Combustion equation
When fuel is combusted, oxygen is supplied, and the products are CO2 and H2O. We often use equation to represent 1 mole of fuel (an Avogadro constant-6.02 x 1023 formula units per mole) reacting, even though the number of moles of oxygen molecules may not be a whole number.
Propan-1-ol C3H7OH (g) + 4 1/2O2→ 3CO2 (g) + 4H2O (g)
Butan-1-ol C4H9OH (g) + 6O2 → 4CO2 (g) + 5H2O (g)
Butan-2-ol C4H9OH (g) + 6O2 → 4CO2 (g) + 5H2O (g)
Cyclohexanol C6H11OH (g) + 8 1/2O2 (g) → 6CO2 (g) + 6H2O (g)
The table
I have decided to repeat my experiment three times on each of the alcoholic fuel so as to increase the reliability the results. I would then use the average on the increase of water temperature to calculate the energy transferred using the equation mentioned earlier on. Also I decided that I am going to measure the temperature every time before every trials because a drop in temperature while I am weighting the fuel would lead me to inaccurate results as well!
Preliminary trial
I have carried out the test earlier on as a task. These are the results I have got:
But later I found out that in fact this preliminary test is very inaccurate due to:
-a poor draught shielding system, therefore energy is lost to the surrounding as heat, this would lower the value for my enthalpy change
-water might have been evaporated into the air because I didn’t put a lid on
-position of the flame varies every trials
-water not stirred evenly
So learning from this, I have to make sure that:
-least amount of energy is lost as heat
-water stirred more evenly
-cover the can with a lid
-using a fixed position from the base of can to the flame
Preliminary test
I carried out similar experiment to find out if several changes would lead me to a different results from the experiment:
- Testing with the size and height of can:
Can 1) Diameter: 7.6 cm
Height: 10.1cm
Can 2) Diameter: 6.7cm
Height: 10.3cm
What I needed:
-no draught shielding system*
-ethanol fuel**
-50ml distill water inside the can
-original water temperature inside the can-21C
The results showed that in fact it does matter for the size of can. However, I have decided that I am going to use Can 2, that is the one with diameter: 6.7cm and height: 10.3cm, so that less heat is lost to the can. Can 1 has got a larger surface area.
- Testing with the position of flame
What I needed:
-ethanol fuel
-50ml distill water
-Can 2) Diameter: 6.7cm
Height: 10.3cm
-original water temperature inside the can-21C
You can see that as the flame is further from the base of the can, the time needed for the temperature to reach 30C increase, more heat is lost to the surrounding.
- Testing with the wick length (the part where it is burnt)
What I needed:
-Can 2) Diameter: 6.7cm
Height: 10.3cm
-filled with 50ml distill water in the can
-tip of wick 3cm from the base of the can
-original temperature
You can see that as wick length increases, time for the temperature in distill water to reach 30C decrease. I guessed that as the wick length increases, more fuel is being burnt, so more energy is ready to heat up the water particles in the can. In the experiment, I am going to use a wick with length of 2cm, because it would heat the can up quickly.
- When should I remove the burner from the flame
-I have decided that I will remove the burner from the flame at the moment when I read off the temperature of distill water of the can (I would try to read the maximum temperature as well). The reason is that I would try to ignore any subsequent drops in temperature or rise. For me the easiest way would be detecting the maximum value, so I have to remove the heat source.
*-although some of the heat is lost to the surrounding and this may affected the time for temperature to reach 30C, but it is only systematic error, that means it happens to every trials I carried out. Systematic errors are bad if you are interested to find accurate results, but here I am just trying to look out for a pattern, same amount of heat is lost to the air, table, me, except for the cans in ‘Testing with the size and height of can’, but only only a small amount is considered so it can be neglected.
**-I used ethanol fuel throughout so that it is a fair test
Enthalpy change from the data sheet
From the table above, I can see that increasing the hydrocarbon chain has an effect of increasing the energy given out (comparing between propan-1-ol and butan-1-ol). ‘Burying’ the OH group in between chain (comparing between butan-1-ol and butan-2-ol) has also an effect of increasing the energy. While cyclohexanol gives out most energy. The values are all negative because the reaction is exothermic, that is why there will be a rise in temperature in the distill water in the can, energy (mainly in form of heat) is lost from the molecule to the surrounding.
Prediction
My prediction would be similar to the results I got from the data sheet,
Errors
These are factors that leaded to inaccurate results:
- Although the draught system is designed, there is still some heat lost to the surrounding that is unable to being measured up.
- Some of the heat is used to heat you and the equipment up.
- Limitation in equipments. E.g. If I have used a more accurate electronic balance, one that is corrected to more decimal places; and if I have used a more accurate thermometer. Then I should get more accurate results.
- Unequal tilting of the base of the can (as explained in ‘Testing with the position of flame’ in preliminary test), the trial with the flame closer to the base of the can, the temperature would increase quicker.
Disposal and storing
-as all the alcohol fuels we used in the experiment is very flammable, a cap should be used to cover up the spirit burner to prevent evaporation. It should also be kept separate from a source where it can easily catches fire.
-Washings and water soluble alcohol may be washed down the foul-water drain with plenty of water (distill water most appreciated). Water-insoluble alcohols should be removed by an authorised waste contractor.
-Bottles used regularly in the laboratory should be no larger than 500 ml capacity. Propan-2-ol should be in a dark bottle and not kept longer than 2 years.
What if
Swallowed-wash out the mouth and give a glass or two of water. Seek medical attention.
Vapour inhaled-remove the victim to fresh air to rest. Keep warm.
Liquid splashed in eyes-flood the eye gently running tap water for 10 minutes. Seek medical attention.
Split on skin or clothes-remove contaminated clothing. Wash affected area thoroughly with cold water. Soak contaminated clothing.
Split in laboratory-Shut off all sources ignition. Open all windows. Cover with mineral absorbent and clear up into a bucket. Rinse area with a cloth. Wash the absorbent with dispersing agent and pour down the foul-water drain.
Source of information
- Internet
-
CLEAPSS, 1995. Hazcards, 4th edition, reprinted 1998 with updates.
- SE, 1996. Safety reprints. Association for science Education
- Salters Advanced chemistry-Chemistry Ideas, Heinemann, by George Burton, John Holman, John Lazonby, Gwen Pilling and David Waddington, second edition
Results table
*- temperature is corrected to 1 decimal place because human eyes are not accurate enough to detect readings less than 1 decimal place
**- readings on the electronic balance is corrected to 2 decimal places so all the readings in the table are corrected to 2 decimal places
Analysis
From my results, I work out:
-the mass of water used
-the temperature rise of water
-the mass of fuel used
I assume that all the energy from the burning fuel is transferred to the water, so I use the enthalpy change equation to calculate:
Energy transferred from the fuel=cm∆T
where c is the specific heating capacity of water (4.17 Jg-1K-1-it means 4.17J is needed to raise the temperature of 1g of pure water by 1C)
-m is the mass of water, in g = 200cm3
-∆T is the change of temperature of the water
E.g. for the 1st trial:
Energy transferred = 15.6C x 200g x 4.17 J/C/g
= 13101.4J
E.g. for the 1st trial:
Energy transferred = 16.75C x 200g x 4.17 J/C/g
= 13969.5J
E.g. for the 1st trial:
Energy transferred = 9.5C x 200g x 4.17J/C/g
= 7923J
E.g. for the 1st trial:
Energy transferred = 18C x 200g x 4.17J/C/g
= 15012J
Now I want to find out the enthalpy change of energy that would be transferred to the water by burning 1 mole of fuel:
Propan-1-ol
-
Formula of propan-1-ol: C3H7OH
- Mass of 1 mole of propan-1-ol: 60
-
Number of moles of propan-1-ol used: 0.696 (the average weight)/60 = 0.0116 mol
-
Energy transferred by this number of moles of propan-1-ol: 12359.88J (the average energy transferred)
-
Energy transferred by 1 mole of propan-1-ol: 12359.88J/-0.0116 mol = -1,065,507J/mol (corrected to the nearest integer)
- Enthalpy change of combustion: -1,065,507J/mol
-
Energy released when 1 propan-1-ol is burnt: 1,065,507/6.02205 x 1023 (Avogadro constant) = 1.77 x 10-18
Butan-1-ol
-
Formula of butan-1-ol: C4H9OH
- Mass of 1 mole of butan-1-ol: 74
-
Number of moles of butan-1-ol used: 0.68/74 = 0.009189 mol (corrected to 6 decimal places)
-
Energy transferred by this number of moles of butan-1-ol: 13969.5J
-
Energy transferred by 1 mole of butan-1-ol: 13969.5J/-0.009189 mol = -1,520,187J/mol (corrected to the nearest integer)
-
Enthalpy change of combustion: -1,520,187J/mol
-
Energy released when 1 butan-1-ol is burnt: 1,520187/6.02205 x 1023 (Avogadro constant) = 2.52 x 10-18
Butan-2-ol
-
Formula of butan-2-ol: C4H9OH
- Mass of 1 mole of butan-2-ol: 74
-
Number of moles of butan-2-ol used: 0.366/74 = 0.009189 mol (corrected to 6 decimal places)
-
Energy transferred by this number of moles of butan-2-ol: 8456.76J
-
Energy transferred by 1 mole of butan-2-ol: 8456.76J/-0.009189mol = -920,313J/mol (corrected to 6 decimal places)
-
Enthalpy change of combustion: -920,313J/mol
-
Energy released when 1 butan-2-ol is burnt: 920,313/6.02205 x 1023 (Avogadro constant) = 1.53 x 10-18
Cylohexanol
-
Formula of cyclohexanol: C6H11OH
- Mass of 1 mole of cyclohexanol: 100
- Number of moles of cyclohexanol used: 0.72/100 = 0.0072 mol
- Energy transferred by this number of moles of cyclohexanol: 16402J
-
Energy transferred by 1 mole of cyclohexanol: 16402J/-0.0072mol = -2,278,056 J/mol (corrected to the nearest integer)
-
Enthalpy change of combustion: -2,278,056 J/mol
-
Energy released when 1 cyclohexanol is burnt: 2,278,056/6.02205 x 1023 (Avogadro constant) = 3.78 x 10-18
Compare them in a table form:
Although all of the fuels above belong to the same alcohol homologous series, they all have different enthalpy change due to different molecular structure. Butan-1-ol have an additional CH2 group compare to propan-1-ol, and gives out 454J more of energy per mole, the trend would be the bigger the mass, the more energy it yields. The reason is the longer the chain, the more products will form. And because the C=O and O=H bonds in products CO2 and H2O are more stable than the C-H bonds in alcohol fuel, because they are more stable so they gives out more energy.
Butan-1-ol has the OH group attached to C1, and butan-2-ol has the OH group attached to C2, in addition, butan-1-ol gives out 600J more energy per mole, the trend would be the further the carbon the functional group is attached to, the less energy it gives out. Therefore the position of the functional group has a great effect on the enthalpy change of combustion.
Cyclohexanol generally gives out more energy, because it contains more carbon than other fuels. So this means that although more C-H bonds have to be broken, more stable C=O and O=H bonds are formed in the products, therefore gives out more energy.
Evaluation
I think although this technique is not perfect, it gives me a rough approximation of the enthalpy change of different alcohol fuel. The way heat lost is roughly systematic, in other word, it loses the way in every trials, similar amount is lost to warm the room, similar amount is lost to the apparatus, etc. , therefore there seems to be an accuracy of overall result, and it won’t affect the reliability of looking for a trend. In the results there doesn’t seem to contain any clearly anomalous results, because they generally fit in the trend. However they are some main sources of errors:
- heat loss to air
- heat loss to apparatus
- incomplete combustion due to inadequate oxygen supply
- different burners and different wicks for different alcohol fuels
Other sources of errors included:
- evaporation of water through lid
- inaccurate reading on thermometer
- inaccurate reading on measuring cylinder
- uneven water temperature in the can
- friction between water molecules, the can and thermometer when stirred, and increases the temperature
- minority of fuel being evaporated
- wick length not accurate
- tilting of the can so that the flame position is not the same every trial
All the errors explain above (except for the friction between water) would lower the temperature of the water and therefore the enthalpy change.
Precision and reliability of measuring
Apparatus itself has errors, below are some of the % errors of some apparatus:
-measuring cylinder ±0.5ml
-thermometer ±0.5C
-electronic balance ±0.005g
Here are some of the % uncertainty of some measurement:
Measuring 200g of distill water using a measuring cylinder, the % uncertainty would be:
0.5/200 x 100%
= 0.25%
I think the overall % uncertainty is very difficult to calculate because the fact that most of the heat is lost to the surrounding, there are so many ways and factors that heat can be lost. The only way I think the most accurate to calculate the % uncertainty is to compare the true reading from the data sheet:
Propan-1-ol:
Enthalpy from my experiment: -1066
Actual enthalpy change: -2021
% uncertainty = [-2021-(-1066)]/-1066 x 100%
≈90%
Butan-1-oll:
Enthalpy from my experiment: -1520
Actual enthalpy change: -2676
% uncertainty = [-2676-(-1502)]/-1502 x 100%
≈78%
Butan-2-oll:
Enthalpy from my experiment:
Actual enthalpy change:
% uncertainty =
Cyclohexanoll:
Enthalpy from my experiment: -2278
Actual enthalpy change: -3728
% uncertainty = [-3728-(-2278)]/-2278 x 100%
≈64%
The % uncertainty overall of all the alcohol fuel is more than 50%, in reality it seems that the results are rubbish, but again looking for pattern in this experiment is more important than caring for the actual, true results.
The smaller the result, the bigger the % uncertainty. The procedural errors accumulated and affect the results.
Improvement
If I am to do the experiment again, I would:
- use a more accurate thermometer showing more decimal places
- use a more accurate electronic balance showing more decimal places
- a more suitable lid that can fit perfectly onto the can without any chances that water molecule can escape
- make sure the wick length is exactly the same
- make sure the flame position is the same
- make sure the base of the can is horizontal
- use a more accurate measuring cylinder showing more decimal places, I don’t think using a pipette or burette is ideal here because we are dealing with 200g of water
- same kind of burner every trial
- stir gently and more evenly
- carry out the experiment in a bomb calorimeter, this is done at constant volume in a close container so not heat can escape, in fact they can all be measured.
