A preliminary experiment was done to get practise at doing the method and to find out what was wrong with the method. A candle was weighed and then burned under a Coke™ can filled with 300cm2 of water. Once the temperature of the water had risen by 20oC, the candle was weighed again. The experiment was done twice to get more accurate results. The difference in the initial mass and final mass was the amount of candle burned in the experiment and this was recorded to be an average of 1.06 grams. The preliminary experiment brought out errors in the suggested main experiment method:
- The Coke™ can was filled too highly with water so the candle took too long to raise the temperature of the water by 20oC. Next time 250cm3 of water could be used instead.
- The water inside the can needed to be stirred so that the water heated up quicker. Next time the water should be stirred at a constant rate.
- Parts of the candle fell off on the way to the balance. In the main experiment this will not happen but spirit from the burners might evaporate. Next time the caps of the spirit burners should be kept on and the burners weighed quickly to minimise an offset caused by evaporation.
- Duplicates did not provide enough accuracy. Next time triplicates should be used.
The variables to be keep the same are:
- The height of the flame (using wick adjustment)
- The surrounding conditions of the experiment (e.g.) temperature)
- The height of the can from the wick (a higher height will mean less heat will get to the can)
- The amount of water used (more water will take longer to heat up to a certain temperature)
- The type of can used (a different material might insulate the can more)
- Stirring – yes or no? (if the water is stirred it will heat up faster but the rate of stirring is hard to keep constant).
The variable to be changed is:
- The type of alcohol used
Variables that were indirectly changed were:
- The mass of the spirit burner and contents
- The starting temperature of the water
Keeping the test fair and accurate
In order to keep the test fair, all of the above variables were kept constant throughout the experiment (apart from the variable to be changed).
Volume of water: If a large a volume of water is used the temperature will only rise a small amount, which allows large inaccuracies in reading the thermometer. If too little water is used, then there will be a too big increase in temperature, and the water might boil, therefore no more heat increases will be read from that point on and all the burning alcohol will be doing is sustaining the boil. Also as the temperature rises, transfer of heat to the air will happen exponentially, causing more evaporation and as a result, a lower heat rise than expected.
A constant volume of 250cm3 should be used.
Type of container: If copper is used, then transfer of heat into it and the water will happen quickly, but conversely heat is lost quickly out of the water through the copper and into the air, this is because the heat conductivity of copper is very high. However, if aluminium were to be used, the results recorded would be average as aluminium is an average conductor of heat.
Am aluminium container should be used. A Coke™ can would be ideal.
Carbon deposits under the container: This affects the experiment because the carbon soot (caused by incomplete combustion) acts as an insulator and slows down the rate at which heat is transferred to the container. Consequently the water will not heat up as much as expected causing inaccurate results.
The carbon deposits should be washed off after each experiment.
Distance between wick and container: This matters as if the container is half a meter away then little heat is going to reach it. However if the container is directly above the flame, almost all the heat will reach container.
The container should be as close to the wick as possible. The can will be placed just above the tip of the flame.
Starting temperature of water: This should be similar all the way through, because of factors leading to evaporation of water due to higher temperatures. This should not matter too much as room temperature should remain constant.
This cannot be changed and is not very significant either.
Draft shields: Can affect how much heat gets to the container.
A draft shield will be used.
Stirring: Stirring the water in the can makes the water heat up faster. Therefore stirring can affect the results obtained.
The cans should be stirred at a constant rate throughout the heating.
Safety goggles were worn throughout the experiment. Care was taken not to ingest the alcohol.
The apparatus was set up as above. The starting temperature of the 250ml of water was noted. The water was put into an aluminium Coke™ can. The spirit burner was weighed with its cap on and then put under the can so that the tip of the flame was just touching the bottom of the can. The cap was taken off and the wick lighted as soon as possible (so that the alcohol would not evaporate). During the heating of the water, the water was stirred at a constant rate so that it would heat the water evenly and the water would heat up faster. Once the water reached 20oC above it’s starting temperature (usually around 42oC), the flame of the spirit burner was blown out, the cap replaced and weighed again as quickly as possible. The results were recorded. This was done for the alcohols methanol, ethanol, propan-1-ol butan-1-ol and pentan-1-ol. After every test, the carbon deposits at the bottom of the can were washed off. Each test was triplicated to increase accuracy.
Anomalous results can be seen clearly, as there are dips in the graph that is out of pattern.
NB: The delta H value on the y axis was calculated by multiplying the relative molecular mass of 1 mol of the alcohol by the amount of heat lost.
We can see that the results in the graph and table support my hypothesis that larger chains of carbons in am alcohol produce an alcohol that releases more energy when combusted. This is because they will have more bonds that contain energy, and when they break (when combusted) they will release most of their energy in the form of heat. On the graph we can see a trend from the trend line – alcohols with a larger RMM will combust to release more energy than alcohols with a smaller RMM.
Knowing the general trend in this experiment, we can now get an accurate graph by plotting recognised delta H values on a graph – these delta H values must first be worked out:
The delta H of methanol is 5606-5144=462J/mol
The delta H of ethanol is 4719-4218=501J/mol
The delta H of butan-1-ol is 8551-7510=1041J/mol
The delta H of butan-1-ol is 13270-11729=1541J/mol
The delta H of pentan-1-ol is 20,934-18,312=2622J/mol
The graph can now be plotted:
From the results we can see that the experiment was quite inaccurately done. This is shown by the slight dip in the graph, on the proepan-1-ol, which is highlighted with an arrow. This dip is clearly not in line with the other results as it quite a long way away from the line of best fit. The anomalous result is also highlighted on the table with a red line of numbers, and the exact triplicate result number (3) is highlighted with blue. The results jump from an average of 1.40g for the first two triplicates to 2.38g on the third triplicate. The inaccuracy of the experiment is also demonstrated in the general shape of the curve, which should be a curve where a continuation of the line would never touch the x-axis (as this would indicate an alcohol that released heat but was never used up). The inaccuracy of the graph can also be seen by the following graph, which plots the results obtained against recognised results.
From the graph it is clear that the results obtained are very inaccurate, as the line does not resemble the line of recognised results. The line is a lot lower as it shows that a lot of heat coming from the burner escaped and was not absorbed into the water or some of the alcohol evaporated. The line also has a very low range which could indicate a faulty method as highlighted below.
If the experiment were to be done again, more care should be taken to avoid anomalous results such as the ones obtained. This can be done by identifying mistakes and how to correct them:
- Some burners burned faster than others but still were heating up cans of water that were stirred at the same pace as a slow burning burner. This meant that a fast burning spirit burner would use more alcohol to heat the water up by 200C than a slow burner. This could be corrected by using the same spirit burner for every alcohol.
- At the bottom of the can was carbon, which showed incomplete combustion and insulated the water from the heat of the burner. This meant that not all of the alcohol was being combusted to produce heat, carbon dioxide and water, some was also producing carbon. This also meant that an insulator had been formed and results after the first experiment would be inaccurate as a result. This could be corrected by using a different can for each experiment so there would be no insulating carbon layer at the start.
- Alcohol was still lost after the initial weighing and before the final mass weighing through evaporation, as sometimes, due to disorganisation, the wicks were not lighted immediately after the mass reading had been taken. This meant that some of the results were inaccurate. Ensuring organisation and keeping a ready supply of spills and a burning Bunsen on the table could correct this.
- Stirring was not entirely accurate due to human error. This meant that when the water was being stirred faster the water heated up faster and less alcohol was combusted than should have been. This could be corrected next time by not stirring the water at all (but this would take a long time for the water to heat up) or using a machine to stir the water at a constant speed.
- Some of the heat from the burner was lost to the surroundings. This cannot be corrected.