Standard free energy, chemical reactions

The algebraic sum of the standard chemical potentials is called the standard free energy of reaction and is denoted as ArG°. In our case ... 

Linear free energy relationship (LFER) — For various series of similar chemical reactions it has been empirically found that linear relationships hold between the series of free energies (-> Gibbs energy) of activation AG and the series of the standard free energies of reactions AGf, i.e., between the series of log fc (k -rate constants) and log K (Kt - equilibrium constants) (z labels the compounds of a series). Such relations correlate the - kinetics and -> thermodynamics of these reactions, and thus they are of fundamental importance. The LFER s can be formulated with the so-called Leffler-Grunwald operator dR ... 

The purpose of defining standard free energies of formation is to use these individual reference values to determine standard free energies of reaction, AG°, where the subscript r indicates reaction. By definition, AG° is the free energy change attending a balanced chemical reaction that involves substances at their standard states... 

The standard free energy of reaction AG° may be calculated from standard free energy of formation data in a manner similar to that for the standard enthalpy of reaction. The following equation is used to calculate the chemical reaction equilibrium constant AT at a temperature F ... 

Free energy changes associated with a biochemical reaction are determined at a standard state, with all reactants and products at 1M. Many biomolecules are unstable in acid, so the biochemical standard state is set at pH = 7.0 rather than at pH = 0 (IM acid), the standard state for chemical reactions. Biochemical standard free energies of reaction are given as AG° to where the indicates this change in standard conditions. 

To proceed fiirther, to evaluate the standard free energy AG , we need infonnation (experimental or theoretical) about the particular reaction. One source of infonnation is the equilibrium constant for a chemical reaction involving gases. Previous sections have shown how the chemical potential for a species in a gaseous mixture or in a dilute solution (and the corresponding activities) can be defined and measured. Thus, if one can detennine (by some kind of analysis)... 

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,... 

STRATEGY We write the chemical equation for the formation of HI(g) and calculate the standard Gibbs free energy of reaction from AG° = AH° — TAS°. It is best to write the equation with a stoichiometric coefficient of 1 for the compound of interest, because then AG° = AGf°. The standard enthalpy of formation is found in Appendix 2A. The standard reaction entropy is found as shown in Example 7.9, by using the data from Table 7.3 or Appendix 2A. 

What Do We Need to Know Already The concepts of chemical equilibrium are related to those of physical equilibrium (Sections 8.1-8.3). Because chemical equilibrium depends on the thermodynamics of chemical reactions, we need to know about the Gibbs free energy of reaction (Section 7.13) and standard enthalpies of formation (Section 6.18). Ghemical equilibrium calculations require a thorough knowledge of molar concentration (Section G), reaction stoichiometry (Section L), and the gas laws (Ghapter 4). 

A catalyst speeds up both the forward and the reverse reactions by the same amount. Therefore, the dynamic equilibrium is unaffected. The thermodynamic justification of this observation is based on the fact that the equilibrium constant depends only on the temperature and the value of AGr°. A standard Gibbs free energy of reaction depends only on the identities of the reactants and products and is independent of the rate of the reaction or the presence of any substances that do not appear in the overall chemical equation for the reaction. 

In the introductory chapter we stated that the formation of chemical compounds with the metal ion in a variety of formal oxidation states is a characteristic of transition metals. We also saw in Chapter 8 how we may quantify the thermodynamic stability of a coordination compound in terms of the stability constant K. It is convenient to be able to assess the relative ease by which a metal is transformed from one oxidation state to another, and you will recall that the standard electrode potential, E , is a convenient measure of this. Remember that the standard free energy change for a reaction, AG , is related both to the equilibrium constant (Eq. 9.1)... 

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 free energy of chemical reactions may be estimated both under the standard conditions and under real, or physiological, conditions. The standard free energy, AG°, of a biochemical reaction is defined as a free energy change under the standard conditions, i.e. at the concentration of reactants 1 mol/litre, temperature 25 °C <298 X), and pH 7. 

An important advance in the understanding of the chemical behaviour of glasses in aqueous solution was made in 1977, when Paul (1977) published a theoretical model for the various processes based on the calculation of the standard free energy (AG ) and equilibrium constants for the reactions of the components with water. This model successfully predicted many of the empirically derived phenomena described above, such as the increased durability resulting from the addition of small amounts of CaO to the glass, and forms the basis for our current understanding of the kinetic and thermodynamic behaviour of glass in aqueous media. 

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

The standard free energy change that accompanies a chemical reaction is the difference between the sum of the free energies of formation of the products minus the sum of those of the reactants, i. e. 

Biochemical reactions are basically the same as other chemical organic reactions with their thermodynamic and mechanistic characteristics, but they have the enzyme stage. Laws of thermodynamics, standard energy status and standard free energy change, reduction-oxidation (redox) and electrochemical potential equations are applicable to these reactions. Enzymes catalyse reactions and induce them to be much faster . Enzymes are classified by international... 

Table 2-2 lists some important chemical reactions along with their standard state enthalpies and free energies of reaction, all in kj/mole. 

Gibbs showed that AG for any chemical reaction is a function of the standard free-energy change, AG°— a constant that is characteristic of each specific reaction—and a term that expresses the initial concentrations of reactants and products ... 

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