Standard-state temperature

In electrochemistry, the electron level of the normal hydrogen electrode is important, because it is used as the reference zero level of the electrode potential in aqueous solutions. The reaction of normal hydrogen electrode in the standard state (temperature 25°C, hydrogen pressure 1 atm, and unit activity of hydrated protons) is written in Eqn. 2-54 ... 

Lastly, note that equation 1.86 includes the expansion work [(1 -I- n)RT] on the gaseous ions from absolute zero to standard state temperature (this energy term should be included in equations 1.80 and 1.81, but the energy variation associated with this term is negligible, being well within the experimental incertitude involved in magnitudes Tn, Eg, E, Ex, and U). 

From the foregoing discussion, it follows that the standard exergy of one of the reactants can be estimated by use of the standard affinity of the reaction, provided that we know the values of the standard exergy of the other reactants and products. The numerical values of the molar exergy thus obtained of various chemical substances in the standard state (temperature T° = 298 K, pressure p° = 101.3 kPa, activity a° = 1) are tabulated as the standard chemical exergy of chemical substances in the literature on engineering thermodynamics [Ref. 9.]. 

Strictly, both T and K in Eq. 1.41 should be written T° and K° to denote the fact that they refer to the Standard States chosen for the reactants and products in a chemical reaction. As discussed in Special Topic 1, Standard states include a prescription of both temperature and applied pressure [usually T° = 298.15 K and P° = 0.1 MPa (1 bar) or 101.325 kPa (1 atm)], and it is under this condition lhat the chemical reaction described by K is investigated at equilibrium. The issue of temperature effects on K, then, is actually the problem of finding how K changes when the Standard-State temperature is changed at fixed Standard-Slate pressure. Evidently, according to Eqs. 1.41 and 1.42,... 

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... 

The chemical potential fn can be identified with the molar Gibbs energy Gi at the standard- ate temp aturc and pressure or with the molar Helmholtz energy Ai (= Gi-pVi Ui-TSi) at the standard-state temperature and molar volume. The first alternative is usual in thermodynamic tables it gives the relations... 

The standard state temperature 25 C is often not the temperature of the system of interest, so we need to convert from 25 C to the system temperature. To accomplish this conversion, the temperature dependence of AG is discussed in the next section. 

Because of the large scatter of solubility data for amorphous Th(lV) precipitates, this review did not cany out temperature corrections for the hydrolysis constants used and the solubility constant calculated from the data at 17°C [1964NAB/KUD] are compared to the standard state temperature of 298.15 K (25°C). 

Standard state - A defined state (specified temperature, pressure, con-cenlration, e1e.)fortabulatingthermodynamic functions and carrying outthermodynamic calculations. The standard state pressure is usually taken as 100,000 Pa(l bar), but various standard state temperatures are used. [2]... 

Here f° is the pure-component value of the intensive quantity/at the standard-state temperature and pressure. A particular example of (10.4.14) appears in (10.3.11). For reaction equilibria, we are interested in situations for which/= g, ft, and Cp. We can consider many ways for obtaining values for the quantities so long as sufficient data are actually available. But here we consider one way that in which the / are computed from properties of formation. 

Let T° and P° be the standard-state temperature and pressure, and let P be the pure-f vapor pressure at T°. Then the difference between the vapor and liquid enthalpies of formation at P gives the latent heat of vaporization,... 

From Eq.l2.5.10and, since the G values are given at the standard state temperature of the reaction and pressure of 1 atm ... 

---