Standard Free Energy Change, AG

Although the Gibbs-Helmhohz equation is valid under all conditions we will apply it only under standard conditions, where— 

You will recall that the sign of AG can be correlated with the spontaneity of reaction. We can do the same thing with AG° provided we restrict our attention to standard conditions (1 atm, 1M). 

//AG° is negative, the reaction is spontaneous at standard conditions. For example, the following reaction is spontaneous at 25°C  

If AG9 is 0, the system is at equilibrium at standard conditions there is no tendency for the reaction to occur in either direction. An example is the vaporization of water at 100°C and 1 atm  

Calcium sulfate, CaS04 is used as a drying agent and sold under the trade name Drierite. For the reaction 

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The dissociation constant, K., and standard free energy change, AG°, for this reaction are related by the equation. 

Cell Volta.ge a.ndIts Components. The minimum voltage required for electrolysis to begin for a given set of cell conditions, such as an operational temperature of 95°C, is the sum of the cathodic and anodic reversible potentials and is known as the thermodynamic decomposition voltage, is related to the standard free energy change, AG°C, for the overall chemical reaction,... 

But spontaneity depends on the concentrations of reactants and products. If the ratio [Bl YCA] is less than a certain value, the reaction is spontaneous in the forward direction if [Bl YCA] exceeds this value, the reaction is spontaneous in the reverse direction. Therefore, it is useful to define a standard free-energy change (AG°) which applies to a standard state where [A] = [B] = 1 M. 

Though LFERs are not a necessary consequence of thermodynamics, their occurrence suggests the presence of a real connection between the correlated quantities, and the nature of this connection can be explored. This treatment follows Leffler and Grunwald. - PP Standard free energy changes AG° will pertain to either... 

Triose phosphate isomerase catalyzes the conversion of dihy-droxyacetone-P to glyceraldehyde-3-P. The standard free energy change, AG°, for this reaction is +7.6 kj/mol. However, the observed free energy change (AG) for this reaction in erythrocytes is +2.4 kj/mol. 

The standard free energy change (AG° ) for hydrolysis of frnc-tose-l,6-bisphosphate (FBP) to frnctose-6-phosphate (F-6-P) and Pi is -16.7 kJ/mol ... 

The fixation of carbon dioxide to form hexose, the dark reactions of photosynthesis, requires considerable energy. The overall stoichiometry of this process (Eq. 22.3) involves 12 NADPH and 18 ATP. To generate 12 equivalents of NADPH necessitates the consumption of 48 Einsteins of light, minimally 170 kj each. However, if the preceding ratio of l ATP per NADPH were correct, insufficient ATP for COg fixation would be produced. Six additional Einsteins would provide the necessary two additional ATP. Prom 54 Einsteins, or 9180 kJ, one mole of hexose would be synthesized. The standard free energy change, AG°, for hexose formation from carbon dioxide and water (the exact reverse of cellular respiration) is +2870 kj/mol. 

All of the free energy calculations to this point have involved the standard free energy change, AG°. It is possible, however, to write a general relation for the free energy change, AG, valid under any conditions. This relation is a relatively simple one, but we will not attempt to derive it. It tells us that... 

Key Terms enthalpy, H free energy of formation, AG standard entropy change, AS° entropy, S spontaneous process standard free energy change, AG° free energy, G... 

As pointed out previously, the value of the standard cell voltage, E°, is a measure of the spontaneity of a cell reaction. In Chapter 17, we showed that the standard free energy change, AG°, is a general criterion for reaction spontaneity. As you might suppose, these two quantities have a simple relation to one another and to the equilibrium constant, K, for the cell reaction. 

Rearrange to relate Zgq to the standard free energy change AG° — -R ZlnZeq... 

Fig. 10. Relationship between (AC i,2 —0.5 ACJi.i) and the standard free energy change (AG ij) of the redox reactions at 25 °C. Open circles, Ce(IV) + Fe(phen)3 reactions in 0.50 Af HjS04. Closed circles, Fe + +Fe(phen)3 reactions in 0.50 M HCIO4. Numbers refer to complexes in Table 32. (From Dulz and Satin, by courtesy of The American Chemical Society.)...

The standard free energy change for this reaction is generally positive at all temperatures because oxides are invariably stabler than chlorides. An exception to this rule occurs in the case of copper because cupric chloride is more stable than cupric oxide. At 500 °C, the standard free energy change (AG°) for the reaction... 

The Relationship Between the Equilibrium Constant and the Standard Free-energy Change, AG°... 

In addition, the standard free-energy change AG° is related to the change in standard redox potential E by... 

The equilibrium constant of a chemical reaction is related with standard free energy change AG° as... 

The model of Fig. 1, in combination with the assumptions mentioned in the text, led Marcus to derive a quadratic expression for the free energy of activation AG of the ET reaction (17), namely (20), where the effective standard free energy change AG° for the ET step is given by (21). 

Thus a useful parameter (AG ) can be calculated for any reaction (not only those operating under standard conditions) providing the relative concentrations of r and p, the temperature and the standard free energy change AG° are known. 

We have shown that standard free energy change (AG ) for a reaction can give information about the equilibrium position in a reversible reaction. 

Determine the spontaneity (feasibility) of the following reactions by determining the standard cell voltage and the standard free-energy change AG for the following cell reactions ... 

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