Standard Property Changes of Reaction

2l standard property change is defined as the property change resulting when a mol ofAandhmolofBin their. standard states at temperature 

T react to form I mol of L and m mol of M in their standard states also at temperature T. A standard state of species i is its real or hypothetical state as a pure species at temperature T and at a standard state pressure P°. The standard property change of reaction is given the symbol AMj, and its general mathematical definition is 

For species present as gases in the actual reactive system, the standard state is the pure ideal gas at pressure P°. For liquids and solids, it is usually the state of pure real liquid or solid at P°. The standard state pressure P° is fixed at 100 kPa. Note that the standard states may represent different physical states for different species any or all the species may be gases, liquids, or solids. 

The most commonly used standard property changes of reaction are 

The standard Gibbs energy change of reaction AGJ is used in the calculation of equilibrium compositions. The standard heat of reaction AH is used in the calculation of the heat effects of chemical reaction, and the standard heat capacity change of reaction is used for extrapolating AHJand AGJwith T. Numericm values for AHJand AGJare computed from tabulated formation data, and ACp, is determined from empirical expressions for the T dependence of the Cp [see, e.g., Eq. (4-52)]. 

---

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... 

Since the standard-state temperature is that of the equilibrium mixture, the standard property changes of reaction, such as A G° and AH°, vary with the equilibrium temperature. The dependence of AG° on T is given by Eq. (15.16), which may be rewritten as... 

Molar or unit-mass value of any extensive property of pure species i Partial molar property of species i in solution Property change of mixing Standard property change of reaction j Mass... 

Standard Property Changes of Reaction A standard property change for the reaction... 

Thus the value of AG° is fixed for a given reaction once the temperature is established, and is independent of tire equilibrium pressure and composition. Othev standard property changes of reaction are similarly defined. Thus, for tire general properly M ... 

Multiplying by v, summing over all species, and invoking the definition of a standard property change of reaction yields ... 

With Eqs. (4-365) and (4-366) this equation easily extends to relate standard property changes of reaction ... 

---