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Comparing the enthalpy changes of combustion of different alcohols.

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Introduction

Rebecca Johnson Mr Smith Comparing the enthalpy changes of combustion of different alcohols I am going to investigate the enthalpy change of combustion of different alcohols. This will be the 'enthalpy change when one mole of substance is burned completely in oxygen under stated conditions.' I will compare the alcohols by seeing how the number of carbon atoms an alcohol contains affects the enthalpy change of combustion. During the experiment I will take two measurements, which are the mass of the alcohols burned, and the temperature increase. I will then use these to work out the overall energy transferred, which will give me the enthalpy change. I will then compare the enthalpy changes for each alcohol. I will do this by using the equation, Energy transferred to water by burning fuel (Q) = mass of water (m) x temperature rise ( t) x 4.2J (Q) = The energy transferred to the water by burning one mole of the fuel (m) = The mass of the water will be a constant each time at 100cm3 ( t) = The temperature rise will be 200C each time This will give the sum, Q = 100cm3 x 200C x 4.2J = 8400J I will use this answer when working out the enthalpy change of combustion and then I will compare the enthalpy change of the different alcohols. I think that the alcohol that will burn the most will be the one that contains the greater number of carbon atoms. For any reaction to take place bonds must be broken and then made. The bond breaking requires energy (endothermic) whereas the bond making gives out energy (exothermic). Bonds between different atoms require or release different amounts of energy when broken or made because they vary in strength. By looking at the equations for the reactions and the bonds been broken/made estimation can be made for the amount of energy released in the reaction. ...read more.

Middle

Number of times experiment is repeated-I will repeat the experiment three times for each alcohol to improve the accuracy of my results. Same copper can- I will use the same copper can each time so that this will improve the conductivity of heat from the flame to the water. Purity of the alcohol-all of the alcohols will be 99% pure. Risk Assessment There are some risks with my experiment, which I will need to identify and prevent from becoming hazardous. These are: 1. It will be vital that I keep the lid on the spirit burner, because the alcohols ignite above 130C. 2. The vapours of the alcohols produce narcotic effects if inhaled therefore victims should be removed to fresh air. 3. If swallowed the mouth must be washed out with water. 4. If it is spilt on eyes, flood with running tap for 10 minutes, then seek medical attention. 5. If spilt on clothing or skin remove clothing to prevent fire risk and wash the affected area. 6. If spilt shut off all sources of ignition and wash with mineral absorbent. Results *The results are shown in the table over the page. I worked out the total enthalpy changes of the different alcohols by firstly using the equation from the beginning, which gave me 8400J as the enthalpy transferred to one mole of all of the fuel. I then used this to work out the total enthalpy change of combustion. This was done using a certain number of steps, which are shown below in working out the enthalpy change for methanol. 1. Write the formula of the fuel-Methanol = CH3OH 2.Work out the mass of 1 mole of the fuel using the equation, mass/molar mass = 1.10/32 = 0.03moles 3.Work out the energy transferred by using one mole of the fuel = 1mole/0.03 x 8400J = 280 000J 4.The enthalpy change of combustion is worked out by doing, 280 000/1000 = -280KJ mol-1 The same steps were conducted for each of the alcohols and the answers are shown in the table below. ...read more.

Conclusion

If I were to do this experiment again I would make a few alterations. To prevent less heat escaping I would shorten the distance between the burner and the copper can. However this would not prevent all of the heat form escaping therefore I would propose that I would insulate the can and the spirit burner to ensure that the water does not lose any heat and that all of the heat for the burner goes to the water in the can and doesn't escape. As there may have been incomplete combustion I would overcome this next time by ensuring that the wick is long enough and that there is a good air supply as\oxygen is a vital component in combustion. The measuring of the water temperature in the can was a problem therefore if I had al longer thermometer that read to the nearest 0.1 0C then this would make my readings more accurate. Despite the fact that stirring the water will have made a difference, it will have been done in an enclosed beaker, which will have increased the chances of evaporation and consequently reducing the temperature of the water. Also the time taken to reweigh the fuel after burning may have affected the results because there may have been evaporation as the fuel cooled down therefore this will have decreased the mass. Another way in which I could improve the reliability of my results would be to user a bomb calorimeter, because this would prevent any of the problems occurring which will have affected the results from before. By using this type of equipment I would expect the results to be closer to the predicted enthalpies of combustion. Below is a diagram of a bomb calorimeter. There were also other areas of this experiment that I would have liked to have investigated but time was limited. If it was possible I will have investigated into whether the position of the -OH group within the molecule affects the enthalpy of combustion and also if branching in the molecule has any effect on the enthalpy of combustion. ...read more.

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