Reactions to Drive a Nonspontaneous Change

When studying a multistep reaction, chemists often find that a nonspontaneous step is driven by a spontaneous step in a coupling of reactions. One step supplies enough free energy for the other to occur, as when the combustion of gasoline (spontaneous) supplies enough free energy to move a car (nonspontaneous). 

Look again at the reaction of copper(I) oxide with carbon. Previously, we found that the overall reaction becomes spontaneous above 352 K. Dividing the reaction into two steps, we find that, even at the slightly higher temperature of 375 K, decomposition of copper(I) oxide to its elements is not spontaneous  

Coupling these reactions allows the reaction with the larger negative AG to drive the one with the smaller positive AG. Adding the reactions together gives 

Many biochemical processes—including the syntheses of proteins, nucleic acids, and fatty acids, the maintenance of ion balance, and the breakdown of nutrients—have nonspontaneous steps. Coupling these steps to spontaneous ones is a life-sustaining strategy that is common to all organisms. A key spontaneous 

CHAPTER 20 Thermodynamics Entropy, Free Energy, and the Direction of Chemical Reactions 

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The Eree Energy Change and the Work a System Can Do 671 The Effect of Temperature on Reaction Spontaneity 671 Coupling of Reactions to Drive a Nonspontaneous Change 674... 

Derive the free energy change (AG) from the second law, and explain how AG is related to work explain why temperature (7) affects the spontaneity of some reactions but not others describe how a spontaneous change drives a nonspontaneous one calculate AG°xn from A// and S° values or from AGf values and quantify the effect of T on AG° obtain the T at which a reaction becomes spontaneous ( 20.3) (SPs 20.4-20.6) (EPs 20.35-20.50)... 

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. 

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