Spontaneous change coupled reactions

We can predict whether pairs of coupled reactions will proceed spontaneously by simply summing the free energy changes for each reaction. For example, consider the reaction from glycolysis (discussed in Chapter 19)... 

Free Eneigy Change and Reaction Spontaneity Standard Free Energy Changes Free Eneigy and Work Temperature and Reaction Spontaneity Coupling of Reactions... 

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... 

Ai-C and K as Functions of Temperature 13-7 Coupled Reactions 13-8 Chemical Potential and Thermodynamics of Spontaneous Chemical Change... 

We have seen two ways to obtain product from a nonspontaneous reaction (1) change the reaction conditions to ones that make the reaction spontaneous (mostly by changing the temperature), and (2) combine a pair of reactions, one with a positive AjG and one with a negative Afi, to obtain a spontaneous overall reaction. Such paired reactions are called coupled reactions. Consider the extraction of a metal from its oxide. 

As an example of two reactions that are coupled, look at the phosphorylation reaction of glucose to yield glucose 6-phosphate plus water, an important step in the breakdown of dietary carbohydrates. The reaction of glucose with HOPO 2- does not occur spontaneously because it is energetically unfavorable, with AG° = + 13.8 kj/mol. (The standard free-energy change for a biological reaction is denoted AG0 and refers to a process in which reactants and products have a concentration of 1.0 M in a soiution with pH = 7.)... 

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. 

Remember that G, H and S are all thermodynamic functions of state, i.e. they depend only on the initial and final states of the system, not on the ways the last is reached. As we have seen, for AG = 0 the reaction has reached equilibrium (and in isolated systems AS has reached a maximum). If AG < 0 the reaction was spontaneous, but if AG > 0 the reaction could not have taken place unless energy was provided from other coupled source. If the source is external then the system is not isolated it is closed if there is no exchange of material or open if there is such exchange. In both cases the environmental changes must be taken into account. 

Redox reactions are more conveniently described in terms of relative electrical potentials instead of the equivalent changes in Gibbs free energy. The electrons in Equation 6.8 come from or go to some other redox couple, and whether or not the reaction proceeds in the forward direction depends on the relative electrical potentials of these two couples. Therefore, a specific electrical potential is assigned to a couple accepting or donating electrons, a value known as its oxidation-reduction or redox potential. This redox potential is compared with that of another couple to predict the direction for spontaneous electron flow when the two couples interact—electrons spontaneously move toward higher redox potentials. The redox potential of species /, ), is defined as... 

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