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Comparing the enthalpy changes of combustion of different alcohols

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Introduction

Kate McAllister 12 VK Chemistry Coursework Planning assessment- Comparing the enthalpy changes of combustion of different alcohols Aim: I am going to investigate the difference in enthalpy of combustion for a number of different alcohols. My aim is to find out how carbon chain length affects the enthalpy change that occurs during the combustion of alcohols. The enthalpy change of combustion of a fuel is a measure of the energy transferred when one mole of the fuel burns completely in oxygen. I will be able to calculate a value for enthalpy change by burning different fuels to heat a specific amount of water using the fact that 4.2J of energy are required to raise the temperature of 1g of water by 1oC. I will produce a wide range of results and will be able to compare them, calculate their enthalpy change of combustion and effectively find an answer my brief. Apparatus: Spirit burner (75 cm3) 500 ml copper can (diameter of 10 cm) with specially adapted lid Boss, clamp and retort stand Electronic balance (to 2 d.p.) Heatproof mat Goggles Measuring cylinder 100 cm3 Measuring cylinder 200 cm3 Mercury thermometer Insulation chamber Stirring propeller Distilled water Methanol (50 cm3 ) *3 Ethanol (50 cm3 ) *3 Propan-1-ol (50 cm3 ) *3 Butan-1-ol (50 cm3 ) *3 Pentan-1-ol (50 cm3 ) *3 Hexan-1-ol (50 cm3 ) *3 Hazard Cards Combustion is principally the oxidation of carbon compounds by oxygen in air to form CO2 if there is a sufficient amount of oxygen. The hydrogen in a compound forms H2O. Combustion produces heat as well as carbon dioxide and water. The energy contained in the bonds of the products is less than the energy contained in the bonds of the reactants. The difference in energy is released as heat. Energy releasing reactions are called exothermic reactions. Calorimetery is a way to determine the amount of heat produced in a reaction. ...read more.

Middle

Notably draughts could have an catastrophic effect on the factor of efficient heat transfer and could be detrimental to the outcome of the results. This is why I have placed a insulation chamber around the experiment and used a specialised lid for the calorimeter. The adverse effects of heat loss would be terrible. With a lid on the calorimeter it means that heat that rises as the liquid changes state to gas condenses back and will be part of the calculation. Loss of the alcohol its self due to evaporation has also been carefully considered. I have decided to ensure that the lid remains on at all possible times, in-between weighing and after the experiment has finished. In regarding the position of certain equipment. I have already stated that the calorimeter should be placed at 1 cm above the flame at all times to ensure accurate heat transfer. The size and shape of the spirit burners is essential. The wick especially needs to have a constant length, width and surface area, so that the same amount of alcohol is in contact with the air to react at all times. I have also decided that the distance the thermometer is suspended in the calorimeter should remain constant as an extra precaution to stirring. The temperature of the water may differ at the top to that at the bottom of the can because heat rises so I have decided to suspend the thermometer at an equal distance of half way up in the centre of the can to ensure fairness. Insulation has been another priority of mine to minimise heat loss. I have silvered the sides of the copper can to minimise heat loss by radiation and insulated the sides of the copper can and the insulation chamber to minimise heat loss by conduction and convection. Heat loss risks affecting the end results of the experiment. ...read more.

Conclusion

by the no. Of moles used (answer from step 4.) The answer to this final sum will give the overall enthalpy of combustion value for the particular alcohol I have used. These values should be worked out for all the alcohols and submitted into a table. An average enthalpy change for each alcohol should be calculated by using the mean. This is because we repeated the experiment for each alcohol three times to minimise the chance of anomalous results. Then the information should be plotted onto graphs with the no. Of carbon atoms in the chain for each alcohol plotted against the enthalpy change of combustion. This will allow me to compare the data more easily as a line of best fit can be drawn. As well as my own results I aim to plot the theoretical values I have stated to see if there is much difference. The equipment I have chosen is what I feel is suitable for my task. I feel I have adequately chosen equipment with enough precision and accuracy for my task. I think a preliminary test is in order however to support my views on the use and size of equipment and I shall use it as a chance to alter and adjust anything which perhaps does not fit precisely into my plan of thought. As a further addition to my experiment I hope to perhaps investigate the effect of branching and position of the functional OH group and see what affect it has on the enthalpy of combustion values. With this extra work I would perhaps consider the use of a bomb calorimeter. This would enable me certainly to increase my accuracy. The principle is the same as the experiment I have actually designed. Energy is transferred from the combusted fuel to the surrounding water, and the temperature rise is measured. This apparatus just gives a more accurate value, provided the readings are taken quickly, because energy losses to the surroundings are reduced to almost zero. Bomb calorimeter Positional isormerism Bracnching. Preliminary testing ...read more.

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