13. How To Measure Disorder   Previous PageNext Page
       THIRD LAW ENTROPIES AND COMMON SENSE

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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