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# 'Enthalpy of Combustion'.

Extracts from this document...

Introduction

Planning Alcohols are very similar to alkanes and alkenes; they are a family of related compounds - a homologous series. Each member of the series has the O-H group in its molecule. The general formulas for alcohols is Cn H2n+1 OH, where n is the number of carbon atoms. There are six alcohols that I will be using and they are Methanol (CH3OH), Ethanol (CH2H5OH), Propan-1-ol (C3H7OH), Butan-1-ol (C4H9OH), Pentan-1-ol (C5H11OH) and Octan-1-ol (C8H17OH). In this experiment I will be burning a range of alcohol's to heat up a container of water. I will be trying to get reliable results that will tell me how the number of carbon atoms relate to the energy released as heat , otherwise known as the 'Enthalpy of Combustion'. The combustion process is the making and breaking of bonds. In this experiment, the alcohols will have an exothermic reaction because heat is given out. The breaking of bonds is endothermic and the making of bonds is exothermic. In an exothermic reaction the products are at a lower energy level than the reactants, the difference is the heat energy. The energy is given out when forming the bonds between the new water and the carbon dioxide molecules. The complete combustion of an alcohol is when it reacts with oxygen in the air to form water and carbon dioxide. The basic formula for this reaction is Heat Transferred = Mass of Substance (g) x Temperature Change (�C) x Specific Heat Capacity (J) The specific heat capacity is the number of joules required to heat one gram of water by 1�C. Water is used because it is safe, easily found and has a reliable heat capacity of 4.2. The bonds that are formed in an exothermic reaction can be of two types. The first could be ionic, where a metal is produced. Ionic bonding involves two electrons transferring from one atom to the other leaving an electrostatic force between them. ...read more.

Middle

The burner was then weighed with the cap held firmly on and the new weight recorded. The water was emptied and the calorimeter washed and dried. Another 50cm3 of fresh water was measured and a second repeat of each alcohol was taken. The whole series of events was repeated for each of the different alcohols. Fair Test To ensure a fair test certain variables must be kept constant throughout the experiment: Mass of water 50cm3. If a too large a volume of water is used, then ensure a constant temperature throughout the volume would be difficult. Allowing a range of range and large inaccuracies of the thermometer readings, also there would be more heat loss due to a larger surface area, therefore a lower energy level is measured. If too little water is used, then the thermometer is not cover completely by the water and therefore takes measurements of the air temperature, or the water may overheat too quickly and boil. If the temperature did become higher, transfer of heat to the air will occur exponentially, and evaporation would decrease the mass of water. Type of Calorimeter: Copper and the same one used throughout. As copper is used, the transfer of heat to it and the water occurs quickly, but heat is also lost from the water and copper to the air because it is a good conductor. Although this is not perfect for use, it is substantially better than glass, which is a poor conductor and therefore it would that a longer time for the heat to reach the water giving an inaccurate result. Temperature rise: 10�C (as accurate as possible). This allows easier comparison, but due to the nature of this experiment it is impossible to stop the heat from transferring a short time after the burner has been removed. Therefore removing it as soon as the temperature rises 10�C and recording the maximum final temperature allow the formulas to be adjusted and a close comparison made. ...read more.

Conclusion

The limitations in this experiment were bad, because heat is not an efficient energy to measure, because of the heat loss. Any molecule of anything will conduct heat to a greater or lesser extent, radiation happens and can be reduced but not stopped, and the most limiting factor of this experiment is the convection of air, and to a lesser extent of water. In all the transfers of heat through the apparatus, you are giving energy to things other than the water. The wick, air , colorimeter, thermometer, burner, the non combusted alcohol, clamp, boss and stand, are all given energy that, ideally should go to the water and be retained by it. All of these would decrease the temperature. During the experiment, some water will evaporate, and so your temperature/water mass reading will change; because of this, your temperature reading might increase the temperature. Using a wider range of alcohols could have given a better graph reading and a wider range of results to support a conclusion. Next time reducing heat loss is the main priority. Alternatively, we can remove most faults in planning by using an advanced technique such as a bomb calorimeter or Nutfield calorimeter. This is the most accurate way of measuring bond energies and this will be as accurate as we can get our results. Overall, the method was sufficient to provide results, but improvements are necessary to improve the accuracy and prevent heat loss. I thoroughly enjoyed this experiment and I feel that I have gained knowledge of different alcohols and their burning properties, as well as understanding how to calculate bond energies. I think I have collected sufficient results to enable a firm conclusion to be drawn. To extend this investigation, I would use more alcohols to give a wider range of results and then compare these to the heat produced by Alkanes and Alkenes. I would also like to be able to use more precise and efficient equipment if it is available. ...read more.

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