Formation reaction standard Gibbs free energy change

AG° is the standard Gibbs free energy change of reaction obtained from the standard energy of formation of pure components AG at 25 °C, P = 1 atm and a suitable aggregation state. Once having determined IQq in standard conditions, the van t Hoff equation may be used to calculate fQq at other temperatures, as follows ... 

Calculate AG° and Kfor each independent reaction. This may be done as in the relevant examples earlier in this section, with determination of AG° as a function of temperature. An easier route, however, is to use the standard Gibbs free-energy change of formation A Gy for each compound at the temperature of interest in the relationship... 

Strategy To calculate the standard Gibbs free energy change of a reaction, we look up the standard free energies of formation of reactants and products in Appendix 2 and use these values in Equation 8.40. Remember that AGf for elements such as 02(g) and Mg(i) is zero because they are stable allotropes of their respective elements at 1 bar and 25°C. Check that the chemical equations are balanced so that you use the correct stoichiometric coefficients in Equation 8.40. Finally, aU stochiometric coefficients are unitless, so AG xn is expressed in units of kJ moP. ... 

The standard Gibbs free-energy change for a reaction, AG°, can be calculated from the standard Gibbs free enCTgies of formation of reactants and products. 

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... 

The solubility product constant may be determined by direct measnrements or calcnlated from the standard Gibbs free energies of formation (JsGf) of the species involved at their standard states. For the reaction scheme (Equation 8.13) the Gibbs free energy change is ... 

Calculation of Standard Change of Gibbs Free Energy for Chemical Reactions from Gibbs Free Energy of Formation... 

The principle of microscopic reversibility allows one to express the backward rate constant in terms of the forward rate constant divided by Kp, which is the equilibrium constant based on gas-phase partial pressures. Kp has units of pressure to the power 5, where 5 is the sum of the stoichiometric coefficients (i.e., 8 = —2 for this problem). Handbook values for standard-state free energies of formation at 298 K are used to calculate the Gibbs free-energy change for reaction at 298 K (i.e., 29s) thi to calculate a dimensionless... 

Even for the most favorable cases, however, the error bars that have to be accepted are larger than one would wish. This is illustrated in Table 1.2, adapted from Cohen and Benson [64] who give references to the archival literature. Here one sees that the best available standard enthalpy of formation values for the small hydrocarbons come with error ranges that imply significant uncertainty in equilibrium constants (a 1 kJ/mol uncertainty in the enthalpy or Gibbs free energy change of a reaction at lOOOK implies an uncertainty of 12% in its equilibrium constant). 

If we know the standard Gibbs energies of formation of each substance for the original system AfG and the reaction system AfG, the change in standard Gibbs free energy AG is calculated by... 

The maximum electrical work in a fuel cell is obtained when all reactions are reversible with no losses and is equal to the change in the Gibbs free energy of formation at the reference standard temperature and pressure (STP), and it is given as... 

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