In the combustion of ethanol to liquid water, the products are
more ordered than the reactants by 33.1 cal deg-1, so
the reaction is impeded by entropy. The reaction is less spontaneous
because order is generated in the products. Of the 327 kcal of heat
given off by the reaction, only 317 kcal are capable of being harnessed
to some other useful process. Ten kilocalories of heat are unavailable,
the price paid for creating order in the products. This is why G
is called "free" energy. It represents the portion of the energy
released that is free and available for other purposes. In this
reaction the free energy is less than the total heat evolved because
of the cost of creating order. In the HC1 explosion the free energy
drop was even larger than the heat evolved; the added energy came
from the production of disorder. Entropy, as well as enthalpy, is
a driving force for or against reactions.
The vector diagram for ethanol combustion is shown above. DH0
is negative and is represented by a downward arrow. Because DS0
is negative, the quantity (-TDS0)
is positive and is shown by an upward arrow, which works against
DH0. The resultant arrow,
DG0, is negative, but
less so than DH0. However,
the reaction remains spontaneous.
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