The alcohols I will use:
I will use various alcohols to help me discover which out of these is the best fuel. I will experiment with:
Methanol CH OH
Ethanol C H OH
Propanol C H OH
Butanol C H OH
Pentanol C H OH
These five alcohols will allow me to produce a good set of results that I can make accurate conclusions from.
Apparatus list
The equipment I need to perform this experiment is:
- A clamp – to hold up the tin can
- A tin can – to store the water in
- Measuring cylinder – to measure the amount of water accurately
- Thermometer – to measure the temperature
- Four bench mats – to use blocks
- Stopwatch – to time the experiments
- The alcohols in the burners
Observations I will make:
I will make a number of observations, before, during and after the experiments. I will observe the properties of the flame. I will observe and record the mass of the alcohol in the burner before and after the experiment, so that I can calculate the difference between the two and then work out how much fuel was used up in the experiment. This will allow me to calculate the energy used and how efficient the different alcohols are. I will measure the amount of water that I put into the tin can; this will be 150 ml each time. I will also observe and record the temperature of the water before and after the experiment as this will allow me to see how much heat has been produced from burning the alcohols and by how much the water has changed in temperature.
How many repeats will I do?
I will repeat the experiment for each alcohol three times and I will have three sets of results for each of the five alcohols. This will help me to see if my results are continuous and if all my repeats are all similar. From these results I will be able to make several calculations and spot any anomalous results that I might have. Also it will allow me to take an average result which will make my results more accurate.
How will I ensure accuracy?
I want to achieve the best most accurate results that I possibly can. To do this I will ensure that I follow my method properly carrying out all the correct instructions. I will carry out the experiments the same each time so that it remains fair and accurate. I will record all measurements properly and precisely. I will use a stopwatch to time the experiments each time as this will be accurate and ensure that each experiment has the same amount of time. I will use a measuring cylinder, rather than a beaker, to measure the 150-ml of water that I need, as this will improve accuracy. I will also take the temperatures with a thermometer and I will take the ending temperature, not as soon as the experiment is finished but when it reaches its highest point on the thermometer. Also when stirring the water in the tin with the thermometer I will be careful not to touch the thermometer on the sides or bottom of the tin as it may be hotter than the water and could change my results. I will take the measurements of the mass of the burner and alcohol to two decimal places as this is a good degree of accuracy. I will also be careful when using the blocks so that they remain in the same position preventing the same amount of heat loss each time.
Prediction
I have come up with two theories as to what could happen in this experiment. Here they are:
- When the alcohol is heated, the heat is providing the activation energy to break the bonds between the molecules within the alcohol. Also the heat will be breaking the bonds of the oxygen in the air surrounding it too. This is so that a reaction can happen. Eventually the bonds in the alcohol will break due to the heat and so will the bonds in the oxygen atoms in the surrounding air. This leaves these atoms free to move about and form new bonds. Forming the new bonds will make the products and will produce energy in the form of heat. This is because the molecules that are formed as the products, don’t need all the energy that their individual atoms had when they were on their own and free. So making new bonds releases energy and if making the new bonds in the products releases more energy than it took to break the bonds in the reactants, then the spare energy is given out as heat and the reaction is exothermic.
This leads me to predict that Pentanol – C H OH will release the most heat energy. This is because this alcohol has the largest amount of bonds and so in the end more bonds will be formed, giving out more energy in the exothermic reaction.
This energy level diagram helps to show how the heat is released in an exothermic reaction.
Energy level diagram – an exothermic reaction.
Here the products have less energy than the reactants, so the energy left over is given out as heat and heats up the surroundings and so heating up the water in the tin. Therefore with more bonds being formed and more energy being given out, the heat produced is greater and so the water is heated up further.
Theory 2
- Complete combustion requires the fuel (the alcohol in this case), heat (from the splint to light the alcohol and burner) and oxygen (from the air). Therefore the alcohols with the least amount of atoms will be able to use the supply of oxygen in the air more efficiently and will not need as much oxygen as the alcohols with more atoms. This will mean that the alcohols with less atoms can use more oxygen to burn to its full potential, therefore producing more heat to heat water, whereas the alcohols with more atoms will require more oxygen and won’t be able to burn as fully. This will mean that they won’t be able to produce as much heat and they may not burn properly and even produce the dangerous gas carbon monoxide or soot (carbon). This is also because the alcohols with more atoms will be using more oxygen from the air and more carbon dioxide will be produced, which reduces the amount of oxygen in the surrounding air so that the burning cannot continue as fully due to lack of oxygen.
Here are the chemical symbol equations to show how much oxygen is required by methanol with a smaller amount of atoms, compared to pentanol with more atoms.
Methanol burning
Methanol + oxygen Carbon dioxide + Water + heat energy
CH 0H + O CO + H O
This equation is at present unbalanced and requires balancing so that there is an equal amount of each atom on each side of the equation.
CH OH + O CO + H O
This is now balanced, but to tidy it up and get rid of the fraction, everything can be multiplied by 2 to get;
2CH OH + O CO + H O
This is balanced as there are two C (carbon atoms), eight H (hydrogen atoms) and eight O (oxygen atoms) on each side of the equation.
I will now look at the equation for the combustion of Pentanol. I will see if I am right and if more oxygen is needed for the complete combustion of Pentanol.
Pentanol burning
Pentanol + Oxygen Carbon dioxide + Water + Heat energy
C H OH + O CO + H O
This is unbalanced, I will now try to balance it;
C H OH + O CO + H O
This is now balanced, but again I will tidy it up by multiplying everything by 2 to get rid of the fraction;
C H OH + O CO + H O
This is balanced as there are 10 Carbon atoms, 24 Hydrogen atoms and 32 Oxygen atoms on each side of the equation.
When I compare the balanced symbol equation for the combustion of methanol to that of pentanol I can show that more oxygen is needed for the burning of pentanol and so proving my theory:
Methanol
CH OH + O CO + H O
Pentanol
C H OH + O CO + H O
This shows that more oxygen is needed for the burning of Pentanol so methanol will burn more fully therefore justifying my theory.
Overall I think that theory 1 is correct. Therefore I predict that Pentanol will produce the most heat because it has the most atoms and the most bonds. I think that there will be a pattern, with the amount of heat produced increasing as the amount of atoms in the alcohols increase.
Results:
I have now carried out the experiments and have recorded my results from each of the repeats of the experiment into one table. This is so that it is easy to access and read all the results simultaneously. It is also set up in a clear and logical order.
All temperatures were taken to the nearest degree from the thermometer and all the mass measurements were all taken to two decimal places to keep continuity and fairness at a good degree of accuracy. Units of measurement are included in the table.
Results from the three experiments:
From these results it is difficult to spot an immediate pattern or reoccurrence in the results. To analyze these results properly there are many further calculations that I need to do.
Analysis
I will now make some further calculations in order to analyse my results. I will calculate the amount of energy in Joules needed to heat the water by 1°C, the number of Joules taken to produce the temperature rise and the amount of energy produced from burning 1g of alcohol.
I know that it takes 4.2 J of energy to heat 1g (1cm ) of water by 1°C. I will now calculate how much energy it would take to heat the amount of water that I used which was 150 ml by 1°C.
It take 4.2 J to heat 1 cm so 4.2 J x 150ml = 630 J
It would take 630 J to heat the 150-ml of water that I used by 1° C.
From this I can now calculate how many Joules of energy it took to produce the temperature rise in my experiments. To do this I will multiply the 630 J of energy that it takes to heat the water by 1°C, by the amount of °C that the temperature rose by in each experiment.
For example: Results 1 - Methanol
630 J x 18° C = 11340 J
I will continue this for all the results of each alcohol.
Here are the results in a table:
From these results I will now calculate the amount of energy produced from 1g of alcohol. I will do this by dividing these results by the amount of mass that was burned for each alcohol in each experiment.
For example: Results 1 - Methanol
11340 J was the amount of energy needed to heat the water by the 18° C so I divide this by the 1.35g of alcohol that was burned in this experiment.
11340 J = 8400 J/g of alcohol, of energy produced.
1.35g
I will continue doing this for all the results and then take the average as this will make my results more reliable as it is an average from all three experiments.
Here are the averages in a table:
I have taken all the averages to 2 decimal places to keep continuity and accuracy, as this a good degree of accuracy.
I have now plotted a graph of these averages, to help me see a pattern or a trend:
Analysising the graph:
I have plotted a graph of the joules of energy produced from one gram of alcohol against the number of carbon atoms that the alcohols contained. I used the number of carbon atoms in the alcohol rather than the number of hydrogen atoms because the number of carbon atoms goes up in a simple consecutive pattern of 1,2,3 etc, whereas the hydrogen atoms are irregular numbers. Also the results would be the same or very similar so carbon atoms were the easier option.
My graph shows a general positive correlation pattern as the points run up from the bottom left to the top right. It shows a pattern and a relationship between the energy produced and the number of carbon atoms contained in the alcohol. I can see that as the number of carbon atoms increase the energy produced also increases showing that more carbon atoms in an alcohol will produce more joules of energy per gram of alcohol burned. However from my graph it is difficult to see exactly the line that the points should follow because I have two results that appear anomalous. To help me see these anomalous results clearly, and if they were to do with any of my repeats being extremely anomalous and out of pattern with the others, I will now plot a graph of all three results along with the averages. This is so that I can see if any results drastically affected my averages so I can interpret the pattern that I received off of my graph.
Here is the graph:
From this graph I can see that most of my repeats from each experiment were fairly close showing them to be quite accurate and that there is continuity within the results. However, from this graph I can see that there were some anomalous results in the repeats that would have affected my averages and made these results anomalous. I have circled these results on the graph and can see that these results would have made the averages too high or have dragged it down too low. Now that I have discovered this I am going to discard these results and re-take the averages again to be plotted onto another graph, which should hopefully improve the correlation on my graph.
Here is the graph of the average results without the anomalous results from the repeats of the experiments.
This graph clearly shows more of a pattern and again there is positive correlation. The results now follow more of a trend with the amount of energy (J) per gram of alcohol increasing as the number of carbon atoms in the alcohols increase. I have drawn a line of best fit and all the results now fit onto the line nicely and I can draw a smooth line. The graph is mainly linear but does have a slight curve. This may be due to the fact that as the temperatures increase the water uses more energy, so at higher temperatures the results are less accurate and more volatile. This means that at higher temperatures heat is lost from the water faster into the surrounding air due to the bigger heat difference between the water and the surroundings, making the higher temperatures more inaccurate and leading to a slightly shallower curve on the graph at these points.
The graph shows positive correlation with results going from low to high. This is because the burning of alcohols is an exothermic reaction, meaning heat is produced. This matches my initial prediction. Here is an energy change diagram for the burning of an alcohol – an exothermic reaction.
This diagram shows how heat is produced in an exothermic chemical reaction, as well as the products. This shows that the reactants, i.e. the alcohols, started off with a high energy level, but in order for them to react with the surrounding oxygen, some activation energy was needed to get the reaction going and to start breaking the bonds. This activation energy in my experiment was the burning of the splint to light the alcohol. This then gets the reaction going and provides the molecules with kinetic energy so that the bonds can be broken and they can separate. It requires energy to break bonds, which is the activation energy, but then when the molecules join back together, bonds are formed and energy is released in the form of heat – making it an exothermic reaction. (See prediction – theory 1 for detail.)
So when the alcohol burns, heat is produced which heats up the water. Also produced are the products of the reaction, which are carbon dioxide in the form of gas,water and also carbon or soot which is the black substance that forms on the bottom of the tin can.
The heat energy comes from the making of the bonds in the reaction. The heat energy is transferred form the chemical energy in the reactants (the alcohols), into heat. This is because the products contain less energy than the reactants, so the excess energy is given out as heat to the surroundings. The change from chemical energy to heat energy was brought about by the original heat source, the burning splint that started the reaction.
Evaluation
My experiments went fairly well and gave me sufficient enough results to analyse and spot a pattern. I was able to plot a graph and see the correlation of my results. I did not really come across any problems and I carried out my experiments with consistency and I tried to ensure accuracy and fairness throughout. My group and I co-operated and worked well together with a systematic approach to doing the experiment.
I think that my results are reasonable, but are good enough for me to see a pattern and make conclusions about what is happening and why it is happening when you burn alcohols. My results also allowed me to understand and interpret which alcohol is the best fuel. Also my results are sufficient enough for me to be able to support a firm conclusion in saying that the energy produced per gram of alcohol increases with the more carbon atoms present in the alcohol and that Pentanol was the best fuel because it had the most carbon atoms present and produced the greatest amount of energy.
I do not think that all of the heat from the burning alcohol went into the water. I think that although I tried to minimize the heat loss into the surroundings through the use of blocks surrounding the experiment, some of the heat energy was transferred to the air in the surroundings. This is very difficult to combat and with the apparatus that I had, using blocks was the best way to try and reduce the amount of heat lost to the surroundings. Also not all of the heat went into the water because the tin can got hot suggesting that some of the heat was transferred there. This would have had a slight affect on my results, as it would make the readings slightly lower because not all of the heat would have gone into the water. But this was not a major factor because in each experiment heat would have been lost in this way and it would only have affected the temperatures by a few °C.
Although my experiment went well and was quite successful, I did have some anomalous results that didn’t fit the pattern. My first graph of the average amount of energy produced from 1g of alcohol showed two anomalous results. I then plotted a second graph of all three repeats and this allowed me to see which of my repeats were anomalous and were affecting and making the averages appear anomalous. I circled these results on this graph and then excluded them and took the average results again without these extremes. I plotted these results into a third graph and the pattern shown was much better and demonstrated a clear pattern and trend. From this graph it was difficult to see any clear anomalous results and my results were then more accurate. I drew a line of best fit onto this graph and they all fit the line very well to form a slight curve.
The anomalous results in the repeats were either too high and brought the average up or they were too low and brought the average down. There are many factors that could have made these results anomalous. Here is a list of possibilities;
- Energy was given off as heat and light so not as much was transferred into heat energy.
- Heat was conducted and convected away through the air even with the use of blocks so again not as much heat went to the water. Also on the particular anomalous results the blocks may have not been positioned as well and so more heat may have escaped making the temperature reading lower and not as accurate.
- The fact that the beaker gets hot, means that not all the heat was transferred to the water and this could make the reading lower like on the two anomalous results that were too low.
- Also the fact that at higher temperatures heat is lost faster to the air and out of the tin can because of the bigger heat difference between the water and the surrounding air, making the higher temperatures more inaccurate.
- Incomplete combustion may have played a role on the anomalous results that were too low.
- The availability of alcohol for the wick to burn. If there was not enough, the wick would burn not the alcohol and would therefore give an inaccurate result.
- I may not have measured the amount of water precisely enough and there may have been too much water on some occasions which would take longer to heat and so the temperature difference would be lower.
- I may not have measured the mass or temperatures accurately, which would affect the outcomes.
- The flame size varied due to the different alcohols, meaning it was a different distance away from the beaker each time, so affecting the results.
- I may not have measured the distance from the wick to the bottom of the tin can properly and this would have affected the heating of the water.
- I may not have stirred the water thoroughly enough in some experiments, therefore affecting the temperature of the water.
- Also I may not have timed the experiment accurately enough so again altering the results again.
All of these possibilities could have caused the anomalous results and I cannot be sure which it was.
I repeated the experiment for each alcohol three times so that I could take an average and achieve more accurate results. My second graph shows the results from all three experiments and most were quite similar with little difference between them. This shows that I was quite accurate in most experiments and that when repeated the same sort of results are obtained.
I think that the alcohols burned completely to complete combustion. Complete combustion means burning in an excess of air (more than enough), when all the carbon turns to carbon dioxide. With complete combustion the products will be carbon dioxide, water and energy (see prediction – theory 2), and I think that these were given off in my experiments. I don’t think that it was incomplete combustion because carbon monoxide, a very poisonous gas would have been given off and a lot of soot would have been produced, and neither of these were produced in my experiments.
Improvements I could make
If I were to repeat this investigation there are some things I may have changed in order to improve the results that I got. I could have made sure that my group and I were more precise in taking all measurements to ensure that all experiments were fair and very accurate. I would be careful with all variables in the experiment to make sure that the outcome of the experiment was as accurate as possible. I would also make reducing heat loss from the experiment main priority and improve insulation techniques, as this would be valuable in helping me to have more accurate results. I would also make sure that a sufficient supply of oxygen was involved in the reaction to cut out any incomplete combustion, which makes less heat be given out because fewer bonds are forming from lack of oxygen. Other than that I think my experiment worked fairly well.
Conclusion
I think my results are reliable and provide me with enough evidence to be certain of my conclusion that the alcohols with more bonds and carbon atoms present provide the greatest amount of energy per gram of fuel. I have discovered that Pentanol was the best fuel as it produced the largest amount of energy and had the most amount of atoms. I am able to support this conclusion because I obtained enough evidence and was able to prove my original prediction and hypothesis through my results and scientific theory. Therefore I am able to make firm conclusions about what I discovered. My results are reliable also because they form a smooth curve on my third graph and show a clear trend and positive correlation that allow me to make conclusions from.
I have learnt many things from this investigation. I have learnt about energy change diagrams and how breaking bonds requires energy and forming bonds releases energy. I have learnt about exothermic reactions and about complete combustion. I have also learnt about the combustion of fuels and which is the best and why.
Next time I would perhaps extend the investigation by looking at other alcohols such as hexanol and heptanol as these would give me a wider graph reading and a better range of results. I would also experiment with different heights above the wick, different times and perhaps without blocks or by using different insulating apparatus. So these are some ways in which this investigation could be extended, and I’m sure some interesting results could be obtained from these.
Emma Keller 11T