Method:
The method that I am going to use to carry out my experiment is:
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Firstly I am going to measure out 100cm3 cold water, using the measuring cylinder (so that the water I put into the calorimeter is accurately measured out) rather than using an ordinary beaker, as an ordinary beaker is less precise compared with a measuring cylinder.
- Then I will record the temperature of the water in the calorimeter, using a thermometer to give a good degree of accuracy.
- After that I will use the clamp to support the calorimeter.
- Then I will place the spirit burner with one of the six alcohols that I am going to test on a heatproof mat.
- In order to make it a fair test and make sure that no energy is lost through surroundings, and to prevent heat from being evaporated I will use 2 draught shields and place them around either side of the calorimeter and spirit burner.
- I will then weigh the spirit on the electronic balance with the lid on to make sure that the test is fair because if I took the lid off the fuel would have evaporated very quickly making my investigation not precise and accurate.
- Then I will place the fuel that I will be using under the calorimeter on top of the heatproof mat.
- Then I will cover the calorimeter with foil to avoid any heat loss.
- I will then pierce a hole into the foil big enough for the thermometer to fit through so I can stir the water and record the temperature of the water.
- Then I will take the lid off the fuel and light the fuel with matches.
- I will constantly stir the water with the thermometer ensuring the temperature is the same throughout the whole calorimeter.
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Once the temperature of the water has reached around 45oC on my thermometer I will immediately put the top back onto the fuel, so that no more fuel is lost via evaporation in the air.
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I will then go and once again weigh the fuel on the electronic balance to see how much fuel has been used to make the temperature of the water in the calorimeter reach about 45oC.
- I will then work out the difference between the two measurements that I have of the fuel and work out the amount of fuel that has been used.
- To work out the mass used I will use the formula below:
Mass used (g) = Original Mass (g)
Final Mass (g)
I have also done a preliminary test by using my plan to guide me, and by following my plan and then making changes to my plan where it was necessary to get accurate results. At the start of the investigation I wasn’t too sure of weather to do my experiment with the lid on or the lid off. However after doing a preliminary test with the lid on and lid off on Butan-1-ol, I got the following results:
With lid = Enthalpy of combustion KJ mol
-956.8782816
Without lid = Enthalpy of combustion KJ mol
-660.5992318
I have worked out the enthalpy of combustion, using the formula below:
Energy transfer per mole = Mr Fuel* energy transferred in experiment
Mass of fuel used
After doing my preliminary test I decided to do my experiment with the lid on rather then off, as with having the lid on I was more likely to get precise results because when the lid is off fuel is evaporated into the air and is lost. So therefore by the time you can weigh the fuel again some of the fuel has been already evaporated into the air, which makes the results less precise.
Risk Assessment:
The Risk Assessment that I am doing is to ensure that I am able to identify the hazards and precautions that I will have to take while doing this experiment.
Chemical exposure occurs through ingestion, inhalation and direct contact with the skin or eyes. Accidental ingestion of chemicals in the laboratory occurs when food or drink become contaminated. It also occurs when people transfer chemicals from their hands to their mouths. Ingested chemicals may damage the digestive tract or they may be absorbed through the digestive tract and transported to organs where they can exert a toxic effect. Inhalation exposure occurs when people inhale airborne gases, vapours, particulates or finely divided liquid droplets. Inhaled chemicals can damage the nasal passages and the upper and lower regions of the respiratory tract. Contaminants - particularly fine particles - may be deposited in the lung where they can cause damage. Or the inhaled contaminants may be absorbed through the lung. The blood stream can then transport the absorbed contaminants to other parts of the body. A small amount of inhalation of certain chemicals is not life threatening however, as the dose increases; the chemical begins to exert an effect. But, as the body is capable of repairing the damage that may have been done, the exposure in this range is without adverse consequences. At still higher doses, the intense exposure overwhelms the defense mechanisms causing irreversible damage. Finally, at even higher doses, the exposure can be fatal. The graph below shows this:
Risk Assessment.
The table below shows the risks of the 6 alcohols that I am going to use, it also shows how accidents can be avoided.
Throughout the investigation I must make sure the area that I am working in is well ventilated, so that there is enough windows open so that any harmful fumes in the room can escape and not be breathed in by people in the room. I must also wear my safety goggles at all times. All lose clothing’s and long hair should be tied back and kept well away from the flame.
Analysing:
The enthalpy of combustion can only be calculated by the formula, which I have already mentioned above. These calculations can simply be done using a PC, however if there is not access to a PC they can always be done manually.
To work out the MASS USED you should use the following formula;
Mass used = original mass – final mass
To work out the temperature difference you should use the formula below:
Temperature difference = final temp – original temp,
For example ethanol the original temp of ethanol is 50oC and the final temperature of ethanol is 70oC then I would do the following calculation to work out the temperature difference:
70 – 50 = 20oC
To work out the energy transfer/J you use the following formula:
Energy transfer/j = mass of water * specific heat capacity of water * temperature difference.
To work out the energy transferred per mole/j you must use the following formula:
Energy transferred per mole/j = molecular mass of fuel * Energy transferred J
Mass used
The formula for working out the energy transferred per mol/KJ you must use the following formula:
Energy transferred per mol/KJ = Energy transferred J
1000
The Final enthalpy of combustion is minus energy transferred per mol KJ.
My results (Done on Microsoft Excel)
Look at graph 1. By looking at this graph you can see that there are 2 anomalous results. One of the anomalous points has a molecular mass of 32, which means that it is methanol, because methanol has a molecular mass of 32. The second anomalous point has a molecular mass of 88, which means it is pentan-1-ol. As my results for methanol and pentan-1-ol are anomalous I have decided to repeat the experiment for both these alcohols.
Now that I have repeated my experiment for both methanol and pentan-1-ol alcohols you can see on my new graph 2, that there are no anomalous points, which means that my results are correct. My errors occurred in the mass used. From my 2nd graph I can see that there is a correlation between the molecular mass and the enthalpy of combustion, I can see that as the molecular mass increases so does the enthalpy of combustion. In my prediction I predicted that there would be a positive correlation between the enthalpy of combustion and molecular mass, now if I compare my prediction to my results I can see that my prediction was correct.
Evaluation:
There are many limitations that can be caused during the practical procedure of the experiment. Many of the obvious and most common limitations that may easily occur without greatly noticeable effects. However some of these mishaps may cause great effect in your final result of the enthalpy of combustion. For e.g. if the temperature of the water is lower then the enthalpy of combustion would be affected, because the enthalpy of combustion will also become lower than it should be. This would mean that my results would not be reliable and would be incorrect, as it will have a greater effect on my overall final result. Another problem for inaccurate results would be the soot at the bottom of the calorimeter. While the water in the calorimeter is being heated, the soot at the bottom may act as an insulator so therefore it will make the temperature of the water lower than it actually should be. This again will affect my results and make the enthalpy of combustion lower than it should be.
I had foil on top of the calorimeter, this was to ensure that no heat was lost, however, while I had the foil on the calorimeter I also had to stir the water init to ensure that the temperature of the water is the same. While I had to stir the water the foil was easily damaged and torn which resulted in minor excessive heat loss into the surroundings. This will also make the temperature of the water go down which means that the enthalpy of combustion will be affected. Another important factor is to ensure that the lid of the burner is closed properly, this is to reduce the amount of energy from being lost. If the lid is not securely put on this will result in inaccurate results when the burner is reweighed, as there has been energy lost.
To reduce the amount of heat lost it is important to cover the water with foil, however this makes it difficult to stir properly which would give inaccurate results of enthalpy of combustion. So therefore in order to reduce these limitations it is important to stir the water properly and avoid tearing the foil as this will mean that heat is lost.
Finally once the experiment is done and the burner has to be weighed it is important that the lid of the burner is put back on straight away, this is to ensure that no vapour is lost via evaporation. In order for my experiment to go well and to gain accurate results as I possibly could, it was important that I used draft shielding to reduce any heat from being lost into the surrounding areas.
Errors:
The errors that occurred in my investigation were only minor errors that didn’t have a great affect on my overall results. The errors that did occur are shown below:
Errors for hexanol:
Errors due to mass readings:
Start: 0.01/228.962=0.0000436
Finish: 0.01/228.064=0.000438
Errors due to temperature readings:
0.1/19=0.00526
Errors due to volume of water measurements:
1cm3/100cm3=0.01
Total percentage of errors due to equipments:
0.0000436+0.000438+0.00526+0.01*100=1.0%
Errors for butan-1-ol:
Errors due to mass readings:
Start: 0.01/240.541=0.0000415
Finish: 0.01/239.364=0.0000417
Errors due to temperature readings:
0.1/20=0.005
Errors due to volume of water measurements:
1cm3/100cm3=0.01
Total percentage of errors due to equipment:
0.0000415+0.0000417+0.005+0.01*100=1.0%
Errors for propan-1-ol:
Errors due to mass readings:
Start: 0.01/256.323=0.000039
Finish: 0.01/255.201=0.0000392
Errors due to temperature readings:
0.1/20=0.005
Errors due to volume of water measurements:
1cm3/100cm3=0.01
Total percentage of errors due to equipment:
0.000039+0.0000392+0.005+0.01*100=1.0%
Errors for ethanol results:
Errors for mass readings:
Start: 0.01/183.114=0.0000546
Finish: 0.01/182.059=0.0000549
Errors due to temperature readings:
0.1/20=0.0005
Errors due to the volume of water measurements:
1cm3/100cm3=0.01
Total percentage of errors due to equipment:
0.0000546+0.0000549+ 0.0005+0.01*100=1.0%
Errors for methanol results:
Errors for mass reading:
Start: 0.01/183.114=0.0000546
Finish: 0.01/182.059=0.0000549
Errors due to temperature readings:
0.1/19=0.00526
Errors due to volume of water measurements:
1cm3/100cm3=0.01
Total percentage error due to equipment:
0.0000546+0.0000549+0.00526+0.01*100=1.0%
Bibliography:
The sources that I used to gain information and knowledge for my coursework on The Enthalpy Changes Of Combustion Of Different Alcohols were:
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Salters Advanced Chemistry – Chemical Ideas.
- Chemical Storylines.
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The internet, using search engines such as: - www.google.com
- www.askjeeves.com
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Hazard cards: - cards with information about different alcohols and their properties etc.
