
Keeping any of the variables constant is relatively simple, although variables such as oxygen supply will change very slightly. Varying any of the variables is also relatively simple except again, the oxygen supply will pose a problem.
I have chosen to vary the number of carbon atoms in an alcohol. I will do this by using different alcohols each time, which contain different numbers of carbon atoms. Although I will measure out the amount of alcohol by its volume, I will later work out its moles using several techniques. This will give me larger scope for my results and graphs. I will use the following alcohols in my experiment: Methanol, Ethanol, Propan-1-ol, Butan-1-ol, and Hexan-1-ol. I will have to make do with no Pentan-1-ol between Butan-1-ol and Hexan-1-ol. I have chosen to change the number of carbon atoms as my input variable because I felt that it would be the one variable that would have the greatest effect and therefore give me better results to work from.
Output Variables
The output variables are the variables that have changed as a result of my input variable. These are what I will have to measure and they will therefore lead me on to my finding my results. As this experiment is to find out the energy released when an alcohol is burnt, therefore is exothermic, I will have to be involving the measurement of heat or heat changes in my output variables. Measuring the temperature of the flame straight from the burning alcohol will not prove to be very accurate therefore, there is only one way ...
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Output Variables
The output variables are the variables that have changed as a result of my input variable. These are what I will have to measure and they will therefore lead me on to my finding my results. As this experiment is to find out the energy released when an alcohol is burnt, therefore is exothermic, I will have to be involving the measurement of heat or heat changes in my output variables. Measuring the temperature of the flame straight from the burning alcohol will not prove to be very accurate therefore, there is only one way of measuring the heat given off by the burning alcohol and that is by the heat change it gives to a certain amount of water. I would suspend a beaker containing probably about 30ml of water, I'd measure the temperature of the water and then I'd begin the experiment; when the alcohol was fully burnt, I would then measure the temperature of the water again and then work out the temperature rise by subtracting the start temperature from the end temperature. After calculating the temperature rise in the water we must calculate the energy produced by the alcohol itself by using the following formula:
Heat capacity Mass
of x of x Temperature Rise (oC) = Energy produced (joules)
liquid Water (g)
The heat capacity of a liquid is the amount of energy required to make 1cm3 of water rise by 1oC, and for water, the heat capacity is 4.2 joules per gram per degrees Centigrade.
Now that I have found a way in which to measure the variable that I will investigate, I must try to keep all of the other influential variables constant. These include the amount of water used in the beaker for heating; the distance between the burning alcohol and the beaker; and the position of the burning alcohol under the water, whether directly under the heating water or slightly off, it must be kept the same during all experiments.
Although I would have worked out the energy produced by the alcohol, I will not know how many moles this relates to, therefore I will have to work out the number of moles are in the 1cm3 of alcohol by using the following procedure for each type:
) Finding out the chemical formula,
e.g. Methanol - CH3OH
2) Finding out the relative atomic mass or r.a.m for each element
e.g. Carbon (C) - 12
Hydrogen (H) - 1
Oxygen (O) - 16
3) Working out the relative molecular mass or r.m.m
e.g. Methanol - CH3OH
(1x12) + (3x1) + 16 + 1
r.m.m = 32
4) Finding out the weight for each type of alcohol relative to 1cm3
e.g. Methanol - 0.791 grams
5) Working out how many moles the alcohol contains
e.g. Methanol - 0.791g
32 r.m.m
= 0.025 moles
Now that I know the amount of alcohol produced and the number of moles in each alcohol, I can generate better, more reliable results and graphs; this is because the number of moles and the energy produced by the alcohol should, in theory, be directly proportional.
Preliminary Work
Before carrying out my actual investigation I had some time to plan out the procedure I was going to use by doing some pilot experiments. During these pilot experiments, I decided things such as, what size beaker I would use, how much alcohol I would burn, how much ceramic wool I would use, how much water I'd heat in the beaker and what distance the burning alcohol would be from the beaker of water.
Safety is a factor in all laboratory-based experiments; therefore this investigation was no exception. For the investigation, I wore suitable safety equipment; this was a) an apron, to protect my clothing and therefore some body protection and b) a pair of safety spectacles, to protect my eyes from any harmful substances or most importantly, flames.
Choosing all of the necessary equipment and deciding where and how to use each item was the next thing to do for my preliminary investigations.
Firstly, I chose a 100ml beaker to hold the water, which was going to be heated, in. This is because a beaker is the most practical container to use as it is small, easy to fill with water, easy to clean after each experiment, easy to measure the temperature of the water inside it and also has a reasonably large base. The large base of the beaker should cover as much of the flame as possible so as not to lose any of the heat given off from the burning alcohol. I also chose a beaker because not much of the heat can escape around it, therefore I will get more reliable results. To hold the beaker of water above the burning alcohol, I decided to use a boss and clamp and a retort stand; these would hold the beaker firmly above the crucible giving a smaller probability of an accident. It would also give the beaker better positioning so as to receive as much of the flame as possible from the burning alcohol below; this would therefore help me to obtain better results for my investigation.
Next I had to choose what to burn the alcohol in. As I was only burning approximately 1cm3 of alcohol, I decided to use a crucible. This is because a crucible can handle very high temperatures and is an ideal size for the experiment I am doing as it will not be sticking out around the beaker of water and therefore non of the heat will be lost.
As I had chosen to use a crucible, I needed to then find the amount of ceramic wool that would nicely cover the base of the crucible. I tried different amounts and I found that approximately 0.28 grams of ceramic wool was just right.
The next step was to choose what distance to put between the crucible and the beaker of water, but before I was able to do this I had to decide how much alcohol I was going to use and how much water I was going to put in the beaker for heating.
For the water, I first decided to use about 100cm3 but after doing one or two test runs, I discovered that it was too much water and that it didn't heat up very much and therefore would not give very reliable results. I then decided to use 30cm3, which worked out just fine; any less would boil and any more wouldn't heat up.
Taking measurements for the amount of alcohol (1cm3) and for my water (50cm3) would have to be very accurate for each and every experiment so that my investigation would be fair; therefore I would have to use correctly sized measuring cylinders for each measurement. I decided to use a small 10ml measuring cylinder to measure out my 1cm3 of alcohol, and a 25ml measuring cylinder to measure my 30cm3 of water. I will take my temperature measurements by keeping a thermometer in the beaker of water for the duration of the experiment; when the alcohol has stopped burning totally, I will quickly stir the water with the thermometer and take the final temperature before the water begins to cool down.
As I mentioned in my Output Variables, I have already decided to use 1cm3 of each type of alcohol; this was just the right amount as it didn't produce as big a flame as 2cm3 of alcohol and therefore would not overheat the water.
Now that I had established how much alcohol and how much water I was going to use, I could start deciding what decided I would keep the crucible from the beaker. After a test run, I found that 2cm was a good distance, spanning from the top of the crucible to the base of the beaker. A larger distance would mean that not much heat would reach the water and would therefore give me unreliable results. A smaller distance would give the burning alcohol a small oxygen supply and therefore will not carry out complete combustion.
Now that I had decided on all of the equipment and volumes of the substances I was going to use, I carried out one final test run; this gave me a better idea on whether or not I had to change anything else before I started my investigation.
I carried out my test run, or 'pilot' experiment, using the following apparatus:
. Boss and Clamp
2. Retort Stand
3. Heat resistant mat
4. Crucible
5. 0.28g of Ceramic wool
6. Different types of alcohols (1cm3)
7. Splints
8. 100ml beaker
9. 30cm3 of Water
0. Bunsen burner
1. Thermometer
2. Electronic balance
3. Safety Specs
4. Apron
5. Pipette
6. 10ml Measuring Cylinder
7. 25ml measuring cylinder
8. Ruler
This is how the apparatus was set up:
I carried out the following procedure for each different type of alcohol and compiled some raw data. First, I fitted a boss and clamp to a retort stand to hold my beaker; then, I measured out 30ml of water in my 25ml measuring cylinder and poured it into the beaker and inserted a thermometer to start measuring a start temperature. Then I weighed 0.28 grams of ceramic wool and inserted it at the bottom of my crucible, which was placed on a heat resistant mat. I then measured out 1cm3 of an alcohol (e.g. Methanol), and poured it onto the ceramic wool in the crucible. Now that all of the apparatus was in place, I placed the top of the crucible two centimetres away from the base of the beaker and then I checked the start temperature and recorded it. The next stage was to light a splint by inserting it into a Bunsen burner, and then, set the alcohol in the crucible alight! After the alcohol had finished burning completely, I stirred the water and took an end temperature. Now that I had a start temperature and an end temperature, I could deduce the temperature rise and use it in the formula to work out the energy produced by the alcohol in Joules. The following are the results that I obtained from my preliminary work experiments.
Type of
Alcohol
Start
Temperature (oC)
End
Temperature (oC)
Temperature
Rise (oC)
Methanol
24
89
65
Ethanol
23
91
68
Propan-1-ol
24
92
68
Butan-1-ol
24
95
71
Hexan-1-ol
24
Boiled
-
As we can see from these results, the temperature rise is very high, there was not much of a range between the end temperatures, and so therefore I would need to change some of the set-up to make my investigation more reliable and fairer. In the case of Hexan-1-ol, there is no temperature rise because it boiled. Although there was no Pentan-1-ol to test, we can see that it would have had an end temperature of approximately 97oC. Therefore, to make my test fairer and more reliable, I must increase the distance between the top of the crucible and the base of the beaker, and increase the amount of water inside the beaker. This will insure that the temperature rise will not be too high and that the water will not boil.
I have therefore decided that for my investigation, I will change two things:
a) The amount of water in the beaker to 50ml
b) The distance between the top of the crucible and the base of the beaker to 2.5cm.
Background Knowledge
Hypothesis
The amount of energy released by the burning alcohol will be directly proportional to the number of carbon atoms in that specific alcohol.
Predictions
As the number of carbon atoms in the alcohol increases, so will the energy that it releases. I predict that they will be in direct proportion and therefore if one doubles, so will the other, and so on. As the number of carbon atoms in the alcohol defines the number of moles, along with other factors, the energy released by the alcohol will also be proportional to the number of moles in the alcohol.
Energy released
by the alcohol
Number of Carbon
atoms in the alcohol
Planned Method
Apparatus
. Boss and Clamp
2. Retort Stand
3. Heat resistant mat
4. Crucible
5. 0.28g of Ceramic wool
6. Different types of alcohols (1cm3)
7. Splints
8. 100ml beaker
9. 50cm3 of Water
0. Bunsen burner
1. Thermometer
2. Electronic balance
3. Safety Specs
4. Apron
5. Pipette
6. 10ml Measuring Cylinder
7. 25ml measuring cylinder
8. Ruler
Method
The method I will use to carry out my investigation is the exact method I used in my preliminary except for a few minor adjustments that I will make in my measurements. Instead of using 30cm3 of water as I did in my preliminary investigation, I have decided to use 50cm3, so as not to make the water boil. I also decided that the distance between the top of the crucible and the base of the beaker was too small, so I will change it to 2.5 centimetres for my investigation. Other than these two minor modifications in my set-up, the procedure I used in my preliminary work is the exact same procedure I will use in my investigation.
Obtaining Evidence
After carrying out my investigation using the exact procedure mentioned in the Preliminary Work and Planned Method, I compiled and processed the following data.
Alcohol
Number of carbon atoms
Methanol
Ethanol
2
Propan-1-ol
3
Butan-1-ol
4
Hexan-1-ol
6
Alcohol
Start Temp. (oC)
End Temp. (oC)
Temp. Rise (oC)
Methanol
7
49
32
Ethanol
7
55
38
Propan-1-ol
7
60
43
Butan-1-ol
6
62
46
Hexan-1-ol
4
64
50
Repeats
Averages
Alcohol
Start Temp. (oC)
End Temp. (oC)
Temp. Rise (oC)
Methanol
23
56
33
Ethanol
23
61
38
Propan-1-ol
24
67
43
Butan-1-ol
23
70
47
Hexan-1-ol
23
74
51
Alcohol
Temp. Rise (oC) 1
Temp. Rise (oC) 2
Average Temp. Rise (oC)
Methanol
32
33
32.5
Ethanol
38
38
38
Propan-1-ol
43
43
43
Butan-1-ol
46
47
46.5
Hexan-1-ol
50
51
50.5
Energy Produced
Alcohol
Average Temp. Rise (oC)
Energy produced (Joules)
Methanol
32.5
6825
Ethanol
38
7980
Propan-1-ol
43
9030
Butan-1-ol
46.5
9765
Hexan-1-ol
50.5
0605
Analysing and Considering Evidence
Method
The method I mentioned in the planned method and preliminary work was the exact method I used in my final investigation; there were no further alterations to my set-up or procedure.
Conclusion
After carrying out my full investigation, obtaining my results, drawing out full tables of processed data and drawing one final conclusive graph, I have found that changing the number of carbon atoms in an alcohol will affect the energy that it produces when it is burnt. This is because more carbon atoms means a larger molecular structure and therefore more bonds within the molecule. Therefore if there are more carbon atoms reacting with the oxygen during combustion, there will be more carbon dioxide and water produced and therefore it will require a lot more energy to form bonds within these molecules. As shown in the results tables and the graph, the amount of energy produced by the alcohol and the number of carbon atoms within it was proportional and therefore nearly backs up my previous hypothesis. Except for the fact that in my hypothesis I stated that the number of carbon atoms in the alcohol and the amount of energy it would produce on burning would be directly proportional, not just proportional, and therefore my hypothesis would have been fully supported, had it been for the incomplete combustion of the Hexan-1-ol. Although some alcohols such as Hexan-1-ol required a larger oxygen supply than the other alcohols to undergo complete combustion and therefore did not produce the amount of energy it was meant to produce and therefore was expected to produce. The trend of the results and the graph show that the energy for Hexan-1-ol should have ideally been higher than it was to produce a nearly perfect set of results.
Evaluation
The evidence I obtained from my investigation was fairly reliable considering the fact that it was carried out in a school laboratory with limited resources. The procedures I used were also fairly adequate but could have been improved. There wasn't much variation in the experiment, as the number of carbon atoms in each alcohol used didn't have any range. There was also a lot of heat loss around the side of the beaker and from the top of the beaker, which could have been the cause of slight anomalies in my results. The only good point of the experiment was that every other variable except for the number of carbon atoms in the alcohols was kept constant. All of the apparatus
