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Comparing the enthalpy changes of combustion of different alcohols.

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Introduction

Comparing the enthalpy changes of combustion of different alcohols. All alcohols have different amounts of energy contained in them. This is released when they are burned. To demonstrate this I will burn 5 different alcohols and use them to heat a specified amount of water. I will then measure the enthalpy change for each one. I will then compare them to see how the molecular structure affects the combustion enthalpies. The 5 alcohols I have chosen presented in the table below, along with some data about each of them. I have chosen these 5 because they are the first 5 in the series. Alcohol Formula Melting point (K) Boiling point (K) Methanol CH3OH 179.2 338.1 Ethanol CH3CH2OH 155.8 351.6 Propan-1-ol CH3CH2CH2OH 146.6 370.5 Propan-2-ol CH3CHOHCH3 184.6 355.5 Butanol CH3(CH2)2CH2OH 183.6 390.3 I have included the melting and boiling points to show that the alcohols are liquid at room temperature (298 K). The method I will use to conduct this experiment is a relatively simple one, but one that will produce precise and reliable results. As I have conducted the experiment before, I know the results to be reliable. Risk assessment. Methanol and ethanol are both HIGHLY FLAMMABLE. The vapours catch fire at temperatures above 13�C. The narcotic effect of ethanol is well known, and may come about from inhalation of the vapour. ...read more.

Middle

I also used the Hazcards for help with information for my risk assessment. Method 1. Measure 100cm3 into the calorimeter and record the temperature in �C (100cm3 is a universal measurement). 2. Clamp the calorimeter so that the base is 40 mm above the top of the wick of the spirit burner. This distance must be kept the same throughout to ensure a fair test. 3. Weight the spirit burner including the lid on a balance and record the mass to 2 d.p. Before you place the spirit burner on the balance press the "on/tare" button. 4. Replace the burner under the calorimeter and place the draught shield around the apparatus to eliminate draughts. 5. Light a splint using the Bunsen burner and use this to CAREFULLY light the wick of the spirit burner. 6. Use the thermometer to stir the water during heating. Carry in heating until the water temperature has risen by about 20�C. 7. CAREFULLY replace the lid on the spirit burner to extinguish the flame. Keep stirring the water and not the highest temperature reached. Calculate the difference between the start temperature and the end temperature. 8. Weigh the burner again and record the end mass. Calculate the difference between the start mass and end mass. ...read more.

Conclusion

Evaluation Unfortunately, as neither propan-2-ol nor pentanol were available to use during the experiment, I had to use hexanol. My results are quite different from the correct data. This is because there were several limitations of the practical procedure. One of the main reasons the results are so very different is because of heat convection. A lot of energy was lost due to the heat escaping over the draught shields. This could have been prevented by placing something on top of the draught shield. However this would have made it difficult to stir the water. Also, it was a very simple procedure, so would not be expected to produce precise results. However there were things that could have increased the probability of gathering precise results. More accurate equipment could have been used, for example to measure the water. If a slightly different volume of water was measured for each fuel the results would be affected. There is also a certain amount of error associated with each piece of equipment, for example the error of the balance used in this experiment is 0.005g. To show how to calculate the percentage error, I will use the value for the initial mass of the methanol. Percentage error = error x 100 = 0.005 x 100 = 0.5 = 0.003% Reading 197.13 197.13 Harriet Stubbs 25/11/03 ...read more.

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