Gibbs standard free energy change

Having calculated the standai d values AyW and S" foi the participants in a chemical reaction, the obvious next step is to calculate the standard Gibbs free energy change of reaction A G and the equilibrium constant from... 

The standard Gibbs free energy change per mole accompanying vaporization (the conversion of a substance from the liquid state into the vapor state), standard hydrogen electrode (SHE) A hydrogen... 

Once the standard states for the various species have been established, one can proceed to calculate a number of standard energy changes for processes involving a change from reactants, all in their respective standard states, to products, all in their respective standard states. For example, the Gibbs free energy change for this process is... 

In order to have a consistent basis for comparing different reactions and to permit the tabulation of thermochemical data for various reaction systems, it is convenient to define enthalpy and Gibbs free energy changes for standard reaction conditions. These conditions involve the use of stoichiometric amounts of the various reactants (each in its standard state at some temperature T). The reaction proceeds by some unspecified path to end up with complete conversion of reactants to the various products (each in its standard state at the same temperature T). 

The enthalpy and Gibbs free energy changes for a standard reaction are denoted by the... 

The most useful expression for describing the variation of standard Gibbs free energy changes with temperature is ... 

The standard Gibbs free energy change for a reaction refers to the process wherein the reaction proceeds isothermally, starting with stoichiometric quantities of reactants each in its standard state of unit activity and ending with products each at unit activity. In general it is nonzero and given by... 

As equation 2.4.8 indicates, the equilibrium constant for a reaction is determined by the temperature and the standard Gibbs free energy change (AG°) for the process. The latter quantity in turn depends on temperature, the definitions of the standard states of the various components, and the stoichiometric coefficients of these species. Consequently, in assigning a numerical value to an equilibrium constant, one must be careful to specify the three parameters mentioned above in order to give meaning to this value. Once one has thus specified the point of reference, this value may be used to calculate the equilibrium composition of the mixture in the manner described in Sections 2.6 to 2.9. 

At 25 °C the standard Gibbs free energy change for the reaction... 

An equilibrium constant is simply related to a standard Gibbs free energy change, as indicated by equation 2.4.7. 

The Gibbs Free Energy change accompanying the transfer of dnB moles of B from a reservoir in which it is present in its standard state to the equilibrium mixtures is... 

Here AGr° is the Gibbs free energy change in the ideal gas phase reaction system when all the gases are in their respective standard states. The equilibrium constant Kp is given in terms of the partial pressures at equilibrium by... 

The partial equilibria of equations 8.165 8.168 reveal the usefulness of standard potentials the Gibbs free energy change AG of the redox equilibrium is always given by applying Faraday s equation to the algebraic sum of the standard potentials of the redox couples in question. For equation 8.163, the bulk potential is thus... 

E Energy AG° Standard Gibbs free energy change... 

Cesium, Cs, and rubidium, Rb, form ideal solutions in the liquid phase, and regular solutions in the solid phase. Their standard state Gibbs free energy changes of melting as a function of temperature, AG c. = (G°cs.l 0,5) and = (G° - G°, 5) respectively, are... 

It can be shown (see Chapter 3) that the standard state Gibbs free energy change for this reaction is given by the difference between the standard state free energies of the products minus the reactants and can be related to the partial pressure of the gaseous species ... 

The variation of the standard state Gibbs free energy change for the oxidation reaction at any temperature from experimentally measured variations in Po2,e r can be fitted to an equation of the form ... 

We represent A///° and A/G° as the standard enthalpy and Gibbs free energy changes for the reaction in which the chemical substance is formed from the elements in their stable form, as they occur in nature at T = 298.15 K.rr For ions in solution, the values tabulated are relative to the standard enthalpy and Gibbs free energy of formation of the H+ ion being set equal to zero.ss... 

Since ArG° is the standard state Gibbs free energy change, the pressure is specified. Hence, (dArG°/dp)T = 0, and from equation (11.102) we get... 

AG° standard Gibbs free energy change at pH = 7 (also called AG8)... 

Standard half-cell potentials can be used to compute standard cell potentials, standard Gibbs free energy changes, and equilibrium constants for oxidation-reduction reactions. 

The standard electrode potential E° of a redox reaction is a measure of the potential that would be developed if both reductants and oxidants were in their standard states at equal concentrations and with unit activities. The units of E° are volts and ° can be calculated from the Gibbs free energy change (AG ) of the redox reaction from the relationships... 

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