What mass of enthanol must be burned to supply 500 kJ of heat?

Question

Ethanol-----> (C2H5OH (liquid)

The standard enthalpy of combustion of ethanol at 298 K is -1368 . mol ^-1.

Please explain.

Thanks in advance!!!!

Answer 1

ethanol has molecular weight = 46.07 g/mole

500 kJ x (1 mole / 1368 kJ) x (46.07 g / mole) = 16.84 g

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are you sure about that answer of 16.12g?   This is a very straightforward calculation.   According to wikipedia, the heat of combustion of  ETOH is about -1370 kJ / mole and the molecular mass = 46.07 g/mole.  So those numbers check out.   Maybe the kJ you need is off?...478.8 kJ instead of 500 kJ gives 16.12 g.  

Answer 2

Heat produced by burning 1 mol of ethanol = 12.6 kJ / (109/46) kJ mol-1= 644kJ

Therefore DHc [CH3CH2OH] = -644kJ mol-1

Answer is much smaller than the accepted value of -1371 kJ mol-1.

    * Heat is lost to surroundings despite precautions

    * Some heat is given to the calorimeter (the glass beaker). We could allow for this if we calibrated the calorimeter.

    * Incomplete combustion of reactants, leading to formation of soot and carbon monoxide

500/644=0.776 mole should be burnt.