can get the formation reaction we are seeking by subtracting the
first reaction from this one:
2C(s) + 3H2(g)
+ ŻO2(g) «
= (+715.1) - (+771.7) = -56.6 kcal per mole of ethanol
Compare this with the value of -56.3 kcal mole-1 from the table
of measured heats of formation given on Page 22. Accuracy to within
one kilocalorie is considered quite good. We can see what is going
on physically by means of the energy level diagram opposite.
The isolated C, H, and O atoms are in a state 715.1 kcal higher
in energy than graphite crystals and H2
and O2 gases, because
715.1 kcal are required to produce the separated atoms. In turn,
because the calculation from bond energies showed that 771.7 kcal
are required to tear an ethanol molecule apart, the intact molecule
must be 771.7 kcal lower in energy than the separated atoms. The
observable heat of formation of ethanol molecules from elements
(not atoms), unfortunately, is a small difference between two large
= +715.1 - 771.7 = -56.6 kcal mole-1
This is one reason for the relative inaccuracy of bond-energy calculations.
A small percent error in a number the size of 700 means a much bigger
percent error in the difference of 57.