Hypothesis
This hypothesis is based on the knowledge that I have of fuels and alcohols and the knowledge I have about energy in reactions. This hypothesis will include my predictions as to which alcohol is the best one and why.
In order for a reaction to take place energy has to be taken in to break the bonds of the molecules being reacted. When the products of the reaction are created, they are being made by the re-forming of the broken bonds but in a new arrangement. When the new bonds are formed energy is released again. In combustion reactions the energy given out by the reaction exceeds the energy taken in to break the bonds. This leads me on to believe that the alcohol which will produce the most energy will be that which forms the most bonds.
I could also predict methanol to be the best alcohol because it has the fewest bonds to be broken, therefore it will take in the least energy. However, it will also make fewer products per molecule of the alcohol that is combusted, so less energy will be given out. That is why I predict the largest alcohol to be the best one because it will form the most bonds when the products are made per molecule of the alcohol combusted.
The bigger the alcohol, the more energy is released.
The preliminary experiment brought back a wide range of different times to heat the water by 20ºC. I was looking for conditions that would give back a time that would be suitable for accurately obtaining results. Therefore the heating of the water should not be so spontaneous that the time is too quick to record accurately, and on the other hand the heating should not be too long for the obvious reasons of wasting time.
Equipment/Apparatus
- Spirit burner, to burn the alcohol.
There will be varying spirit burners that shall be used throughout the experiment and each one must be weighed with the lid attached to the container so that an initial mass may be recorded, also the amount of alcohol being vaporised has to be kept as low as possible to help ensure accurate results and fair testing. If alcohol was to be lost in the atmosphere it would have an affect on the number of grams used for the temperature increase.
- Aluminium calorimeter, a calorimeter made of Aluminium shall be used as Aluminium is a metal and metals are very good metal conductors as of their delocalised electrons.
- Clamp stand, so that the calorimeter may be suspended above the spirit burner freely without the need for anyone to hold it causing heat conduction from the hand and also it is less hazardous. Also minimal energy will be lost in heating the tripod. The distance between the tip of the wick and calorimeter must be kept equal to allow a fair experiment. A distance of 5 cm is suitable, as the amount of vaporisation of alcohol must be kept as low as possible.
- Electronic balance which will be able to read up to 2 decimal places, this will allow the mass to be accurate within one hundredth of a gram.
- Heat proof mat, so that the working surface shall not be burnt, also use the heat proof mats to act as a wind block around the spirit burner, to allow as much conduction as possible, therefore once again reducing vaporisation of the alcohols being used.
- Goggles, to protect the eyes while the experiment is taking place.
- Measuring cylinder, to keep liquid measurements within 1 mm
- accuracy.
- Thermometer within a one degree accuracy reading so the margin of error should not be greater than one degree centigrade.
Quantities of Materials
- Methanol
- Ethanol
- Propanol
- Butanol
- Water (200 ml per experiment)
How to carry out the experiment from the start?
- Place a heatproof mat onto the working surface.
- Weigh the spirit burner with the lid using the scales; this shall be your initial mass (g).
- Then place the spirit burner containing your first alcohol onto the heatproof mat.
- Place the calorimeter onto the tripod and slightly push in into the hole so that the calorimeter may not move.
- Use the clamp stand and clamp the copper calorimeter into the clamp firmly.
- Then place two or three heatproof mats around the tripod reducing the amount of spirit being evaporated, remember to allow one side of the tripod to be open which will allow you to ignite and extinguish the flame.
- Measure out 100ml of water using the measuring cylinder the empty this into the calorimeter ensuring that no water is lost to allow a fair experiment. To gain more accurate results first measure the mass of the measuring cylinder by itself and then measure it with 100ml of water inside it, this shall give you an accurate measurement of the water being used.
- Allow only a few cm of distance between the calorimeter and spirit burner approximately 5 cm to allow fair testing throughout.
- Place the thermometer in the water and leave it in for 1 minute to gain an initial temperature.
- Leaving the thermometer in place remove the lid of the spirit burner and then ignite using a lit splint.
- You must also stir the water that is being heated repeatedly until the desired increase in temperature has been achieved this helps maximise conduction and transfer of energy.
- When the water temperature has risen by 15 degrees C replace the spirit burner lid to extinguish the flame and stop any evaporation.
- Then take the spirit burner and record its mass with the lid after completing the experiment.
Possible to reproduce the value
The method which I have used should allow the reproduction of precise and reliable results as it has been devised to be a fair test and takes into account the major factors which may affect the experiment.
Details of sources
The source which was used to help devise my plan was a worksheet which had been given out in class to help get the experiment started.
After carrying out a preliminary experiment using Methanol the following changes should be implemented, to allow a greater degree of accuracy.
- The distance between the wick and calorimeter needs to be reduced further to allow more accurate results based on our preliminary results which were the percentage error compared with the standard enthalpy change of combustion was 55%, but a target of 45% and below should be achievable therefore I think the wick should be a distance of 2cm below the calorimeter.
- To allow less vaporisation of the alcohol it may be a good idea to sellatape the corners of the heat proof mats which are acting as wind blocks to allow less vaporisation of the alcohol instead of using the tri-pod to support them as we did previously.
- In the preliminary experiment the range between initial and final temperature was between 15 and 17 degrees centigrade, after viewing other peers calculations and percentage errors, it was possible to conclude that when a temperature range between 20 and 25 degrees centigrade was used there was a smaller percentage error.
- The copper calorimeter we had used did not have a lid, it may be useful to attach a small piece of cardboard to the calorimeter with a small hole indented for the thermometer therefore reducing the amount of heat loss, and therefore reducing the amount of alcohol used. It may also be possible to insulate the calorimeter around the outside with wool to insulate the can and allow less heat loss.
- When measuring the water instead of using a measuring cylinder it may be possible to use a Burette as this piece of equipment allows a more accurate reading and therefore smaller margin of error allowing more accurate results.
With the following changes implemented I think that the percentage error should be reduced, as the main sources of inaccuracy were targeted and improved.
To allow fair testing certain measurements and conditions must be kept as precise as possible to allow a fair test environment.
It is also required to complete all of the experiments in one session so that the room temperature may remain as close as possible to a constant.
HAZARDS
Certain hazards that must be identified and kept to a minimum are
- The alcohols that shall be used are all highly flammable therefore the highest degree of safety must be used so that the alcohols being used must not be spilt.
- The alcohols being used are also highly volatile so the lid must not be kept of the spirit burners as the alcohol may be vaporised into the surroundings.
- The alcohols being used cause a lot of irritation if they come into contact with the skin it may be therefore a good idea to use rubber gloves when handling the alcohols.
- When using the thermometer it must be place on a level surface so that it may not drop off as the mercury inside the thermometer is extremely hard to clean up and is poisonous.
- When using the Bunsen burners it must be made sure that the gas taps are not left on without being connected to the burners, once the gas taps have been turned on they should be lit as soon as possible.
FAIR TESTING
- To allow fair testing all of the alcohols mass must be measured with the lid on and rounded off to two decimal places.
- Every time the Aluminium calorimeter is going to be used, the build up of carbon soot must be removed to allow a fair conduction of the heat from the alcohol being used.
Analysis
From the results of the main experiment there seems to be a varied selection of values. For each alcohol the quantities are within a small range, which is good and hopefully not drawing out any anomalous results. I cannot be sure as to whether they are “correct” or not because they are merely showing the mass of fuel burnt needed to release the same amount of energy and as I explained before in my hypothesis, the alcohols have to be compared by how many moles of each were burnt. Now I need to convert the results so that they show the number of moles of each alcohol burnt rather than the mass of each burnt.
Regarding my hypothesis, the point I made was that the bigger the alcohol molecule, the more energy is released. My results seem to be in accordance with this statement as the graph shows. There is a positive correlation, which means that as the number of carbon atoms in the alcohol molecule increases, so more energy is released with the combustion per mole of alcohol. I know that the values I got back from my experiment are not going to be accurate because of several factors affecting it. Loss of energy is the main concern. There is obviously going to be a certain amount of energy lost from the combustion of the alcohols because in reality, not all of the energy released is going to go into heating the can and the water. A lot of the energy would be lost to the surroundings via conduction or convection. The draught shield was put in place to try and reduce loss through convection but it would still not be an immaculate countermeasure.
Energy is also lost through light and radiation from the flame as well as the heat expelled. However, those values would be minute in comparison. I wanted to find out the exact amount of energy that was expelled by each reaction.
As the homologous series increases there is an added CH molecule onto each alcohols this therefore creates and makes more bonds which in turn releases more energy, which is why the enthalpy change of combustion systematically increases.
Each C-H bond is equal to 413kjmol-1 therefore a steady increase between molecules should equal 826kjmol-1
Below is the calculation of one of my results for the enthalpy change of combustion.
I shall only display the method for one of the alcohols as the others uses the same method.
Relative Atomic Masses:
Carbon(C) = 12, Hydrogen (H) = 1, Oxygen(O) = 16
Methanol (CH3OH)
1) 1 mole of methanol = 12 + 1 + 1 + 1 + 16 + 1
= 32 grams
Fuel burnt = (186.19-184.22)
= 1.97 grams
Moles burnt = 1.97 / 32
= 0.062 moles
Q=(Mw x Cw x ΔTw)
Energy absorbed/kJ=4.2*150*20
=12600 joules
Enthalpy Change of Combustion=12600/0.062
= 203.23 kj mol-1
2) Fuel Burnt
=2.13 grams
Moles Burnt
=0.067
Energy absorbed/kJ=4.2*150*21
=13230 joules
Enthalpy Change of Combustion=13230/0.067
=197.5 kj mol-1
Through my research I came upon a textbook where they had published a list of values for the energy of combustion for alcohols. They were as follows:
The energy of combustion is written as a negative value because it is showing the decrease in enthalpy for the combustion reaction and is therefore exothermic. Enthalpy is thermodynamic property and a measure of the internal energy of a substance usually measured in kj mol-1. In an exothermic reaction (e.g. combustion of alcohols), there is a decrease in enthalpy for the reaction mixture; in other words heat is given out. So the values can be interpreted as the positive energy released by the combustion. These values plotted on a graph looks like this:
However, these values would probably also have been obtained through practical experiment and there could always be the possibility of flaws in its design. This means that the values may not be the exact amounts of energy released. In order to find the exact amount you would have to calculate the values manually by using theory rather than experiment. Calculations using bond energies would be the answer to this problem. The formula for the combustion of a fuel is;
Fuel + oxygen ——› carbon dioxide + water.
Adding up the energy needed to break the bonds in the fuel and oxygen and then deducting the energy needed to reconstruct the bonds in CO2 and water can seek the theoretical values.
The bond energies required for these calculations are:
C – C 347 kj mol-1
C – H 413 kj mol-1
C – O 358 kj mol-1
C = O 805 kj mol-1
O – H 464 kj mol-1
O = O 498.3 kj mol-1
In order to know how many of each bond there are, I need to write out balanced equations for each complete combustion reaction. This will tell me how many moles of oxygen are needed to fully react a mole of alcohol and how many moles of CO2 and water are produced.
methanol + oxygen ——› carbon dioxide + water
2CH3OH + 3O2 ——› 2CO2 + 4H2O
ethanol + oxygen ——› carbon dioxide + water
C2H5OH + 3O2 ——› 2CO2 + 3H2O
propanol + oxygen ——› carbon dioxide + water
2C3H7OH + 9O2 ——› 6CO2 + 8H2O
butanol + oxygen ——› carbon dioxide + water
C4H9OH + 6O2 ——› 4CO2 + 5H2O
I will explain in detail how to calculate the values but only for methanol. The rest I will calculate without the method shown because it is the same.
Evaluation
My results show a clear increase in the enthalpy of combustion as the alcohols get larger, Butan-1-ol, the largest alcohol that I have tested shows the highest enthalpy of combustion and methanol the smallest in size has the smallest value for the enthalpy of combustion. This is as I had expected, as the enthalpy of combustion that I estimated earlier using average bond enthalpies and the successive increase of the CH molecules as you go down the series.
My results show the same pattern for the enthalpy of combustion as the average bond enthalpy estimation worked out, but they do not show the same total amount of energy being released per mole. Remembering that the bond enthalpies are only estimations I need to compare my results to other more reliable results to see how accurate the results that I have obtained are.
It is actually possible using something called a bomb calorimeter to measure the exact enthalpy of combustion. This means that I can compare these values against my results and it will be possible to work out how exact the results are. The bomb calorimeter apparatus's are specially designed to avoid heat loss by completely surrounding the 'bomb' with water. Heat losses can be eliminated altogether if the thermo chemical investigation is coupled with an electrical calibration. First of all, the chemical reaction is carried out in the calorimeter and the temperature is plotted against time before, during and after the reaction.
The experiment is now repeated, but this time an electrical heating coil replaces the reactants. The current in the coil is carefully adjusted so as to give a temperature/time curve identical to that obtained in the chemical reaction. By recording the current during the time of this electrical calibration, it is possible to calculate the electrical energy supplied with great accuracy. This electrical energy is exactly the same as the energy change in the reaction. As it includes both the heat absorbed by the system and the energy lost in the system and the heat lost from the system it eliminates the need for heat loss correction.
The values for the enthalpy of combustion given by the bomb calorimeter are
715
1371
2010
2673
They were measured under the conditions of a temperature of 289 Kelvin and a pressure of 1 atmosphere.
By comparing these to my result I can work out the percentage error/ accuracy of my results
Alcohol Average Enthalpy of Combustion (my results) Percentage of Exact Enthalpy of Combustion (bomb calorimeter)
As you can see from the table above the accuracy of my results from my experiments were not very good. Methanol was the least accurate at 82% error and Methanol at 71.6% was the most accurate.
The results I have got have been consistently inaccurate, the gap between the most accurate and the least accurate being only 10% they are all out by along way, the most accurate is only 30% of what it should be. This gives me the impression that my results are not just wrong because of human error i.e.
*Reading of the thermometer (I can only read to accuracy of nearest degree)
*Measuring of the weight of the Alcohol
*Measuring of water to be heated
* Impurities in the water (may change the specific heat capacity of the water)
* The enthalpy's of combustion that I am comparing my results to were measured under different conditions so this means they would be different any way
By looking at the set up of my experiment it is quiet clear why the accuracy of the results are not very good.
A lot of heat produced in the experiment was aloud to escape before it had even entered the apparatus and even heat that got into the water could escape back out of the calorimeter, as the good conducting aluminium which let the heat in could just as easily let it out again.
*Firstly carbon is a better insulator than aluminium, it has a lower thermal conductivity value and so it will stop as much heat getting through to heat the water as there should be
*Secondly the carbon has come from the spirit burner and for my results I am assuming that all the alcohol that leaves the spirit burner is combusting and releasing heat to contribute to the heating of the water which it is quiet clearly not. This will mean I am assuming that more alcohol is needed than really is to cause the increase in temperature.
A method which may have been tried was to cover the top of the aluminium calorimeter and insulate it to allow less heat loss.
This is an area where the accuracy of my experiment could have been improved a lot, by not allowing any heat escape, but to do this I would have to use a bomb calorimeter, which was not available to me.
Other factors which lead to inaccurate readings where chemical factors such as incomplete combustion of the hydrocarbons, non-standard heat conduct, non-continuous stirring and vaporisation.
There are other aspects of the enthalpy of combustion of alcohols that I could have also investigated. Firstly I could have looked into whether the position of the OH group within the molecule effects the enthalpy change and also whether branching within the molecule also has any effect on the enthalpy of combustion.
To check how much of the error should be attributed to procedural error I shall work out how percentage uncertainty in reading instruments.
Methanol
The uncertainty in reading a thermometer that measures to 0.1 oc is 0.05
Therefore the percentage uncertainty in reading the thermometer is the uncertainty divided by average temperature change of water being heated by the combustion of methanol
0.05 x100 = 0.5%
10
The uncertainty in reading the 100cm–3 measuring cylinder that measures to 1cm–3 is 0.005
The average value of water recorded was 99.67cm3
0.005 x 100 = 5.02x 10–3%
99.67
The uncertainty in reading a balance that meat measure to 2 decimal places it 0.005 the average mass recorded is the average (start mass + average end mass)/2
207.48 +207.08 =207.28g
2
0.005 x100 = 2.41 x 10–3%
207.28
The percentage my result is out is
335 x100=46.14%
726
100-46.14 =53.86
My result is 53.86% out
My total percentage uncertainty is 0.51%
Ethanol
The uncertainty in reading a thermometer that measures to 0.1 oc is 0.05
Therefore the percentage uncertainty in reading the thermometer is the uncertainty divided by average temperature change of water being heated by the combustion of ethanol
0.05 x100 = 0.5%
10
The uncertainty in reading the 100cm–3 measuring cylinder that measures to 1cm–3 is 0.005
The average value of water recorded was 99.84cm3
0.005 x 100 = 5.01x 10–3%
99.84
The uncertainty in reading a balance that meat measure to 2 decimal places it 0.005 the average mass recorded is the average (start mass + average end mass)/2
189.63 + 189.25 = 189.43g
2
0.005 x 100 = 0.0264%
189.43
The percentage out my result is
- x100=37.1%
1367
My result is
37.1-100 = -62.9% of the actual result
My total percentage uncertainty in working out the enthalpy change of combustion of ethanol is 0.53%
Propan-1-ol
The uncertainty in reading a thermometer that measures to 0.1 oc is 0.05
Therefore the percentage uncertainty in reading the thermometer is the uncertainty divided by average temperature change of water being heated by the combustion of propan-1-ol
0.05 x100 = 0.5%
10
The uncertainty in reading the 100cm–3 measuring cylinder that measures to 1cm–3 is 0.005
The average value of water recorded was 100.00cm3
0.005 x 100 = 5.00x 10–3%
100
The uncertainty in reading a balance that meat measure to 2 decimal places it 0.005 the average mass recorded is the average (start mass + average end mass)/2
204.87+204.53 =204.7
2
0.005 x 100=2.44 x 10–3%
204.7
The percentage my result is out is
741 x 100=36.7%
2021
It is unsettling to see that my result is just over 1/3 of the actual result
36.7-100= -63.3% from the actual enthalpy change of combustion of propan-1-ol
My total percentage uncertainty is 0.51%
Butan-1-ol
The uncertainty in reading a thermometer that measures to 0.1 oc is 0.05
Therefore the percentage uncertainty in reading the thermometer is the uncertainty divided by average temperature change of water being heated by the combustion of butan-1-ol
0.05 x100 = 0.5%
10
The uncertainty in reading the 100cm–3 measuring cylinder that measures to 1cm–3 is 0.005
The average value of water recorded was 100.00cm3
0.005 x 100 = 5.00x 10–3%
100
The uncertainty in reading a balance that meat measure to 2 decimal places it 0.005 the average mass recorded is the average (start mass + average end mass)/2
192.17+191.93 =192.05
2
0.005 x 100=2.60 x 10–3%
1920.05
The percentage my result is out is
1300 x 100=48.6%
2676
48.6-100= -51.4% from the actual enthalpy change of combustion of butan-1-ol
My total percentage uncertainty is 0.51/%
As I have established that the percentage uncertainty hasn’t had a considerable effect on my experiment the only way I can think of improving my procedure is eradicate some of the procedural errors.
