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Comparing the enthalpy changes of combustion of different alcohols.

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Introduction

Rebecca Winter November 2003 Plan Comparing the enthalpy changes of combustion of different alcohols I am going to investigate the enthalpy change of combustion of a number of alcohols. The enthalpy change of combustion of a fuel is a measure of the energy transferred when one mole of fuel burns completely under the stated conditions. Enthalpy is defined as the energy of reaction, or the heat energy associated with a chemical change. I will be investigating how and why the enthalpy change is affected by the molecular structure of the alcohol. Therefore, I will attempt to find how the number of carbon atoms that the alcohol contains affects the enthalpy change that occurs during the combustion of the alcohol. Alcohols are derived from alkanes by substituting an -OH group for an -H atom. They are a series of related organic molecules and have a general formula of: CnH2n+1OH, where 'n' is the number of carbon atoms present. Alcohols have physically and chemically similar properties due to the similarities in molecular structure, and the fact that they differ only in the length and structure of the hydrocarbon chain. In my investigation I will be dealing with the first five alcohols in the homologous series; Methanol, Ethanol, Propan-1-ol, Butan-1-ol and pentan-1-ol. I have chosen five alcohols as I feel that it is a suitable range that I can manage in the given time span including repeats. ...read more.

Middle

Record the initial mass or the spirit burner plus alcohol. NB do not take the lid off the spirit burner until you are ready to perform the investigation as he alcohol will evaporate and interfere with your results and safety issues are a concern * Place the spirit burner under the can (acting as a calorimeter) and light the wick. * Continuously stir water with the glass rod, constantly monitor the temperature rise on the thermometer. * As the final temperature approaches a rise of 20?C be prepared to extinguish the spirit burner. The temperature may continue to rise so this must be taken into consideration when extinguishing the spirit burner. * Record final mass of burner and alcohol using the same chemical balance as previously in the experiment. * Ensure that can is thoroughly washed before repeating the experiment and that all traces of carbon are removed. * Repeat experiment using all the other alcohols using the exact same method. NB: when repeating the experiment all equipment must be at it's original starting temperature to maintain constant conditions. As with any scientific experimentation I need to consider the possible hazards which may occur and do all that is possible to minimize the risks of these hazards occurring. Goggles must be worn and hair tied back throughout the investigation. ...read more.

Conclusion

If the initial temperature is too high, the water may reach boiling temperature and although energy would still be being released, I would not be able to monitor this with my equipment. A very important variable that I must monitor is the temperature rise. I must ensure that the temperature rises 20?C from its initial temperature. This is a difficult variable to maintain, as the temperature will continue to rise after the alcohol has been extinguished. However, I must attempt to maintain that the temperature rise is 20?C, correct to the nearest ?C with each alcohol and repeat reading. I will repeat the investigation with five different alcohols and will repeat each alcohol three times to establish and average enthalpy change. This will help to ensure that my results are precise. I must also ensure that I use alcohols with the -OH bond in the same position each time. I have chosen to use Propan-1-ol, Butan-1-ol and Pentan-1-ol and must keep this constant throughout my investigation. I have learnt previously that the bond energies between two atoms will vary accordingly to where in the molecule the pair of atoms is. For this reason it is important to keep constant the position of the -OH group. I must ensure that the experiment is performed under standard conditions. I.e. 298K (25?C), 1 atmosphere and a concentration of 1mol dm -3. If this condition is not kept at a constant, ?H will vary accordingly to the specific conditions. ...read more.

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