Fair Test
- Container size. The container size affects the volume of water inside it, I can't put 300cm3 of water inside a 200cm3 container. Also, if the volume of water I find most appropriate, only fills half the container, then the space not being used will be transferring more heat to air around, than if I used the same amount of water in a smaller, more appropriate sized container. It will be an unfair test if the size of the container differs in each repetition and will have to remain constant throughout.
- Volume of water. If I use too large a volume of water, then it will only heat up a small amount, allowing large inaccuracies in reading the thermometer. If too little water is used, then there will be too big an increase in temperature, and the water might boil, therefore no more heat increases will be read from that point on. From then on all the burning alcohol will be doing is sustaining the boil and not heating the water further. Also as the temperature gets so high, transfer of heat to the air will happen exponentially, and there would be a lot more evaporation this will cause errors in the equations.
- 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 into the copper and into the air, this is because the conductivity of copper is very good. If glass is used, then it will retain more heat before passing it on, thus the water will not get as much heat to start of with, than the alcohol is producing, thus giving misreading due to experimental inaccuracy, as glass is a reasonably good insulator.
- The amount of carbon soot left on the container. This affects the experiment because the carbon soot acts as an insulator and slows down the rate at which heat is transferred to the container and consequently to the water, as explained for glass in the above statement.
- Time. This should too be constant because you should not vary more than one thing in an experiment, but it does not matter, as the result in the end will be the same, because the relative alcohol burnt and temperature increase will produce the same result.
- Distance between wick and container. This matters because, if the container is very far away from the flame then hardly any heat will reach the beaker. If the beaker is in the flame, then lots of carbon will be formed on the container and incomplete combustion will happen, thus less energy released from the same mass loss causing inaccurate results. The distance will remain constant throughout.
- Starting temperature of water. This should be similar all the way through, because of factors leading to evaporation of water, due to higher temperatures. I should only change one variable at a time, but room temperature is reasonably stable, and as I am reading temperature change, this factor does not really matter too much.
Hypothesis
To hypothesise which of the fuels will work the best I must work out which fuel will give out the most energy. To find this out I can use bond energies. I know the formulae for each fuel and the products, as they are all combustion reactions, all that is left to do is balance the equations. Once I have done this I am able to draw out the structures and then substitute the bond energies. The total of the reactants minus the products will tell me the amount of energy given out by the fuel.
As the size of the alcohol increases more energy is released. The reaction that takes place in each one is an exothermic reaction, which increases with the alcohol. This happens because more energy is produced when new bonds are formed than the energy required to break the existing bonds. Therefore there is excess energy. This energy is released, in the form of heat to the surroundings. As the alcohol gets larger the amount of excess energy becomes larger therefore more heat is released to the surroundings. This is clearly seen in the enthalpy reactions where the amount of surplus energy is shown to get larger, as the difference between ‘a’ and ‘b’ gets larger as the size of the alcohol does. I am therefore able to hypothesise that that as the alcohol gets larger the fuel will be more effective and release more energy per mole and per gram.
If you double the number of carbons in the chain, then the energy released will approximately double. I think this is because, when the bond energy before the experiment doubles, the calculated energy released doubles, and when reacted, shorter carbon chains burn slower, because they have less hydrogens. When the carbon chain doubles, the number of carbons double, the hydrogens almost by a factor of two, and the oxygens by two, i.e., two Hydrogens, and three oxygens, are added, when one carbon is added. That is to say that the bond energy being broken is increasing by 1918J every time, so you are almost going to get this doubling effect.
Results
Analysis
These results can now be used to calculate exactly how much energy is given out by each alcohol.
This can be done because of the following. Each time the temperature is increased by 50oC. This means that each time the water receives the same amount of energy. To find which fuel is the most efficient per gram the energy taken in by the water must be divided by the average mass change. This will give the amount of energy released by a single gram and therefore the fuel with the highest figure can be named the best fuel.
Firstly therefore the amount of energy taken in by the water each time must be found. This can be done by using the following equation:
Heat energy = Mass x Specific Heat Capacity x Temperature Rise
Heat energy = 100 x 4.2 x 20
Heat energy = 8400J
To compare the predicted results with the actual results the predicted enthalpy equation must be divided by that alcohol’s relative atomic mass. The relative atomic mass for each fuel must therefore be found. We can work this out using the periodic table.
The elements present in the fuels and their relative atomic masses are shown below:
C = 12
H = 1
O = 16
These numbers must be substituted into each formula to work out the relative formula mass of each substance. This is done by adding together each individual atomic mass of the elements present.
CH3OH = 12 + (3 x 1) + 16 + 1
= 32
C2H5OH = (2 x 12) + (5 x 1) + 16 + 1
= 46
C3H7OH = (3 x 12) + (7 x 1) + 16 + 1
= 60
C5H11OH = (5 x 12) + (11 x 1) + 16 + 1
= 88
The numbers can now be substituted into the formula:
Methanol: CH3OH
Predicted = 739.5 / 32
= 23.1
Actual = 21 / 4.86
=
Accuracy = / 23.1 x 100
= %
Ethanol: C2H5OH
Predicted = 1461 / 46
= 31.8
Actual = 21 / 3.84
=
Accuracy = / 31.8 x 100
= %
Propanaol: C3H7OH
Predicted = 2183 / 60
= 36.4
Actual = 21 /2.92
=
Accuracy = / 36.4 x 100
= %
Pentanol : C5H11OH
Predicted = 3625.5 / 88
= 41.2
Actual = 21 / 2.38
=
Accuracy = / 41.2 x 100
= %
The above graph shows how for each alcohol there seemed to be the same amount of inaccuracy for each alcohol when comparing it’s predicted results with its actual results. The justification for this inaccuracy and why it is constant for each alcohol can investigate in the evaluation.
The above graph shows my hypothesis to be correct. It shows clearly that as the alcohol gets larger the amount of energy releases by the alcohol per gram increases. I can therefore accept my hypothesis.
Alcohol’s when burnt produce energy. This is because when burnt they undergo an exothermic reaction. This means that they release energy to the surroundings, which is present in the form of heat. The reason why the alcohol’s when burnt undergone an exothermic reaction can be seen in the bond energy equation in the hypothesis. In every alcohol more energy is produced from the formation of new bonds than the energy required to break the existing ones. This extra energy is released as heat. As the alcohol becomes larger the difference becomes larger. In each case as the alcohol gets larger the is more surplus energy. This is why as the alcohols become larger they release more energy and become a more effective fuel.
EVALUATION:
The inaccuracy of the experiment was constant for every alcohol. I can therefore justify using the results obtained to prove my hypothesis correct. The fuels came in the same order I predicted which shows that the experiment has been done well as no fuel is in a different position to that which I thought it would be. When comparing the predicted results with the actual there seems to be some inaccuracy. However this inaccuracy is there by roughly the same percentage each time therefore spoiling no trends or harming the hypothesis strength in anyway. I believe the experiment was well done and the results therefore good enough to be used to draw graphs and to be analysed to disprove or in this case prove my hypothesis. As the experiment was repeated three times for every fuel and an average used, the results were made even better and anomalies less likely to be produced.
There were a few errors created in the experiment. These could have produced an anomaly, or given inaccurate results. The most obvious error and the error, which probably created the inaccuracies, would have been due to human error. Each fuel may have had to change the temperature of the water by different amounts each time. Human inaccuracies or forgetting to put out the flame may have caused some of the flames to have given the water more energy than others. Although this seems a small difference if one fuel was stopped 5oC before it should have and another 5oC after the difference becomes 10oC which is a fifth of the constant temperature rise for all the fuels. Another human error would have been using slightly different amounts of water each time. This meant that some fuels were heating up more waters than others, which would result in anomalies.
These two errors would have made a difference due to the formula:
Heat Energy = Mass x Specific Heat Capacity x Temperature Rise
If the mass or temperature were to change each time then a different amount of heat energy would be calculated. However it is taken as a constant so when 21KJ is used later on in the enthalpy equation to work out the amount of energy given out by each mole it may be slightly wrong and therefore give a slightly inaccurate answer.
When comparing the predicted results and actual results the same degree of inaccuracy was had every time. The actual results are around half the value of the predicted every time. This indicated to me that an inaccuracy is occurring in every experiment at around the same extent. This can only be as heat lost to the surroundings may have created another error. Heat-proof mats were used to surround the flame and try to concentrate the heat energy produced by the flame to heat the water and not simply be lost to the surroundings. There may have been, however holes where the heatproof mats connected which allowed heat out. These holes may have been different sizes each time the experiment was performed so therefore allowing different amounts of heat out each time. The heatproof mats also let much heat through them. This will be the same amount of heat for each fuel so the order of the effectiveness of the fuel will be the same. This is the reason to why our results obtained are 20% of the results predicted as that is the amount around that is the amount I would expect to be lost as heat.
An improvement that could be made to the experiment would be to use a kalometer.