Preliminary Work
I have decided to do preliminary work so that I can familiarize myself with the type of trend that I should be looking for when the ethanol and propanol is heated. I achieved this by working out the Nett Energy, which involved using bond energies worked out by scientists who will have used the exact equipment and conditions. I received these bond energies from ‘Chemistry Data Book 2nd Edition’ by J G Stark and H G Wallace. I used these bond energies to find the energy used in breaking and making the bonds of ethanol and propanol. To find out the total energy transferred I will take away the energy needed to break bonds from the energy used making the bonds. I will also use the molecular structure to show what the relative atomic mass for ethanol and propanol is. I will do this by firstly finding the relative atomic masses using the periodic table for all of the properties of ethanol/propanol. Then I will multiply these by the number of elements actually present in the molecular structure and then I will add all of these together to give the relative molecular mass.
The Molecular Structure of Ethanol
Relative Atomic Masses
Carbon =12 (12 X 2) = 24
Hydrogen =1(1 X 6) = 6
Oxygen = 16 (1X16) = 16
Relative Molecular Mass=24 + 6 + 16 = 46
Total Energy Transferred
= 4719 – 5750 = 1031KJ
The Molecular Structure of Propanol
Relative Atomic Masses
Carbon =12 (12 X 3) =36
Hydrogen =1 (1 X 8) = 8
Oxygen =16 (1 X 16) = 16
Relative Molecular Mass = 36 + 8 + 16 = 60
Total Energy Transferred
= 10238 - 8162 = -2076KJ
I can see from the results from my preliminary work that the overall trend is that proponal gives out more heat energy than ethanol. Depending on my results from my actual experiment I should receive a similar trend. There is clear evidence that the chemical reactions are exothermic, because the total energy released in bond making is greater than the energy needed for bond breaking. This is shown in the energy level diagram below.
The overall energy change in a reaction depends on the bond energies in the reactants and the product, which will release energy as heat.
Predictions
Chemical bonds are forces of attraction between the atoms or molecules in a substance. Energy is needed to break these bonds and energy is released when new bonds are made. In a chemical reaction bonds between atoms in the reactant molecules are broken and new bonds are made. The energy released by a fuel depends on two things. Firstly the number of bonds to be broken and made and secondly the type of bonds involved. Therefore from the preliminary work I can see that propanol has similar type bonds to ethanol, but it has more of them. This means that proponal will use more energy to break the bonds and; therefore more energy will be released to make more new bonds. Also, the greater the surface area and the greater the force of attraction between the molecules, will make it harder to vaporise. Therefore more energy will be released.
Fair Test
I would like to gain the most reliable results possible using the equipment and conditions provided; therefore I will have to consider certain factors, which if I do not control, may enforce my results to become less reliable. I will,
- Keep a constant water temperature at the beginning.
- Keep the distance of the copper can away from the flame at 5cm.
- Have a constant water mass.
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Keep the temperature from which the experiment is terminated fairly constant.
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Stir the water with the thermometer before each temperature reading, which will distribute the heat evenly.
Results
Analysis
I will use the averages for ethanol and propanol to work out the total energy transfer and then I will use this to find the mass of fuel burnt per mole. The workings below show what I found out and what this suggests.
Ethanol
Q = M x C x t
The letter Q is the energy in joules and what I want to find out, M is the volume of water in the copper can, C is the specific heat capacity of water and At is the rise in temperature during the experiment. All of these were kept the same throughout the experiment; therefore,
M = 100cm3
C = 4.2
t = 200C
This gives the equation,
Heat produced = 100 x 4.2 x 20 = 8400J
The mass of ethanol burnt is 1.4g and the relative molecular mass is 46, which I have already worked out previously. In order to work out the number of moles in 8400KJ I will use the equation,
Number of Moles = mass/Relative Molar Mass
If I put these numbers into the equation to give,
Moles of ethanol burnt = 1.4g/46 = 0.03 moles.
I will then use this to calculate the energy in one mole,
8400/0.03 = 280000J = -280KJ
Propanol
I will use the same equation using the same numbers as before as they were kept the same in this experiment as well. This should give,
Heat Produced= 100 x 4.2 x 20 = 8400J
The mass of propanol burnt is 1g and the relative molecular mass of propanol is 60, which I again worked out beforehand in my plan. I will use the equation,
Number of Moles = mass/Relative Molar Mass
I will put the numbers in the equation appropriately to give,
Moles of propanol burnt = 1g/60 = 0.02
The above figure can be used to work out the energy in one mole,
8400/0.02 = 420000J = -420KJ
I did not use 0.4 for the propanol average, because it does not follow the general trend; therefore it is anomalous. If I had included this result it may have affected my results slightly.
The results that I obtained clearly show that the propanol did have the greatest energy loss; therefore my prediction was correct and theoretically my preliminary work was very successful in helping me gain the correct prediction. This is shown whereby for ethanol the energy give out per mole is -280KJ, whereas propanol gives out a slightly bigger -420KJ. From this experiment I have learnt that if there are too many carbon atoms present in a fuel then the energy given out as heat through combustion is increased, whereas if there are too few carbon atoms then the energy given out as heat from combustion decreases. If I compare my results with my preliminary work there is an immense difference in figures, whereby for ethanol the energy produced from one mole using the scientific figures is -1031KJ, whereas my results show that there is -420KJ of energy released. Also the scientific results for propanol are -2076, whereas my results show that for proponal -420KJ of energy was released. This significant difference was probably due to the extreme mass of heat loss during the experiment from the copper can, the spirit burner and the water; therefore less heat was transferred to energy. Also average bond energies shown in the data book were taken at 250C, whereas I took my results at a temperature of 200C, which means that a higher temperature the molecules would have had more energy as the bigger the temperature the faster the reaction; therefore more energy would have been transferred as heat after the breaking and making of the bonds.
Evaluation
I think that my experiment was fairly successful considering the limited equipment and conditions, as I managed to gain a set of fairly reliable results in order to draw a firm conclusion. There was only one anomalous result, which happened when testing the propanol whereby there was a result of 0.4; therefore I did not include this in my average taking. This could have occurred for a number of reasons these are,
- The amount of water put into the can may not have been measured completely accurately because of the way the level of the water is seen in the measuring cylinder.
- There may have not been a totally consistent height above the spirit burner.
- The spirit burner was not covered up during experiment; therefore the fuel could have evaporated.
- The length of the wick varied.
- Heat could have escaped out of the sides of the spirit burner during combustion.
- The copper can meant that it absorbed and transferred heat well but it lost heat to the air as well.
These reasons could have had an slight effect on my results overall, whereby it will have made my results lower than the theretical results, as shown in my analysis.
If I were to do this experiment again I would,
- Weigh the water in the copper can using burette instead of a measuring cylinder.
- Measure the distance between the can and the sprit burner between each fuel burnt.
- Cut the wick to the same length after each fuel is burnt.
- Insulate the copper can at the sides.
- User a Bomb Calorimeter so that draughts can be prevented and so that there is restrictions to the heat lost. The diagram below shows what the bond calorimeter would look like and how it would be set up.
I think that despite the problems that occurred I still managed to gain a fairly reliable set of results in order to imply whether the energy loss for ethanol or propanol was bigger. If I were to experiment further I could experiment with a much wider range of fuels to see whether their energy loss is bigger then propanol. Also I could investigate other factors that affect the energy loss from combustion, such as, the length of the wick, the heat source and the heat capacity. In doing this I can broaden my understanding in this section of chemistry.
