Theoretical Heat of Combustion = 13618 – 10488 = 3180 kJ/mole
Octan-1-ol (C8H17OH)
C8H17OH + 12 O2 = 8 CO2 + 9 H2O
Theoretical Heat of Combustion = 21232 – 16248 = 4984 kJ/mole
This data is again reinforced by the results from a preliminary experiment involving Alkanes. Methane, Ethane, Propane and Butane were all burnt using the same method as in this investigation. The results were calculated and are as follows:
The graph produced from these results has help to support my prediction because it is a straight line and proportional. The experiment also helped me to adjust my variables until suitable. These included using a greater quantity of water than originally planned because the calorimeter had a greater volume than expected and this did not submerge the thermometer bulb sufficiently and adjust the desired temperature rise because it took longer than expected.
The theoretical enthalpy of combustion and the preliminary experiment prove that the longer the molecule the more energy is required to break and make bonds
Aim
A fuel is a substance that is generally burned to produce heat energy. Using these alcohol fuels. I am to find out how much heat energy, each alcohol produces per mole.
Once the results have been achieved I also wish to compare the fuels to find out which one is the best and to find out what factors affected the results of the experiment.
Prediction
For my prediction I have used background knowledge and completed a preliminary experiment. As you can see I have worked out the theoretical energy release for each alcohol and from these I would predict that as the number of carbon atoms go up, so will the heat of combustion. I believe that the results will be proportional to the amount of carbon atoms. This is because every time you add an extra carbon bond, you also add two more hydrogen bonds, which means that the relative molecular will increase as well. To break these bonds and to create new ones would require more energy. I predict that this will be a constant amount. When I have my results I will need to figure out the actual amount of energy given out and the molecular mass will be important. To calculate this, I will use the formula stated in the planning:
Heat Transferred = Mass of Substance (g) x Temperature Change (°C)
x Specific Heat Capacity (J)
This number will be slightly varied in my results because although I will try to achieve the same temperature change each time by removing the burner as soon as it reaches the required heat, it is impossible to be completely accurate. The results to the formula above would then be divided by the amount of alcohol burnt as fuel, to give me the amount of energy produced by one gram. Then this would be multiplied by the molecular mass to give me the energy produced by one mole.
Diagram
Equipment List
Spirit Burner and cap containing a liquid alcohol (Ethanol, Methanol, Propan-1-ol, Butan-1-ol, Pentan-1-ol, and Octan-1-ol)
Heat Proof Mat
Clamp Stand
Clamp
Calorimeter
Thermometer (0 - 110°C)
Measuring Cylinder (50 cm3)
Water
Electronic Scales (grams to 2 decimal places)
Bunsen flame with which to light burner.
Method
The apparatus was set up as illustrated above and 50cm3 of water was carefully measured using the measuring cylinder then poured gently into the calorimeter. The cap was held tightly over the wick of the burner, to prevent evaporation, and it was then weighed on the electronic scale to 2 decimal places. The burner was carefully placed under the calorimeter, making sure that the wick was about 15mm below the base of the calorimeter and the temperature of the water was recorded. The cap was quickly removed from the burner and the wick was lit immediately.
The burner was kept alight and the water was stirred until the temperature had risen 10°C, then cap was replaced and held firmly in place. The water continued to be stirred until the temperature stopped rising, this maximum temperature was recorded. The burner was then weighed with the cap held firmly on and the new weight recorded. The water was emptied and the calorimeter washed and dried. Another 50cm3 of fresh water was measured and a second repeat of each alcohol was taken. The whole series of events was repeated for each of the different alcohols.
Fair Test
To ensure a fair test certain variables must be kept constant throughout the experiment:
Mass of water 50cm3. If a too large a volume of water is used, then ensure a constant temperature throughout the volume would be difficult. Allowing a range of range and large inaccuracies of the thermometer readings, also there would be more heat loss due to a larger surface area, therefore a lower energy level is measured. If too little water is used, then the thermometer is not cover completely by the water and therefore takes measurements of the air temperature, or the water may overheat too quickly and boil. If the temperature did become higher, transfer of heat to the air will occur exponentially, and evaporation would decrease the mass of water.
Type of Calorimeter: Copper and the same one used throughout. As copper is used, the transfer of heat to it and the water occurs quickly, but heat is also lost from the water and copper to the air because it is a good conductor. Although this is not perfect for use, it is substantially better than glass, which is a poor conductor and therefore it would that a longer time for the heat to reach the water giving an inaccurate result.
Temperature rise: 10°C (as accurate as possible). This allows easier comparison, but due to the nature of this experiment it is impossible to stop the heat from transferring a short time after the burner has been removed. Therefore removing it as soon as the temperature rises 10°C and recording the maximum final temperature allow the formulas to be adjusted and a close comparison made. Prevent the water from heating to a high temperature prevent the risk of scalding if an accident occurred.
Surrounding Temperature: room temperature 21°C. Heat transfer to the environment is impossible to prevent, but prevent drafts and keeping the room at a constant temperature reduces this effect and prevents fluctuations in the transfer rate, reducing its effect on the results.
The height of beaker from wick: 15mm. The wicks produce different sized flames and although it is reduced by keeping the wicks at similar length it does occur and lead to heat transferring to the environment. 15mm allowed ample oxygen to reach the flame, allowing it to burn without smothering it or hindering its progress; whilst also allowing the heat to reach the calorimeter.
Scales Used. Using the same scales prevents variation in reading, if one of the scales had an error. The scales were also Tared (rezeroed) each time to ensure the measurement started at 0g.
Weight of Spirit Burner with lid (not including contents). This was not kept constant between the different alcohols, but only between repeats to make sure that the weight loss was not due to the burners themselves but only the alcohols burning.
The amount of carbon/soot remaining on the calorimeter. This is maintained by washing and drying the calorimeter. This affects the experiment because carbon soot acts as an insulator and slows down the rate at which heat is transferred to the container and consequently to the water.
The variable aspect to this investigation is:
The type of alcohols used, which varies the number of carbon atoms. The elements have to have only one variable. Therefore a homologous series is ideal, aspects are identical excluding one, the number of carbon atoms. If this were not kept constant, other factors would have to be accounted for in the final results
The also ensure fairness, I decided to repeat the experiment once, to eliminate anomalous results. I chose not to repeat for a third time because of time restrictions. All weights were recorded to two decimal places to reduce the range of inaccuracy.
Safety Precautions
I needed to ensure that the experiment was safe and the preliminary helped to ensure maximum safety. As alcohols as very dangerous and highly flammable, I wore safety goggles and fire resistant overalls at all times. All long hair and loose clothing was tied up and tucked in. The lids were kept on the alcohols at all times to prevent evaporation, inhalation (which can cause dizziness) and any spillages. The Bunsen burner was kept on a yellow flame, so it was visible at all times
General Laboratory rules were followed at all times, these involved keeping workspace tidy, keeping stools under benches, not running etc.
Equipment was always handled carefully and if any apparatus was hot, safety gloves were used.
Results
See Table and Graph on following pages
Analysis
As you can see, I obtained enough results to calculate the amount of energy produced per gram and per mole. It was calculated by the following equation:
Heat Transferred (J) =
Mass of Substance (g) x Temperature Change (°C) x Specific Heat Capacity (J)
This equation remains the similar throughout all the different alcohols because the variables remain within a certain range. This figure is then used to calculate how much energy is produced per gram, by using this equation:
Energy per Gram of Fuel (J) = Heat Transferred (J) x Mass of fuel Burnt (g)
The final column on the table shows the energy produced by a mole of an alcohol, which is calculated by using this equation:
Energy per Mole of Fuel (kJ)= Energy per Gram of fuel (J) x Molecular Mass of Alcohol (g)
1000
This number increased as the number of carbon atoms increased, this is a positive correlation as shown on the graph. The best-fit line is straight with a positive correlation and shows that the results are also proportional and increase by 123 kJ per carbon atom. The graph does not go through 0 and this is because if there were no carbon atoms, the hydrogen and oxygen remaining would still produced energy when burnt and therefore it will always be greater than 1.
The data collected from data books also had a best-fit straight line and was positive and proportional. This showed that my results followed the same pattern and were quite accurate. The very accurate figures in the data book increase at a higher rate of 652 kJ (average) per carbon atom. The middle line on the graph is the line plotted for the theoretical heat of combustion that was calculated in the planning. Again, this has a best-fit straight line and is therefore proportional, it increases at a rate of 618kJ per carbon atom. This line is almost parallel to the data from data books, but it is slightly lower because the energy values are for when all reactant and products are gases. The data from books includes the energy required to turn the liquids into gases. The energy required to do this can be calculated by subtracting the data (from books) and the theoretical values line.
My predictions was correct because my results have shown that as the number of carbon atoms increases so does the heat produced by the alcohol. The results are also proportional and they do produce straight-line graphs as predicted, although the three lines are not parallel. The reason for why the energy produced increase is because the larger number of carbon atoms require more fuel to be burnt and it also provides a larger surface area on which the reaction can take place.
Evaluation
The results that I got from my experiment were accurate enough to give me results I could rely on, but they could never have been as accurate as the theoretical calculations because there were many experimental errors involved. The theoretical results and the ones for a data book are much higher than my own which show that my results were not the true values and therefore are not accurate. My own results are not parallel to either of the other two lines and this indicates that the margin of error increases as the number of carbon atoms increases, a specific reason for this is unknown, but I think that it is due to inaccuracy in measurement readings. All the errors that could account for these problems and the huge loss of heat between the burning alcohol and the water are:
- Energy given off as sound and light.
- Heat conducted and convected away through the air
- Radiation of heat into the atmosphere.
- The variation in water temperature throughout the calorimeter.
- Evaporation of the water so there would have been less water to heat, and therefore make the water temperature rise more quickly
- The heat transfer to the calorimeter so not all of the heat is transferred to the water.
- The rubber clamp transferred heat away
- Higher humidity and temperatures allow for faster heat loss to the air and out of the calorimeter, due to a bigger heat difference, making the higher temperatures more inaccurate, and making a shallower gradient on the graph.
- Incomplete combustion of the alcohols, leaving traces of soot behind on the calorimeter acting as an insulator.
- The energy produces by the burning wick, itself.
- The area of the wick would increase or decrease the amount of fuel burnt at one time. (This would not have a great effect)
- The difference in flame sizes due to each specific alcohol, and the distance between the tip of flame and the calorimeter was not the same throughout.
All of the above makes a difference to the calculated energy change and the results. Insulating the sides of the calorimeter, using a different material for the calorimeter, and using a lid to prevent evaporation and heat loss could have made improvements, although not all aspects could be improved with the equipment available. Perhaps if I started the experiment below room temperature, then the experiment could produce better results. To reduce incomplete combustion a supply of oxygen would have to have been provided to allow all the carbon to form new bonds and not to remain as soot.
The limitations in this experiment were bad, because heat is not an efficient energy to measure, because of the heat loss. Any molecule of anything will conduct heat to a greater or lesser extent, radiation happens and can be reduced but not stopped, and the most limiting factor of this experiment is the convection of air, and to a lesser extent of water. In all the transfers of heat through the apparatus, you are giving energy to things other than the water. The wick, air , colorimeter, thermometer, burner, the non combusted alcohol, clamp, boss and stand, are all given energy that, ideally should go to the water and be retained by it. All of these would decrease the temperature. During the experiment, some water will evaporate, and so your temperature/water mass reading will change; because of this, your temperature reading might increase the temperature.
Using a wider range of alcohols could have given a better graph reading and a wider range of results to support a conclusion. Next time reducing heat loss is the main priority.
Alternatively, we can remove most faults in planning by using an advanced technique such as a bomb calorimeter or Nutfield calorimeter. This is the most accurate way of measuring bond energies and this will be as accurate as we can get our results.
Overall, the method was sufficient to provide results, but improvements are necessary to improve the accuracy and prevent heat loss. I thoroughly enjoyed this experiment and I feel that I have gained knowledge of different alcohols and their burning properties, as well as understanding how to calculate bond energies. I think I have collected sufficient results to enable a firm conclusion to be drawn.
To extend this investigation, I would use more alcohols to give a wider range of results and then compare these to the heat produced by Alkanes and Alkenes. I would also like to be able to use more precise and efficient equipment if it is available.
Bibliography
Encarta 2000 Encyclopaedia
USBOURNE Illustrated Dictionary of Science by C Stockley and C Oxlade
Chemistry by Chris Connoly