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We can think of this reaction as a process in which the atoms of hydrogen and oxygen move from a state of high chemical potential (H2 and 02 molecules) to a state of lower chemical potential (molecules of water vapor). It is reasonable to think of the free energy per mole of a substance as its chemical potential, and to regard a spontaneous chemical reaction as a rolling of atoms down a chemical potential "slope". The free energy given off in a spontaneous process then is just the change in potential (AG per mole) times the amount of substance undergoing the change (number of moles).

In equation form the reaction may be written as follows

free energy emitted = change in potential x amount of reaction

For the water-vapor reaction, the free energy change per mole of water vapor formed is -54.64 kcal mole-1 H2O. This is the chemical potential drop during the water reaction. If fifty moles of water vapor are produced, then the total free energy given off is

DG0 = (-54.64 kcal mol-1) (50 moles) = -2732 kcal

In the language of gravitation, this amounts to rolling 50 balls down a 54.64 kcal hill. The concept of a potential of some kind to explain why spontaneous processes take place is a useful one

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