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Comparing the Enthalpy Changes of Combustion of Different Alcohols.

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Introduction

Michael Taylor Comparing the Enthalpy Changes of Combustion of Different Alcohols: Planning Assessment The enthalpy change of combustion of a fuel is a measure of the energy transferred when a mole of the chosen fuel burns completely. The value for the enthalpy change in a fuel can be found by using the burning fuel to heat water and using the fact that 4.2J of energy are required to heat 1g of water by 1oC. In this activity I am going to find the enthalpy change of combustion for 5 alcohols: Methanol (CH3OH), Ethanol (C2H5OH), Propan-1-ol (C3H7OH), Propan-2-ol (C3H7OH) and Butan-1-ol (C4H9OH). I am doing this to investigate how molecular structure of a molecule affects its enthalpy change. Equipment List: - Spirit Burners containing: Methanol Ethanol Propan-1-ol Propan-2-ol Butan-1-ol - Small Copper Can or other metal container to act as a calorimeter - 0-110oC Thermometer - 50 cm3 Pipette + Pipette filler - Access to a balance - Bunsen Burner with Splints - Draught Shielding - Clamp Stand - Burn-Proof Matt - Distilled Water Plan: -Put 100 cm3 of water in the calorimeter and record its temperature. (The same temperature each time will produce the most accurate results.) ...read more.

Middle

are harmful. -The results will be more accurate if a lid on the burner is used, as it reduces evaporation of the alcohol. I have chosen the named alcohols, as they are all straight chain alcohols progressively gaining a carbon atom from 1 - 4. This will allow for easy comparison of the alcohols. I chose Propan-2-ol to compare structural isomers to other structural isomers. Results Tables: Analysing Evidence: [C=12, H=1, O=16] Methanol (CH3OH): 100 x 4.2 x 14.83 = 6.22 kj per 1.19g of fuel 6.22 = 5.22 kj/g 1.19 Molar Mass: 12 + (3x1) + 16 + 1 = 32 1 mol of Methanol = 32g 5.22 x 32 = 167.04 kj Enthalpy Change of Combustion for Methanol = 167.04 kj Ethanol (C2H5OH): 100 x 4.2 x 14.33 = 6.01 kj per 0.47g of fuel 6.01 = 12.78 kj/g 0.47 Molar Mass: (2x12) + (5x1) + 16 + 1 = 46 1 mol of Ethanol = 46g 12.78 x 46 = 587.88 kj Enthalpy Change of Combustion for Ethanol = 587.88 kj Propan-1-ol (C3H7OH): 100 x 4.2 x 14.83 = 6.23kj per 0.41g of fuel 6.23 = 15.20 kj/g 0.41 Molar Mass: (3x12) ...read more.

Conclusion

This means the Balance has quite a high amount of percentage error. There are other sources of error, such as the positioning of the calorimeter, use of a different batch of distilled water half way through the experiment, and doing the experiment over two days. Overall, however, the experiments results are fairly reliable. They have been made so by doing repeats of the experiment and calculating an average. The more repeats done, the more accurate the results, but time allowed for 3 repeats per alcohol. If I was to do this experiment again, I would more than likely use a longer time period to do it in, rather than spreading out the experiment over 2 days. I would also do more repeats per alcohol, possibly 5 repeats per alcohol. Another way to make the experiment better would be to do it in a small room. You would have to make sure the room however was well ventilated as many of the alcohols are dangerous to inhale. A better draft shield would be helpful, as well as putting the calorimeter closer to the flame. It would make the experiment a lot quicker if a 100 cm3 pipette was supplied, rather than the 50 cm3 pipette, which was supplied, as the majority of the time to do the experiment was taken up by filling the pipette to refill the water in the calorimeter. ...read more.

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