Standard free-energy changes

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,... 

The standard free energy changes for the process graphite and... 

T) is the absolute temperature, and (AGO) is the standard free energy change. 

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... 

At equilibrium, the ratio of concentrations is an equilibrium constant, so we can write the standard free energy change for the process as... 

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 ... 

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

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. 

Remembering that AC = -nEF, it follows that the standard free energy change for the half reaction is AC° = -nE°F. e.g. ... 

The standard free-energy change for a reaction M + X - MX is also related to the equilibrium constant for the corresponding reaction... 

Heat of Precipitation. Entropy of Solution and Partial Molal Entropy. The Unitary Part of the Entropy. Equilibrium in Proton Transfers. Equilibrium in Any Process. The Unitary Part of a Free Energy Change. The Conventional Standard Free Energy Change. Proton Transfers Involving a Solvent Molecule. The Conventional Standard Free Energy of Solution. The Disparity of a Solution. The E.M.F. of Galvanic Cells. 

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.)... 

Using AG values from Appendix 1, calculate the standard free energy change at 25°C for the reaction referred to in Example 17.3. 

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... 

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