Investigating the enthalpy change of different fuels when combusted.
Investigating the enthalpy change of different fuels when combusted.
Aim:
The aim of this experiment is to see how the enthalpy changes vary when different alcohols are combusted in standard conditions. The alcohols used will be: Ethanol, Propan-1-ol, Butan-1-ol, Pentan-1-ol and Hexan-1-ol. These alcohols all have the OH functional group on the first Carbon atom of the molecule.
Background Knowledge
The enthalpy change of combustion (also shown as ?Hc) is a measure of energy when one mole of a fuel burns completely in air, at a standard temperature and pressure. This standard temperature is 298 Kelvin and 1ATM pressure. Maintaining these conditions would be complicated; therefore the experiment will be carried out in normal conditions. Any drastic alterations to these conditions will need to be taken care of to maintain a degree of accuracy.
As combustion is exothermic (heat is transferred to the environment) all of the enthalpy changes will be negative. The formula for enthalpy change is:
Energy Transferred= Heat capacity of water x Change in water temp x mass of water.
Or
E=MC?T
Hess's Law.
"the enthalpy change for any chemical reaction is independent of the intermediate stages, provided the initial and final conditions are the saeme for each route."
This basically means that in an enthalpy cycle, detours can be made to reach the final products and the total energy required for the detour will be equal to the direct route ie.
The enthalpy change for ?H1 = ?H2 + ?H3. as we cannot go directly along ?H1, we must take the detour and from this we can calculate the enthalpy change. This can be calculated by the enthalpy changes of combustion for carbon, hydrogen and the alcohol in question.
Bond Enthalpies:
The amount of energy required to break the bonds between atoms is called the disassociation energy. The higher the disassociation energy, the shorter the bond is in length and the stronger the bond is. Single bonds are relatively easy to break as opposed to double or triple bonds.
Endothermic and Exothermic Reactions:
Exothermic reactions are those that give energy into their surroundings in the form of heat, heating the surroundings. This allows us to heat up the water directly above the spirit burner.
An endothermic reaction is totally opposite to an exothermic reaction as endothermic reactions take in energy from the surroundings and cool the surroundings.
Here are some bond energy diagrams for exothermic and endothermic reactions:
This shows the negative enthalpy change as energy is given to the surroundings. The downward pointing arrow shows the energy lost as heat during the reaction.
This diagram shows the positive energy change of an endothermic reaction. The intake of energy from the environment is represented by the smaller of the 2 arrows.
Alcohols
Alcohols are modified alkane chains with a functional hydroxyl group. E.g.
Ethane Ethanol
The only difference between these molecules is the hydroxyl group. The position of this hydroxyl group will determine if the alcohol is primary, secondary, tertiary and so on. Primary alcohols have the functional group on the first carbon atom of the molecule at either end; Secondary alcohols have the hydroxyl group on the second carbon of the molecule and so on. The number of the carbon atoms is determined by counting from the end nearest the carbon atom in question. These are all isomers and all molecules with more than 2 carbon atoms can form an isomer. The isomers have the same molecular formula but have different chemical structures and therefore have slightly different chemical properties. This is because the molecules shape has changed so they may no longer fit together in this isomer form as they did in the straight chain molecules. This may affect the boiling and melting point of the alcohol as the intermolecular forces are affected.
This experiment will use the 2nd, 3rd,4th,5th and 6th alcohols, which will al be primary alcohols where applicable. Methanol will not be used as it produces poisonous carbon monoxide and the combustion of this alcohol must be undertaken in the fume cupboards. This will affect our results as the fume cupboard produces a current of air near the front of the cupboard which has the tendency to blow the flame at an angle, reducing the amount of heat that is transferred to the water
In the combustion of alcohols, if there is a plentiful supply of oxygen, there will be complete combustion, in which the only waste products are water and carbon dioxide. If there is not a plentiful supply of oxygen, incomplete combustion occurs and the products are water and poisonous carbon monoxide.
Alcohol
Bond Breaking
Bond making
Total
Breaking - Making
Ethanol
C-H= 413x5=2065
C-O= 385x1=385
O-H= 464x1=464
O-O=498x3=1494
C-C= 347x1=347
O-H= 464x6=2784
C=O= 805x4=3220
-1249 kJ/mol
Propan-1-ol
C-H= 413x7=2891
C-O= 385x1=385
...
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In the combustion of alcohols, if there is a plentiful supply of oxygen, there will be complete combustion, in which the only waste products are water and carbon dioxide. If there is not a plentiful supply of oxygen, incomplete combustion occurs and the products are water and poisonous carbon monoxide.
Alcohol
Bond Breaking
Bond making
Total
Breaking - Making
Ethanol
C-H= 413x5=2065
C-O= 385x1=385
O-H= 464x1=464
O-O=498x3=1494
C-C= 347x1=347
O-H= 464x6=2784
C=O= 805x4=3220
-1249 kJ/mol
Propan-1-ol
C-H= 413x7=2891
C-O= 385x1=385
O-H= 464x1=464
O-O= 498x4.5=2241
C-C= 347x2=694
O-H= 464x8=3712
C=O= 805x6=4830
-1867 kJ/mol
Butan-1-ol
C-H= 413x9=3717
C-O= 385x1=385
O-H= 464x1=464
O-O= 498x6=2988
C-C= 347x3=1041
O-H= 464x10=4640
C=O= 805x8=6440
-2485 kJ/mol
Pentan-1-ol
C-H= 413x11=4543
C-O= 385x1=385
O-H= 464x1=464
O-O= 498x7.5=3735
C-C= 347x4=1388
O-H= 464x12=5568
C=O= 805x10=8050
-3103 kJ/mol
Hexan-1-ol
C-H= 413x13=5369
C-O= 385x1=385
O-H= 464x1=464
O-O= 498x9=4482
C-C= 347x5=1735
O-H= 464x14=6496
C=O= 805x12=9660
-3721 kJ/mol
Prediction
I predict, using the results above, that the longer the hydrocarbon chain, the more energy is required to break the bonds, so the more energy is given off as heat.
Variables
Independent variables
These are the factors that we will be changing. In this experiment it is only the alcohol that will be changed. The alcohols will be changed to see how much the enthalpy change varies with each alcohol. The experiment is also made more accurate if there is only 1 factor that is being changed as I do not have to keep tabs on several variables during 1 experiment.
Dependant variables
These are the factors that I will be measuring. In this experiment there is only 1 dependant variable for each alcohol and that is the change in mass of the spirit burner + alcohol + lid. This will allow us to calculate the enthalpy change and therefore how much energy is required to heat up water through 20°C. The measuring of the mass will be done by a set of balances which can show mass changes from as little as 100th of a gram.
Controlled variables
These are the factors that we will wish to keep the same or control. In this experiment there are several controlled variables that need to be maintained in order to produce accurate results.
These variables are:
. The volume of water in the copper colorimeter
2. The height between the spirit burner wick and the base of the calorimeter
3. The change in the temperature of the water
4. The starting temperature of the water must be within 2°C of all other experiments.
5. The amount of soot on the underside of the calorimeter (none at the beginning of each experiment.)
The volume of water in the copper calorimeter must be kept the same as any variations will affect how easily the water is heated through 20°C. An increased volume of water in the calorimeter will require more energy to heat it up. This will affect the results as more mass will be lost from the spirit burner in order to heat up the rest of the water.
The height between the spirit burner and the calorimeter is also important as the closer the two are together, the closer the calorimeter is to the flame and so the calorimeter will be receiving more heat from the flame. This will reduce the amount of energy required to heat the water up, affecting the results.
The change in the temperature of the water must be kept the same to ensure that the energy required to heat the water is accurate. For example, it would take more energy to heat water through 40°C than to heat it through 20°C. For this reason the temperature change will be maintained at 20°C for all the experiments.
The starting temp of the water is also important as less energy is required to heat lower temperature water than to heat higher temperature water.
The amount of soot on the underside of the calorimeter is also important as it will provide a limited amount of insulation from the heat of the flame, reducing the amount of heat reaching the calorimeter and the water. In order to overcome this, the calorimeter should be cleaned thoroughly in order to remove ant of the insulating soot.
Method
Apparatus
* Spirit burners
* Alcohols (ethanol,propan-1-ol, butan-1-ol, pentan-1-ol and hexan-1-ol)
* Copper calorimeter
* Clamp stand
* Clamp
* Boss
* Thermometer/Data logger
* Method of insulation
. Measure the mass of the spirit burner on the balance with lid on. Note the mass
2. Place the spirit burner in the apparatus setup.
3. fill a copper calorimeter with 100 cm^3 of water
4. note the temperature of the water
5. ignite the spirit burner and wait for temperature to rise by 20°C
6. extinguish the spirit burner once the temperature has risen by 20°C
7. Measure the mass of the spirit burner with lid again. Note mass.
8. Empty and Clean the base of the calorimeter to clear up any soot due to incomplete combustion.
9. repeat the experiment for all alcohols
0. repeat experiment 3x for each alcohol.
Safety
All of the alcohols used in this experiment are harmful to human health if ingested or if contact with eyes occurs.
Ethanol vapours are known to have narcotic properties and could be dangerous if inhaled in moderate volumes.
All of the alcohols are irritant to the skin and eyes. To overcome this problem, gloves must be worn when handling alcohols out of the spirit burners and goggles should be worn at all times to prevent irreversible damage to the eyes.
Alcohol vapours may have narcotic effects so they should not be inhaled or ingested.
All of the alcohols are flammable so should be kept away from sources of ignition and incandescent materials. Ignition of the alcohols should only be completed when all of the equipment is set up and there is no risk of any materials catching fire or melting.
Measurements
All temperature measurements will be undertaken with either a thermometer or a data logger. The data logger is digital and therefore more accurate. These can show how much the temperature has changed and can provide a signal to extinguish the flame of the spirit burner.
To maintain a degree of accuracy, all experiments should be carried out at least three times and an average should then be taken. If the mass changes are more than 5% away from each other, the experiment should be retaken to get more accurate results.
The initial temperature of the water should also be kept within 2°C each other as lower temp water is easier to heat through 20°C than water of a higher temperature. Doing this will ensure that the results acquired, are all accurate.
Justification
These alcohols are being used as they are all primary alcohols and they all have similar molecular structures. They also have similar chemical properties and are therefore suitable to be compared against each other. The fact that they are all primary alcohols means that they can easily fit together and the intermolecular forces will be stronger. If they were secondary or tertiary alcohols, the intermolecular forces would be less than those of the primary alcohols and they would have a different boiling and melting points. There is also a slight difference in the energy required to break some of the bonds but this is negligible as the variation is very small.
Analysis:
Alcohol
Mass (g)
Temperature (°C)
Before
After
Change
Before
After
Change
Ethanol
97.93
96.17
.76
5.5
35.5
20
96.17
94.23
.94
5.7
35.7
20
94.23
92.42
.81
5.3
35.3
20
Average:
.836667
Average:
20
Propanol
95.7
94.15
.55
6.1
36.1
20
94.15
92.39
.76
6.2
36.2
20
90.32
88.88
.44
4.7
34.7
20
Average:
.583333
Average:
20
Butanol
201.79
200.62
.17
6
36
20
99.72
98.32
.4
7
37
20
98.32
97.06
.26
6.5
36.5
20
Average:
.276667
Average:
20
Pentanol
87.82
86.64
.18
5.6
35.6
20
86.64
85.21
.43
5.6
35.6
20
85.21
83.88
.33
6.1
36.1
20
Average:
.313333
Average:
20
Hexanol
220.51
219.06
.45
6.2
36.2
20
219.06
217.67
.39
6.4
36.4
20
217.67
216.21
.46
6.3
36.3
20
Average:
.433333
Average:
20
To work out the enthalpy change we will first have to use the formula cm?T to work out the enthalpy released. Then the number of moles will have to be worked out by dividing the mass by the atomic mass. The answer from cm?T will then be divided by the number of moles to get the KJ released per mol. This is where:
c = specific heat capacity of water = 4.17 J g-1 K-1
m = mass of water = 100g
?T= 20
Alcohol
Molar mass
chage in mass (g)
no of moles
cm?T water
energy released per mole (kJ)
Ethanol
44
.837
0.04175
8340
-199.760479
Propanol
60
.583
0.0263833
8340
-316.1086545
Butanol
74
.277
0.0172568
8340
-483.2889585
Pentanol
88
.313
0.0149205
8340
-558.9642041
Hexanol
02
.433
0.014049
8340
-593.6357292
The above results show that the number of moles required to heat the water through 20ºc, decreases as the number of carbon atoms increases, and therefore the molar mass increases. As the alcohols have the same general formula, the number of carbon atoms is related directly to the molar mass. In all, this means that the longer the alcohol hydro-carbon chain, the less moles of a fuel are required to heat the water.
This is due to the increased number of bonds between the atoms of each molecule. There is an increasing difference between the energy required to break and make bonds. This difference determines the amount of energy given out as heat. As the difference gets larger, the more heat is given out.
The results supported my prediction "that the longer the hydrocarbon chain, the more energy is required to break the bonds, so more energy is given off as heat." This is proven by the amount of energy released per mol. The higher the energy released, the more energy is transferred to the water, and so the less fuel is required to heat the water. From my prediction hexan-1-ol should be the fuels that release the most energy per mol, and is supported by my results from the experiments. However, as the graph shows, there was a large proportion of energy lost as heat.
For each alcohol there was an addition of CH2. Example:
H H
H C C OH Ethanol
H H
H H H
H C C C OH Propanol (An extra CH3 has been added)
H H H
This means that each alcohol is different by a small amount from the last alcohol:
CH2 + O2 CO2 + H2O
Knowing this allows us to calculate the predict the difference between an alcohol and both adjacent alcohols, using the bond energy values that I used in my plan. Each alcohol has the value below added on to the previous alcohol:
CH2 + 1.5O2 CO2 + H2O
Values for bonds broken:
C-H x 2 = 413 x 2 = 826
O-O x 1.5 = 498 x 1.5 = 747
There is also a C-C bond that has to be broken. This is between the CH2 and the rest of the molecule. This means that 347KJ/mol needs to be added to the above calculation.
Total= -1920 KJ mol
Value for Bonds formed;
C=O x 2 = 805 x 2 = 1610
H-O x 2 = 464 x 2 = 928
Total= 2536 KJ mol
Enthalpy change = 1920 - 253 = -618 KJ mol
So there is a difference of -618 KJ. This is supported by my calculations made n the plan, which have a difference of 618KJ between that alcohol and both adjacent alcohols.
Evaluation
The results were very inaccurate in comparison to the theoretical results. This was caused by the large amount of heat loss caused by insufficient insulation and containment. There was also a large amount of soot accumulating on the base of the copper calorimeter after each experiment suggesting incomplete combustion.
Due to insufficient insulation, the sides of my heat containing contraption were getting very hot, almost to the point of not being able to touch. This proves that there was a large amount of heat loss, and therefore less energy being used to warm the water. This would have affected the results by increasing the moles of fuel required to heat the water. In order to overcome this, an insulator manufactured from a heat resistant material. A mould could be formed and an expandable foam pumped into the mould and left to set. Expandable foam is flame resistant and a very good insulator of heat. The top of the insulator had an area in which the calorimeter could fit, but there was a small area round which there was no insulation. This allowed heat to escape from the insulator and heat up the surrounding area, once again reducing the energy applied to heating the calorimeter. This could have been reduced by simply having a disc of an insulating material around the calorimeter that rested on the insulator. This would cause the heat to accumulate at the base of the calorimeter, and prevent the surrounding atmosphere being warmed by the energy released form combusting the alcohols. The soot that accumulated at the base of the calorimeter was due to incomplete combustion. This was due to the insufficient supply of oxygen to the base of the flame. This causes there to be uncombusted molecules of carbon in the flame. This settles as soot on the base of the calorimeter. This layer of soot acts as an insulator to the heat and increases the number of moles of fuel required to heat the water.
The above factors all significantly affected the results due to heat loss. There were other contributing factors to the large difference between predicted and actual results.
The distances that the wick stuck out from the top of the burner were all equal, but some of the wicks had differing diameters and amount of fraying at the end of the wick. Both of the factors caused there to be differing surface areas that could contain fuel for combusting. The diameter of the wick caused there to be larger or smaller surface areas through which the fuel could be combusted and the fraying increased the surface area drastically, as each fibre from the wick was fully exposed at the end. This cased there to be more or less fuel burned across the different fuels. The larger flames were in contact with the base of the calorimeter and on one occasion were large enough to ignite the tin foil covered card that was insulating the top of the calorimeter, whereas the smaller flames were barely reaching 5cm from the tip of the wick. This caused there to be more fuel burnt and more heat lost due to flames escaping from the insulation for the larger flames, and more fuel being burnt due to the distance between tip of flame and calorimeter.
There is also a degree of uncertainty. This is calculated by using the following formula:
Predicted - actual
Predicted x100 = uncertainty.
Here are the uncertainty values for my results:
Predicted (KJ/mol)
Actual (KJ/mol)
Uncertainty
Ethanol
249
200
83.98719
Propanol
867
316
83.074451
Butanol
2485
483
80.56338
Pentanol
3103
559
81.985176
Hexanol
3721
594
84.036549
There is a large degree of uncertainty in this experiment caused mostly be the lack of sufficient heat insulation. These figures show that the results are very inaccurate and are relatively useless to science.
Luke Harlow Candidate No.: 1109 Centre No.: 20806