Spontaneous processes coupling with nonspontaneous reactions

C14-0117. The notion of thermodynamic coupling of a nonspontaneous process with a spontaneous process is not restricted to chemical reactions. Identify the spontaneous and nonspontaneous portions of the following coupled processes (a) Water behind a dam passes through a turbine and generates electricity, (b) A gasoline engine pumps water from a valley to the top of a hill. 

The free energy change (AG) of a process is a measure of its spontaneity. The free energy of a system decreases in a spontaneous process that is, if ASuniv > 0, AGsys < 0. In a coupling of reactions, a spontaneous step with a larger negative AG drives a nonspontaneous step with a smaller positive AG. 

Coupled Reactions—Nonspontaneous processes can be made spontaneous by coupling them with spontaneous reactions and by taking advantage of the state function property of G. Coupled reactions, that is, paired reactions that yield a spontaneous overall reaction, occur in metallurgical processes and in biochemical transformations. 

A coupled process links a spontaneous reaction with a nonspontaneous one. In this case, the negative free energy change of the acetyl phosphate reaction drives the conversion of ADP to ATP. 

Fig. 4.12 Iftwo weights are coupled as shown here, then the heavier weight will move the lighter weight in its nonspontaneous direction overall, the process is still spontaneous. The weights are the analogs of two chemical reactions a reaction with a large negative A,G can force another reaction with a smaller A,G to run in its nonspontaneous direction.

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