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An Investigation into the Enthalpies of the Combustion of Alcohols.

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Introduction

An Investigation into the Enthalpies of the Combustion of Alcohols Planning My aim for this experiment is to see the energy produced from different alcohols. This investigation involves burning alcohol in the air. 'GCSE Chemistry' by B.Earl and L.D.R Wilford says that "alcohols form, another homologous series, with the general formula Cn H2n+1OH ". The alcohol reacts with the oxygen in the air to form the products water and carbon dioxide. This reaction is exothermic, as heat is given out. This is because the amount reactant energy is more than the product energy the difference between this is ?H, therefore some energy has been given out in the form of heat. The energy is given out when forming the bonds between the new water and carbon dioxide molecules. This can be shown in an energy level diagram: Reaction co-ordinate ?H is the heat content, which is the enthalpy, which is negative in exothermic reactions as the diagram shows that energy is 'lost' as heat. Enthalpy is defined as the energy of reaction, or the heat energy associated with a chemical change. For any reaction carried out directly at a constant pressure, the heat flow is exactly equal to the difference between enthalpy of products and that of the reactants. To measure the heat energy given off, we must use this energy to heat something, this will be water. This is assuming that all the heat produced by combustion of the alcohol will equal the amount of heat absorbed by the water (q). So I will measure the amount of energy required to do so. This can be worked out by using the formula: q = mass x specific heat capacity x temperature rise or q = MC?H. Where q is the quantity of heat. The specific heat capacity is the amount of energy required to heat the substance, and is calculated using the formula: q =energy supplied/ Mass x Temperature, where q is the enthalpy, c is the specific heat capacity and ?H is the temperature rise. ...read more.

Middle

� Bunsen burner, � Retort stand, � Clamp, � Weighing balance Diagram Safety procedures � Make sure goggles and lab coat are worn through out the whole experiment, � Be ware of the hot water in the calorimeter, � Make sure copper calorimeter is tied tightly to the metal rod, � Make sure alcohol is lit safely with your selves rolled up out of the way, � Make sure the sprit lamp is put out by putting on the glass top after use. Method: I plan to carry out this experiment by using all the safety issues and fair testing procedures to give me the most reliable and most accurate set of results on each alcohol. I plan to record the mass of the alcohol sprit lamp before the experiment and after, which would tell me how much alcohol was used in each experiment, which I can compare to other alcohols. I will place 100cm3 of tap water into the copper calorimeter and clamp it to the retort stand; I will then hold the thermometer in the centre of the water in the calorimeter recording just the water temperature. This will give the most accurate set of results as the thermometer will not touch the copper calorimeter at all therefore only recording water temperature and not the calorimeters. I will then light the sprit lamp with a wooden splint wearing my safety glasses, and making sure the tip of the flame reaches the calorimeter flask at the same time will be starting the stop clock. Then I will then let the alcohol burn until the temperature has risen by 20oC for each alcohol, then it will immediately be put out and weighed. I will then record the finishing temperature of the water as the water will still be heated by the conduction of the copper calorimeter. I will then do the same thing for each alcohol and repeat, giving me the most reliable and accurate results, also ruling out any potential anomalies. ...read more.

Conclusion

The Propanol alcohol should have produce a heat energy between the alcohols Ethanol and Pentanol as Pentanol has 5 carbon atoms and ethanol has 2, where as Propanol has 3 making the results in between the two. The odd result of the Propanol alcohol could have been an anomalous result for a number of reasons. It could have been because the tip of the flame might have not been touching the bottom of the copper calorimeter, making the alcohol produce less heat to the water in the calorimeter. It could have also been because of the flame as it might have been too small, which might have also gave a low temperature reading to heat the 100cm3 of water. The procedure of the experiment was good enough of finding out the energy gained from the individual alcohols, but changes could have been made to make the reliability of the results more accurate. This could have been done by using a better and more accurate thermometer, such as an electric thermometer, which have gave me a much more pin point set of results. If I were to do this experiment again I would make a number of improvements or changes, I could use a Bomb Calorimeter which submerges the reaction inside an insulated container of water. An electrical heating device starts the reaction inside a sealed reaction vessel and the temperature rise of the water which surrounds it is measured. Bomb calorimeters are often used to find the calorific value of foods. Looking at the experiment I did I thought we could have extended the results we achieved by using different flame lengths, this would have told us how much the alcohol uses its energy when the flame is increased. I could have also changed the amount of water in the copper calorimeter, or even change the concentration of alcohol to see the effect. Parag Raval 11AC Chemistry Coursework ...read more.

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