I intend to measure the temperature of the water, the amount of water and the weight of the alcohol burner. A thermometer, a measuring cylinder and a set of scales respectively, will measure these. I intend to test five alcohols, methanol, ethanol, butan-1-ol, pentan-1-ol and hexan-1-ol. The method that I will use will be to weigh the alcohol burner and then place it underneath the tin of water and light it. Once the temperature of the water has risen by 20°C I will stop the alcohol burning and weigh it once again, and then calculate the mass of alcohol burnt. After I have done this, I will repeat the procedure with fresh water but with the same alcohol burner and equipment, this way any inaccuracies in the equipment will be reciprocated throughout all the results, having the same bearing on all. 20°C is the best temperature rise to use since it is easy to read and not very difficult, less than this and it can happen too quickly and more than that would be a waste of alcohol and it would take too long.
In my experiment I will have to control many things to ensure that it will be a fair test and that the results are as reliable as they can be. The things which could affect the results are: uneven heating, loss of heat to the atmosphere, loss of alcohol due to vaporisation after the experiment has finished, inaccuracies in both the scales and thermometer and human error. These are the most important factors, which could influence the experiment, since they are the ones that could have the worst effects. I can control them by various methods, although ones such as human error can have very little done about them. To combat the uneven heating I can stir the water, by insulating the tin with aluminium foil and surrounding the experiment with heatproof mats, I hope to reduce loss of heat to the atmosphere. By putting the lid onto the top of the alcohol burner as soon as the water has risen by 20°C will ensure that the alcohol does not continue to vaporise, but will instead be caught and still count in the weighing. Inaccuracies in the scales and thermometer can have very little done about them, but if the same pieces of equipment are used each time, the fault will be present in all and not just some.
The only variables that will be present are the different types of alcohol being burnt, and the amount that will be burnt. The amount of water, temperature rise, equipment and the height of the tin above the flame will all remain constant. As long as we do not put our hands in the alcohol flame, the experiment is a pretty safe one. We will wear goggles and the heatproof mats will keep us safe from any possible danger.
A preliminary experiment, which was conducted, allowed me to improve this one to a high degree of efficiency. It showed me that I need to insulate the experiment from the outside to ensure that the results can be more accurate, since the ones gathered from it were under half of the theoretical value, so with the changes that I have made the next experiment should have far better results. The experiment produced a result of –672KJ/mol, around half of the theoretical value. This means I have to insulate in order to prevent this loss. The following sheet shows the calculations for the preliminary experiment.
With my results, I would expect to see the delta H to be negative, and the values gained from it to be close to the theoretical values, with around a 40% margin for error. I would also expect the heat of combustion to go up the more carbon atoms are present in the various alcohols.
Results:
This is the table of results, which was gathered from the experiment conducted. Everything went as planned and the results gathered are good, or so I shall have to find out. The next sheet of paper shows all the calculations that were performed in order to discover the mass of energy burnt. The results from that have been put into the following table, and the graph plotted.
Analysis
The results show that are more carbon atoms there are, the higher the heat of combustion is. It is clear to see how the theoretical results go up in regular steps, each time with a leap of around –619.5 KJ/mol. This is because every time we go up the alcohol series we are adding the same number of bonds. Each time it is an addition of 2 carbon-hydrogen bonds, 1 carbon-carbon bond and 1½ oxygen-oxygen double bonds. These all add up to 1918.5 KJ/mol, The energy taken in as 2 carbon-oxygen double bonds and 2 oxygen-hydrogen bonds means that it is around -2538 KJ/mol which is added to the total. If we add the 1918.5 to the –2538, then we get a reading of –619.5 KJ/mol. So every time a new alcohol is introduced, the heat of combustion should go up by –619.5 KJ/mol. The alcohol series is a homologous one, so the formulae of each, goes in a regular progression. This relationship of going up in a regular step should be illustrated in my results.
It is clear to see that it does go up in steps, each alcohol having released more energy than the last, but these steps are far from regular. Instead of going up by –619.5 each time, they do as the following table shows:
This table shows how all of the results were out by either a small, or significant amount. The percentage reading shows how much from the actual reading of –619.5 KJ/mol. Two of the results worked especially well, the Butan-1-ol and the Hexan-1-ol, quite how there was a reading of more for the Butan-1-ol is unclear, but that will be discussed within the evaluation.
The bonds within my experiment have quite clearly broken and released energy, although the amount which has been released does not clearly link in with what should have happened. Again, these discrepancies will be analysed within the evaluation.
All of the results, both theoretical and my own, are negative. When bonds are broken, as they are when the alcohol is burnt, energy is taken in, since it is needed to break them. Once any bond is broken, energy is taken in. This energy is released though when the new bond for the products are reformed. This is why the increase in amount of energy released in the theoretical values is the same each time, since it is always the same number of bonds being broken.
With my results however, various factors mean that they do not correspond to this. They do still break the bonds and reform them, since the calculations show that the delta H is negative. This means that the products have been formed and the reaction has taken place.
With my results, the heat of combustion does go up the more carbon atoms are present just as I predicted. The results are also pretty accurate with only one being far off the predicted increase. So, my predictions were fulfilled by my results.
Evaluation
The experiment went very well and I believe that the method that I employed worked as well as could be expected since it managed to record all the correct information and allowed me to draw some valid conclusions from them. If I were to do the experiment again I would try and use more insulation since I believe that that was the biggest fault with it and the reason that the results were off.
There were not any major errors with the experiment, although there were many factors that mean that the results were off. The major problem was the heat loss to the room; this will always affect the experiment. It means that the results will always be off since it would not have heated the water, which is what I was measuring. There was also soot produced on the later alcohols, this suggests incomplete combustion which I was not measuring, so if the soot was burnt then more energy would be gained.
The results were fairly reliable since I controlled as many factors as I possibly could that may affect the results, so the ones that I gathered should be fairly accurate. The fact that a different increase in the heat of combustion occurred from alcohol to alcohol suggests that the reliability has to be questioned more than normal. This means that they cannot be taken as textbook, and they are by no means what I actually wanted. I had hoped to improve on the preliminary work, and this was only done in a couple.
I also only tested five alcohols. I did not test the one with 3 carbon atoms in – Pentan-1-ol since it was not available at the time the experiment was performed. This means that there is a significant gap in the results, this means that the sufficiency of results is not filled. This would be another improvement should the experiment be performed again. It would have improved my conclusions since it would have made the increase from value to value clearer to see.
With my results there are no major anomalies, this is good since it means that my conclusions will be more reliable and based more on truth than inaccurate results. There are the usual inaccuracies that come with an experiment not performed in controlled conditions, though these do not affect the results much.
More work, which could be done to try and create more conclusions, could be to time the experiment. That way I could discover which alcohol creates more heat more quickly, and if the number of carbon atoms affects the rate at which the heat is produced. I could also use a different homologous series, for example a series of alkanes or alkenes, if it was alkenes then a relationship into the effect carbon-carbon double bonds could be set up. Ones, which can obviously be tested, are methane, ethane, propane, butane, pentane and hexane for the alkanes and methene, ethene, propene, butene, pentene and hexene for the ethenes. These should provide a valuable insight into the heat of combustion and various other factors.
