Calculate the enthalpy of combustion of ethanol. – NABTEB QUESTION

When 4.6g of ethanol, CH3CH2OH were burned in a calorimeter containing 1.0kg of water, the temperature rises from 291k to 316k. If the specific heat capacity of water is 4.18kj-¹k¹ and the water equivalent of the calorimeter is 1.3kjk-¹ and the relative molar mass of ethanol is 46. Calculate the enthalpy of combustion of ethanol. 

SOLUTION

Heat evolved on burning ethanol = the heat gained by the water and the calorimeter

Heat gain by Water = 1.00 X 4.18 X 25 = 104.5KJ
Heat gained by calorimeter = 1.3 X 25 = 32.5KJ

NO of moles of ethanol burnt = 6.4/64 = 0.1

:. 0.1 mole ethanol burns to evolve (104.5 + 32.5) KJ = 137KJ
:. 1 Mole of ethanol burns to release 137.0/0.1 = 1370KJ
:. Heat of combustion of ethanol = -1370KJmol-1