Conclusion: The results show that our experiments did not trap 100 percent of the energy released from each fuel. The graph below shows that the results we obtained do not the follow the pattern of the book of data as they go up and down there as the book of data shows the energy released per j/g increased as the chemical bonds did.
In a molecule there are bond energies that hold the atoms together. When the fuel combusts a chemical reaction takes place, this breaks the bonds, this requires energy, and makes new bonds this gives out energy. So the more bonds you have the more energy is given out to break the bonds. The results shown the average energy released because we did the experiment three times and took the average. Octanol gave out the most energy and methanol gave out the least this is to with the chemical bonds.
Evaluation: The graph and the results shows that the investigation could have a lot of improvements because we got 7 anomalous results. The energy trapped from our experiment was much less than the book of data. The accuracy all the measurements could have one of the reason why the results were so low. When the temperature was measured some groups used alcohol thermometer and some used digital. This meant the accuracy were different. The alcohol thermometer had a accuracy of 0.5 to the nearest centigrade, where as the digital thermometer had a accuracy of 0.05 the nearest centigrade. The digital thermometer gave a much more accurate reading. This could have affected the results because when we were taking the average results it would not have been as good as could have been and when it came to calculating the energy released would not be as accurate as it should be.
The water was measured with an 100ml measuring cylinder which has a accuracy of 0.5 to the nearest centimetre. If the water was not the same amount each time the results could be slightly inaccurate which is why we take a average. If the water was over 150ml the temperature reading would be lower than it should because it would take longer to heat the water in three minutes because there is a bigger mass to heat. We could have uses a smaller measuring cylinder such as a 25ml which would have a accuracy 0.25 to the nearest centimetre. When we weighed the mass of the fuel the mass was very accurate and had a accuracy of 0.005. People have different reaction times and this could have affect the results because the fuel could have brunt slightly more giving a inaccurate result, the temperature could have gone up in this time. Most people have a reaction time of about a second and so the temperature should have been taken 1 second before the actual time. The wick should have been 0.5cm but this was not always the case. This would have made a difference to the results because if the wick is longer energy is lost and so we are not trapping hundred percent energy because it would be incomplete combustion. I think this was definitely one of the reasons our results were low .
The mass of the fuel was worked out by weighing the fuel before and after it was burnt and take away them away from each other to get the mass. This is not a good way of calculation the mass because even though the mass balance is very accurate it is very sensitive to the surrounding and mass could be add on just from the wind blowing on it. This could be solve by closing all windows and stand well back from the balance. The method we used had a lot problems with it. To get complete combustion the tip of the flame has to be at the bottom of the can but not so it is squashed. The can attached to the clamp must be adjusted so that this does not happen. Our experiment was affected by room conditions such as wind blowing the flame which could be solve by closing the windows and having a abstractor on to get rid of any flumes. Room condition could also affect the results because when we did the first run it was a hot day so the temperature rise might be slightly wrong. We could have decided on a start temperature and cool the thermometer and water down to this temperature.
When we did the experiments some groups had longer flame than others. The longer the flame the more energy is lost to the surrounding area and this is one of the reasons why we did trap hundred percent fuel. The idea of the experiment is to transfer the energy to the fuel to heat the water so we knew the energy released. This could be improved by using a calorimeter which is a instrument used to measure the amount of heat transferred. It consists of an insulated container of water, a stirring device and a thermometer. When the heat source is a chemical reaction, such as the burning of a fuel, the reacting substances are placed in a heavy steel vessel called a bomb. The bomb is placed within the calorimeter, and the reaction is started by ignition with an electric spark. The can we used was a steel one which is a metal. Metals conduct heat themselves so some of the heat energy would be trapped in the steel can. Steel is especially a good conductor so you would used a non-metal which didn’t keep the heat in although not much heat would be trapped in the can. The clamp, which is metal, and was touching the can which will mean some of the heat was transferred into the clamp and stand, causing more heat loss form the experiment. You could insulate the can and the clamps.
Incomplete combustion is another problem where there is a restricted supply of oxygen and the alcohol was burning with an orange flame rather than blue. The fuels that we burnt were alcohols which represented hydrocarbons. These consist of only hydrogen and carbon atoms. When they were burnt carbon was given off because there is incomplete combustion so it does not all completely convent to carbon dioxide and forms on the steel can. Carbon is a non-metal which means it is a insulator, this is stopping the energy transfer from the fuel to the water. Conduction and convection are ways heat can escape through the air. The insulator around the can will cut this down.
Our results were very low compare to the book of data so energy is being lost. This means we must improve the method to try and achieve hundred percent combustion. I will modify the original plan by:
- Set up the calorimeter as shown in the diagram below. Weigh the mass of the fuel before it is burnt using the mass balance.
- Check the temperature of the water using a digital thermometer.
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Start the reaction by igniting the fuel with an electric spark.
- Check the temperature after 3 minutes and calculate the temperature rise.
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Weigh the fuel again and calculate the mass burnt.
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Repeat again twice to get a fair test.
Diagram:
