This reaction requires energy to break the bonds within the alcohol and oxygen molecules and gives out energy when it forms new bonds within the carbon dioxide and water molecules. Different bonds between different pairs of atoms require different amounts of energy to be broken and give out the same energy when formed as required when broken. We call the energy required for the breaking or forming of the bonds “bond enthalpies.” The values for bond enthalpies are given in joules per mole and vary for different pairs of atoms. Some pairs of atoms have higher bond enthalpies than others.
The reaction is exothermic, which means that it gives out energy, since the amount of energy given out for forming bonds within the products is larger than the amount of energy taken in to break the bonds with the alcohol molecules. This is because the total bond enthalpies in the alcohol and oxygen molecules are larger than the bond enthalpies in the carbon dioxide and the water molecules. It also means that it gives out heat when burned. This is displayed in the sketch graph on the right.
However, if there are not enough oxygen molecules to react with the alcohol molecules, or the amount of moles of oxygen needed to react with the moles alcohol is less than required, then the alcohol goes incomplete combustion. In such a case, the following happens:
Alcohol + oxygen carbon + carbon dioxide + water
In such a case, the carbon is given off as soot since there is not enough oxygen to react with it to form carbon dioxide. The reaction shall also not give off as much energy since no bonds are formed between the carbon atoms and the unavailable oxygen atoms.
Input variables
In this experiment, there are several variables that can be investigated that will affect the amount of energy produced when alcohols burn. These are the input variables:
Number of carbon atoms in alcohol – The larger the number of carbon atoms present in the alcohol, the larger the molecule of the alcohol, as can be shown in the equation CnH2n+1OH. Therefore, a larger number of carbon atoms in the alcohol molecule will create a larger amount of carbon dioxide and water produced when the alcohol is burned and a larger amount of energy will be produced to create the bonds within these products. Hence, the larger the number of carbon atoms in the alcohol molecule the larger the amount of energy produced when the alcohol is burned. We can also say that the number of carbon atoms in the alcohol molecules is directly proportional to the amount of energy the alcohol produces when burned, as shown in the previous sketch graph.
Purity of alcohol – The purer the solution of alcohol, the larger the amount of alcohol molecules present in the solution that can undergo combustion and produce energy. Hence, the purer the alcohol, the larger the amount of energy produced.
Amount of alcohol – The larger the amount of alcohol, the larger the amount of alcohol molecules present to undergo combustion and produce more products that will more energy due to the larger amount of formation of bonds. Therefore, the larger the amount of alcohol, the larger amount of energy produced.
Oxygen supply – The larger the amount of oxygen supply, the larger the amount of oxygen molecules available for the reaction of combustion to take place between the oxygen molecules and the alcohol molecules to produce carbon dioxide and water that give out energy when the bonds within them are formed. Therefore, the larger the supply of oxygen, the larger the amount of energy given out. However, there is a certain amount of oxygen at which all the molecules, both alcohol and oxygen, are being used in the reaction of combustion and this will be the maximum amount of oxygen needed to supply the alcohol for complete combustion, and it would therefore produce the maximum amount of energy. Any further increase in the amount of oxygen will not have an effect on amount of energy produced. This is shown in the graph on the right.
Varying the oxygen supply will be fairly difficult since the equipment for doing so is not available. So, ultimately, out of number of carbon atoms, purity of alcohol and amount of alcohol, I have chosen to investigate the number of carbon atoms present in the alcohol. This is because it seems to have the greatest effect on the amount of energy produced. I also anticipated that it would be possible to produce suitable results to draw graphs and conclusions from.
In the experiment, the main way in which I can alter the number of carbons in the alcohol is by selecting a range of different types of alcohols with a different number of carbon atoms in their chains. These include methanol and ethanol. However, when I reach largely structured alcohols, such as propanol and butanol, there shall be different isomers to choose as part of the experiment. I have therefore decided to choose alcohol molecules on which the OH molecule has bonded with the first carbon atom in the chain. This will contribute in making the experiment a fair test, even though different isomers will not affect the readings. In my preliminary work, I am going to investigate different alcohols and choose which will be the best to use in the experiment. It is important that I choose a suitable and open range of different alcohols with different numbers of carbon atoms.
I also have to ensure that the experiment is a fair test. To do this, I must keep all the other variables, that could affect the experiment, constant. To keep the amount of alcohol constant throughout the experiment, I am going to keep the amount of alcohol molecules present in each sample of alcohol constant. Therefore, I shall use the mole theory and I am going to use a constant fraction of 1 mole of each alcohol. To do this, I must first calculate the molecular mass of each alcohol, by adding together the relative atomic mass of each atom in every different alcohol molecule. Below is a table of the relative atomic mass of all the three molecules present in alcohols, carbon hydrogen and oxygen:
I shall use the formula CnH2n+1OH for the atoms in alcohols. When we put these numbers into the formula, we get (n represents the number of carbon atoms):
(12 x n) + (1 x 2n+1) + 16 + 1 = molecular mass (g)
C H O H
I shall use the formula above to calculate the molecular of every type of alcohol, just by putting the number of carbon atoms in contains into the formula. I am then going to divide the molecular mass of each alcohol by the constant number that I choose. I will choose this number in my preliminary work.
I am going keep the purity of alcohol constant throughout the experiment by using the same concentration for each alcohol that I investigate.
I am also going to keep the oxygen supply constant by performing the experiment under atmospheric conditions. The supply of oxygen to the reaction will therefore be the 20% in the air. This condition will also keep the temperature constant at room temperature and the pressure constant at atmospheric pressure.
The alcohols cannot be burned alone. They need a kind of wick to help it burn. However, the wool should not be able to burn and give out heat energy, otherwise it would make the experiment unfair. A suitable material for this purpose is ceramic wool. Despite a varying in the mass of the ceramic wool shall not make a difference on the accuracy of the readings, I have decided to keep the mass of the ceramic wool constant for each reading, in order to ensure a fair test.
Output variables
To achieve results from the experiment, I first have to consider what I am going to measure. The test is conducted to measure the amount of energy produced when alcohols are burned. We also know from the background knowledge that the reaction is exothermic, meaning that it gives out energy in the form of heat. We can therefore measure the amount of energy produced by measuring the amount of heat produced from the reaction.
There really is mainly one way of measuring the heat given out by burning the alcohol. This is by measuring how much it heats up a certain amount of water. To do this, we must suspend the water in a container above the burning alcohol and measure the temperature of the water before and after burning the alcohol. Then we can calculate the temperature rise:
After calculating the temperature rise, we can calculate the amount of energy produced by multiplying the amount of energy required, in joules, to raise 1cm3 of water by 1oC (4.2 joules), by the volume of water, in cm3, and by the temperature rise of the water, in oC. Hence, we get the formula:
After working out the energy produced, I divide it by the number of moles burned in order to calculate the amount of energy released by the alcohol per mole. This gives us:
= energy produced per mole (Joule mole-1 or J m-1)
Using this method of measuring energy has also produced us with many factors of the experiment that we must keep constant to ensure a fair test. These include keeping the volume of water in the container constant and keeping the distance between the burning alcohol and the container of water constant. We must also try to keep the burning alcohol directly underneath the container of water for every reading.
Preliminary work
Before actually doing the experiment, I was allowed one session to perform rough pilot experiments in the laboratory in order to obtain an idea of what techniques and equipment shall be best to use.
Safety
While performing these experiments, I discovered that it was necessary to protect myself. The reaction of burning the alcohols is exothermic and gives out a lot of heat that may be quite dangerous since it might lead to a spark or an unexpected reaction. I therefore found it necessary to wear safety spectacles, to prevent any hot substances from coming into contact with my eyes.
I also found it important to protect my clothes from the reaction. I therefore decided to wear an apron to prevent any hot substances from burning my clothes.
Choosing the equipment
I had chosen to use a clamp stand and clamp to suspend the container of water in. I also chose a crucible to burn the alcohols in. This was because the crucible is small, since the amount of alcohol I was dealing with was small, and it is able to withstand high temperatures. I also decided to use 50g of ceramic wool to burn the alcohol in. This was a suitable amount since it filled the crucible fully and accommodated the correct amounts of alcohols needed.
I also had to choose a suitable container to put the water in and suspend above the burning alcohol. This included choosing the correct size and shape.
The shape of the beakers meant that the heat coming from the burning alcohol would pass around the size of the beaker. This is because the beaker has a small base and its cross-sectional area is the same size as the base all the way to the top. However, the shape of the conical flask is more appropriate. This is because it is wide at the base and its cross-sectional area becomes narrower as we rise to the top of the flask. This means that the heat coming from the burning alcohol shall higher probability of reaching the wide based flask, and it is not able to go round the side of the flask since it becomes thinner nearer the top.
The size of the flask also should not be too big since the heat may be lost in the air space in the flask. A small flask may mean that the water is not spread out for the heat to reach it. The size of the conical flask shall be chosen when I choose the correct amount of water to be heated.
Measuring out the water
There were many different sized measuring cylinders in the laboratory that I could use for measuring the water. I discovered that the large measuring cylinders were not very accurate for measuring the amounts that needed to be measured. This was because they only had a few imprecise markings on the side. The small measuring cylinders, however, had markings going up in small numbers. I therefore chose a small 25ml measuring cylinder to use for the experiment.
The distance between the burning alcohol and the water
I also had to choose a suitable distance between the burning alcohol and the conical flask of water. This was very important since the distance determines the amount of heat that reaches the water. If the distance is too large, then not enough heat will reach the water and a lot of the heat will be lost around the sides. If it were too close to the burning alcohol, then it would limit the oxygen supply to the alcohol and stop it burning properly.
Since different alcohols burnt with different sized flames, I therefore decided to choose an average distance for all of the different sized flames to have constant in between the burning alcohol and the conical flask. This distance was 50mm between the bench and the bottom of the conical flask. This distance was to be constant for all the readings to be taken.
Choosing the right amount of alcohol
I mentioned in the input variables that I am going to vary the different types of alcohols according to the ratio of their molecular mass. The alcohols available in the laboratory range from methanol containing one carbon atom, up to hexanol, containing 6 carbon atoms. By applying the formula for molecular mass, mentioned in the input variables, we get the masses of 32g for methanol and 102g for hexanol. These masses are far too large to experiment with and I must therefore choose a suitable number by which to divide the molecular masses by. I decided to experiment with dividing the molecular masses of methanol and hexan-1-ol by 50, 100 and 200 and burning these different masses under 50cm3 of water in a 100ml conical flask and measuring the temperature rise. I allowed each alcohol to burn until it was completely out. These were my results:
(The temperature rise of the mass of 2.04 for hexan-1-ol could not be measured since the water in the conical flask boiled and went off the scale of the thermometer before the alcohol had burned out)
By consulting the results, we can see that using a fiftieth of the molecular mass of the alcohol will be too much, since it produces too much heat that causes the water to heat up off the scale and maybe boil. I will not be able to measure such high temperatures. The temperature rises of two hundredths of the alcohols seem to be too small to use as results and calculate energy changes. However, using one hundredth of the molecular masses of the alcohols produces results that are of suitable and appropriate sizes to measure and calculate energy values from. I have therefore chosen a constant number of 100 to divide the molecular masses of all the alcohols by.
Now that I had chosen how to vary the amounts of alcohols, I had to choose the correct amount of water to put in the conical flask. I experimented with 1.02g of hexan-1-ol and 25cm3, 75 cm3, 100 cm3 and 150 cm3 and I used my results from my previous experiment for 50 cm3. I used a 100ml conical flask for 25cm3 and 75cm3 and a 200ml conical flask for 100cm3 and 150 cm3. Again, I allowed the alcohol to bun out completely before taking the end temperature. These were my results:
(The temperature rise for the water volume of 25cm3 could not be measured since the water in the conical flask boiled and went off the scale of the thermometer before the alcohol had burned out)
We can see from the results that a small volume of water of 25cm3 is too small to accommodate the heat from the burning alcohol. Volumes above 75cm3 also give results that are too small to measure for smaller molecules of alcohols and to calculate energy changes. I have therefore decided to use a volume of 50cm3 of water in my experiment which I shall put in a 100ml conical flask, since this seems to be the most appropriate sized flask.
I also noticed while performing the experiment that it was difficult to determine when to take the end reading for the temperature. It was difficult to see exactly when the flame of the alcohol had completely gone out, since the flame would get smaller and smaller near the end. I found it easier to turn off the lights in the laboratory to see the flame easier and choose the time to take the end temperature reading.
The alcohols available in the laboratory are methanol, ethanol and isomers of propanol, butanol and hexanol. I have chosen to experiment with all the five different alcohols and to keep he isomers of the large alcohols constant, even though varying the isomers mainly has no effect. Therefore, for propanol, butanol and hexanol, I have decided to use the isomers with the OH molecules on the first carbon atom in the chain, which are propan-1-ol, butan-1-ol and hexan-1-ol.
From the preliminary work, I have decided many things:
- To divide the molecular masses of all the alcohols by 100 throughout the experiment
-
To use a constant volume of 50cm3 of water to heat up
- To use a 100ml conical flask
- To use a 25ml measuring cylinder when measuring the mass of the water
- To use 50g of ceramic wool to burn the alcohols in
- To use a crucible to burn the alcohols in
- To keep the distance between the conical flask and the burning alcohol constant by keeping a constant distance of 50mm between the bench and the bottom of the conical flask.
Hypothesis
The amount of energy produced by the alcohol will increase in direct proportion to the increase in the number of carbon atoms in the alcohol molecule.
Predictions
By looking at the effect of the number of carbon atoms in an alcohol and the molecular mass of an alcohol molecule on the energy given out by a burning alcohol in the background knowledge and in the preliminary work, I predict that in the experiment, the amount of energy given out by the alcohol will be directly proportional to the number of carbon atoms in the alcohol molecule. This means that if the number of carbon atoms in the molecule is doubled, the amount of energy given out will double, and if it is trebled, so will the amount of energy given out, and so on. This means that the higher the number of carbon atoms in the alcohol molecule, the higher the amount of energy given out when burned. By looking at the results in my preliminary work, I predict that the graphing displaying the results of the experiment will resemble the one on the right with a positive constant gradient of about 200 000 Jm-1 per increase in carbon atom in the alcohol.
Planned Method
Apparatus
- Range of different alcohols (methanol, ethanol, propan-1-ol, butan-1-ol, hexan-1-ol) ceramic wool
- Electronic balance
- Clamp stand
- Boss and clamp
- 100ml conical flask
- Water
- Crucible
- Ceramic mat
- Pipette
- Splints
- Flame source (Bunsen burner)
- Ruler
- 25ml measuring cylinder
- Thermometer
- Tissue
FOR SAFETY
Diagram
Experiment
As mentioned before, I am going to measure the effect of the number of carbon atoms in an alcohol on the amount of energy it produces when it is burned.
Method
First, 50cm3 of water shall be measured out using the 25ml measuring cylinder and put in the conical flask. The small volume of the measuring cylinder of 25ml will ensure the amount of water measured out will be accurate. The clamp stand and clamp shall then be set up to hold the conical flask 50mm above the bench, using the ruler to measure this distance. This distance shall be kept constant throughout the whole experiment in order to keep a fair test. The temperature of the water in the conical flask shall be measured using the thermometer and noted down.
The first alcohol shall then be prepared to be tested. To do this, 50g of ceramic wool shall be measured out using the electronic balance and put in the crucible. The alcohol to be tested will then be measured out by mass (according to the different hundredths of molecular masses in the table) by tarring the balance with the crucible of wool on it, and then applying the correct mass of the alcohol in the wool. Further precision of the mass of the alcohol will be achieved by using a pipette.
The crucible shall then be put directly underneath the conical flask of water and the alcohol shall be lit using the splint from the flame source. The flame from the alcohol shall heat up the water in the conical flask.
After all the alcohol is used up, when the flame completely goes out, the new temperature reading of the water would be read and noted down by the starting temperature and the number of carbon atoms in the alcohol. The lights The temperature rise of the water shall also be calculated.
The water in the conical flask would be let to cool down slightly as the next alcohol would be prepared using a new piece of 50g ceramic wool. Again the mass of the alcohol would be measured according to the mass given in the table. Before the next alcohol is lit, the bottom of the conical flask would first be wiped of soot using the tissue. This would ensure a fair test in that the soot from previous alcohols will not be able to affect the heating of the water from following alcohols. Also, the temperature of the water in the flask would be measured again before it is heated and after it is heated. The temperature rise shall also be calculated.
This shall be done for all the alcohols, using a new piece of 50g ceramic wool for each alcohol and using the mass of a hundredth of the molecular mass of each alcohol as shown in the table. The bottom of the conical flask shall also be wiped before the burning of each alcohol for a fair test.
After each alcohol has been tested, they shall all be tested for a second time in the same way, each with its own 50g of ceramic wool and under exactly the same conditions, and a second temperature rise reading shall be made for each alcohol. Another replicate of results should have therefore been recorded. The results will then be analysed and if two results of temperature rise for an alcohol are not similar, the experiment will be repeated again for that particular alcohol. The third result will then be compared with the other two. If it is similar to one of them, it will replace the other. This will help to get rid of any unreliable results and replace them with more reliable accurate results.
After this has been carried out, the average temperature rise over the two replicates shall be calculated. Then, these readings shall be used to calculate the amount of energy released per molecular mass for each alcohol. This shall be done by using the following equation:
These final results will be the most accurate and will be used to draw graphs and conclusions from.
