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Effect of the Position of the OH Functional Group on the Enthalpy of Combustion of Alcohols

Divyamshu Surabhi1* and Dr. Nagamanju S

Energy is one of the fundamental units that drive various processes. Among them, heat or thermal energy is quite abundant in nature; usually released outside the system as a byproduct. The total summation of internal energy stored within reactants or products is Enthalpy (H). The amount of heat lost or gained by a system is equal to the Change in enthalpy of combustion or ΔH. In the present study an attempt has been made to investigate whether the positioning of OH functional group in methanol, 2-propanol and 2-methylpropan-2-ol has any effect upon the standard enthalpy change. Calorimeter was used to calculate the standard change in enthalpy of combustion of alcohol, by raising the temperature of 100 g of distilled water by 30oC from its initial temperature. The water was heated using a spirit lamp with 20 ml of specific alcohol; it’s mass before and after combustion was recorded. Procedure was repeated three times with all three alcohols. The final values obtained were -41.8 ± 0.10 KJ mol^-1, -114 ± 0.39 KJ mol^-1, -114 ± 0.38KJ mol^-1 for methanol, propan-2-ol and 2-mehtylpropan-2-ol respectively. The present study showed that standard enthalpy changes of combustion increased from primary to secondary alcohol. This is due to the same number of C-C bonds or same carbon chain length. Only a slight variation of ± 0.01 KJ mol^-1 was observed. This variation exists as the standard enthalpy change of combustion is not only dependent on the nature of the C-OH bond but also neighboring groups as they have different enthalpy of formation. To conclude the standard change in enthalpy of combustion of alcohols is affected by the number of C-C bonds, its R group and by the phenomenon of geometrical isomerism.