Standard state Gibbs energy change reaction temperature

Compute the standard-state Gibbs energy change, enthalpy change, and entropy change for this- reaction for the temperature range in the table. 

At 452.2 K, a total pressure of 95.9 kPa, and with an appropriate catalyst, the equilibrium extent of dissociation of pure isopropanol to acetone and hydrogen is found to be 56.4 percent [H. J. Kolb and R. L. Bur-well, Jr., J. Am. Chem. Soc., 67, 1084 (1945)]. Use this information to calculate the standard-state Gibbs energy change for this reaction at this temperature. 

The reported standard-state Gibbs energy change for this reaction over a limited temperature range is... 

Values of standard-state Gibbs energy changes of formation for a number of substances are included in Table A.8 of Appendix A, and larger tables are available. This table also includes values of the function —— 7fm,298 >)/7, which can also be used to calculate AG° for a reaction, as is done in Example 7.1. This function generally varies more slowly with temperature than does AfG°. If a value of AG° is needed for a temperature that is not included in the table, interpolation of this function usually gives better accuracy than does interpolation in a table of AfG values. 

The practical importance of thermodynamic theory lies in the fact that, given certain basic data, its principles permit the exact calculation of other data and the state of equilibrium of systems under specified conditions. Thus thermodynamic theory will allow one to state the feasibility of a hypothetical reaction and the maximum yield of a desired product which is to be expected. Use of approximate expressions for this latter purpose may sometimes be justified. For example, the defining equation for standard Gibbs energy change at temperature Tis the expression ... 

The first term, AG°, is the change in Gibb s free energy under standard-state conditions defined as a temperature of 298 K, all gases with partial pressures of 1 atm, all solids and liquids pure, and all solutes present with 1 M concentrations. The second term, which includes the reaction quotient, Q, accounts for nonstandard-state pressures or concentrations. Eor reaction 6.1 the reaction quotient is... 

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 ... 

Stability criteria, 449-454 Standard Gibbs-energy change of formation, 510, 512-513 of reaction, 504-510 effect of temperature on, 507-508 Standard heat (enthalpy change) of formation, 118-123 of reaction, 116-133, 505, 507-508 effect of temperature on, 123-127, 508 Standard state, 117-118, 399-400, 504-505, 515-518... 

STANDARD-STATE FREE ENERGIES The change in the Gibbs free energy for a chemical reaction performed at constant temperature is... 

Consequently, by measuring the zero-current cell potential we obtain the standard state Gibbs free energy change on reaction (if all the ions are in their standard states). Now if we continue further and measure how the zero-current standard state cell potential varies as a function of temperature, we have... 

As with all chemical reactions, the standard state Gibbs free energy change for an isotope exchange reaction at a given pressure and temperature is related to the equilibrium constant by ... 

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). 

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. 

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 ... 

The function A G° v,G° in Eq. (15.14) is the weighted difference (recall that the Vs are positive for products and negative for reactants) between the Gibbs energies of the products and reactants when each is in its standard state as a pure substance at the system temperature and at a fixed pressure. Thus the value of A G° is fixed for a given reaction once the temperature is established, and is independent of the equilibrium pressure and composition. Other standard property changes of reaction are similarly defined. Thus, for the general property M, we write... 

Standard States. According to equation (10), Chapter 6, the change of the Gibbs free energy, AZ, of a reaction occurring at constant temperature and pressure is... 

Because AG° is a property of pure species t in their standard state and at constant pressure, its value depends only on the temperature. In fact, AG° represents the quantity v, G°. Therefore, this function is the difference between the Gibbs ffee-energies of the products and reactants and depends on their stoichiometric coefficients. It is independent of the equilibrium composition or pressure and it is fixed for any given corrosion reaction once the temperature is established. The Gibbs free-energy change is defined according to Eqs. (2.29) and (2.31) as ... 

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