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Determination of the enthalpy change of combustion

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Introduction

Chemistry Laboratory Report 5 Date: 4th, November, 2008 Topic: Determination of the enthalpy change of combustion Objective: To determine the enthalpy change of combustion of a series of alcohols Introduction In this experiment, a measured mass of an alcohol is burnt and the heat released is transferred to a calorimeter made from an aluminium can. The heat capacity of the calorimeter is first determined by calibration with methanol of known enthalpy change of combustion (715 kJmol-1). From the rise in temperature of water in the calorimeter, the enthalpy change of combustion of ethanol and propan-1-ol can be calculated. Heat capacitor of the calorimeter can be calculated by, where n is no. of moles of methanol and C is heat capacitor of calorimeter Enthalpy change of combustion can be calculated by: ?Ho[alcohol] Results Methanol Ethanol Propan-1-ol Initial mass (g) 182.06 166.06 164.49 Final mass (g) 180.36 164.78 163.22 Change in mass (g) 1.70 1.28 1.27 Initial temperature (oC) ...read more.

Middle

Therefore, the energy required for 1 mole of alcohol to undergo complete combustion increasing along the homologous series. -Accuracy of Results and Source of Errors The value obtained from the experiment is greater than the actual value. It may due to some sources of errors in the experiment: > Heat may be lost to the surroundings To reduce heat lost to the surroundings, the hole of the can was blocked by tissue paper to avoid heat loss by convection. Moreover, cotton wool or cloth was used to wrap the can to reduce heat loss by conduction. > Some heat is brought away by air instead of absorbed by the calorimeter A shield formed by insulating mats was installed for the experimental set-up so that heat will not be brought away by air current easily. > Incomplete combustion Due to insufficient supply of oxygen, carbon or carbon monoxide may have been produced rather than carbon dioxide. ...read more.

Conclusion

Large proportion of heat from combustion is absorbed by the aluminium can. If the heat capacity of the aluminium can is ignored, the error in calculation would be very significant. The percentage error of the experimental result will be more than 30% which is very inaccurate. -Choice of Container In the experiment, aluminium can is used instead of glass or other materials. This is because aluminium is a good thermal conductor. Also, it has high melting point which can withstand with burning. Moreover, it's light and can be obtained easily. However, it is not transparent so we cannot observe if convection currents affect the measurement of temperature by data-logger. Conclusion In the experiment, the heat capacity of the can is 1.84 kJoC-1. Also, the enthalpy change of combustion for ethanol and propan-1-ol are -1414.9 kJmol-1 and -1926.8 kJmol-1 respectively. The objective of the experiment has been fulfilled as the enthalpy change of combustion of alcohols is successfully determined. ?? ?? ?? ?? P. 1/3 ...read more.

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