Additivity of Free Energy Changes Coupled Reactions

Because free energy changes are additive, it is ofien possible to bring about a nonsponta-neous reaction by coupling it with a reaction for which AG° is a large negative number. As an example, consider the preparation of iron metal from hematite ore. The reaction 

Coupled reactions are common in human metabolism. Spontaneous processes, such as the oxidation of glucose, 

ADP (adenosine diphosphate) and ATP (adenosine triphosphate) are complex organic molecules (Fig. 17.9) that, in essence, differ only hy the presence of an extra phosphate group in ATP. In the coupled reaction with glucose, about 38 mol of ATP are synthesized for every mole of glucose consumed. This gives an overall free energy change for the coupled reaction of 

In a very real sense, your body stores energy available from the metabolism of foods in the form of ATP. This molecule in turn supplies the energy required for all sorts of biochemical reactions taking place in the body. It does this by reverting to ADP, that is, by reversing reaction 17.6. The amount of ATP consumed is amazingly large a competitive sprinter may hydrolyze as much as 500 g (about 1 lb) of ATP per minute. 

Many industrial processes involve coupled reactions. 

7 Additivity of Free Energy Changes Coupled Reactions 

Free energy changes for reactions, hke enthalpy or entropy changes, are additive. That is, if Reaction 3 = Reaction 1 + Reaction 2 then AG3 = AGi + AG2 

This relation can be regarded as the free energy equivalent of Hess s law (Chapter 8). To illustrate its apphcation, consider the synthesis of CUCI2 from the elements 

The negative sign of AG° implies that although Fe203 does not spontaneously decompose, it can be converted to iron by reaction with carbon monoxide. This is in fact the reaction used in the blast furnace when iron ore consisting mainly of Fe203 is reduced to iron (Chapter 20). 

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ADDITIVITY OF FREE ENERGY CHANGES COUPLED REACTIONS... 

As is evident from the additivity of free-energy changes of sequential reactions, any phosphoiylated compound can be synthesized by coupling the synthesis to the breakdown of another phosphoiylated compound with a more negative free energy of hydrolysis. For example, because cleavage of Pi from phospho-enolpyruvate (PEP) releases more energy than is needed to drive the condensation of Pi with ADP, the... 

Under standard conditions, A cannot be spontaneously converted into B and C, because AG° is positive. However, the conversion of B into D under standard conditions is thermodynamically feasible. Because free-energy changes are additive, the conversion of A into C and D has a AG° of — 13kJmol ( —3kcalmoU ), which means that it can occur spontaneously under standard conditions. Thus, a thermodynamically unfavorable reaction can be driven by a thermodynamically favorable reaction to which it is coupled. In this example, the reactions are coupled by the shared chemical intermediate B. Thus, metabolic pathways are formed by the coupling of enzyme-catalyzed reactions such that the overall free energy of the pathway is negative. 

Because the large negative free energy change of corrosion is favored by thermodynamics, it will undoubtedly remain a problem in several areas of modern society. Many scientists are employed in research to find ways to hinder corrosion, either by slowing its kinetics or by coupling additional chemical reactions to the corrosion reactions to make them less thermodynamically favorable. The details of such schemes can become quite complex but are based on the general concepts of electrochemistry that we have explored in this chapter. 

The concept of coupled reactions is of great importance for biological reaction mechanisms. If the transformation of one substance to another by a reaction which is endergonic can proceed by a pathway which involves a highly exergonic reaction, then the net free energy change is simply additive and the reaction may proceed essentially to completion. 

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