Question

Hess's Law

Show how to calculate the standard enthalpy of formation (ΔH^o_f) of ethanol C₂H₅OH(l) from the heat of combustion of ethanol, which is -1368 kJ/mole, by using tabulated standard enthalpies of formation for CO₂(g) and H₂O(l).

ΔH^o_f(C₂H₅OH,l) = ________ ? ______

Answer 1

The balanced equation for the combustion of ethanol (l) is

C2H5OH(l) + 3O2(g) ==> 2CO2(g) + 3H2O(l) . . .delta H = -1368 kJ

The delta H for any reaction can be calculated as

delta H reaction = sum of delta H of formation of products - sum of delta H of formation of reactants

From tables:

delta H f CO2(g) = -393.5 kJ/mole
delta H f H2O(l) = -285.8 kJ
delta H f O2(g) = 0

delta H reaction = ((2 moles CO2)(-393.5 kJ/mole) + (3 moles H2O)(-285.8 kJ/mole)) - ((1 mole C2H5OH)(delta H f C2H5OH(l))

-1368 kJ = (-787.0 kJ - 857.4 kJ) - (delta H f C2H5OH(l)
-276.4 kJ = delta H f C2H5OH(l)