Standard potential state, definition

For a substance in a given system the chemical potential gi has a definite value however, the standard potentials and activity coefficients have different values in these three equations. Therefore, the selection of a concentration scale in effect determines the standard state. 

Analysis of the transition state in terms of energy is certainly a key aspect of the S 2-ET problem. Entropy considerations may, however, bring about additional information, possibly helping us to conceive better the transition between the two mechanisms. It was observed in this connection that, whereas the entropy of activation of both the anthracene anion radical and of the ETIOPFe(O) porphyrin (pp. 99, 100) (which have about the same standard potential) is close to zero in their reaction with s- and t-butyl bromides a definitely negative value, ca. — 20 eu is obtained for the reaction of the porphyrin with n-butyl bromide (Lexa et al., 1988). The same was found for the reaction of two other iron porphyrins, TPPFe(o) and OEP-Fe(i). These activation entropies were estimated from (153), where Z is... 

In this equation, the standard chemical potential MAgjS 6e used because the deviation of 6 from stoichiometry is much less than 1. According to the definition of the standard potential, = 0, is the standard Gibbs energy of the element in the standard state AG . Then... 

The standard potential of equation 8.176 is = 1.228 V. At standard state, the activity of gaseous oxygen is 1 by definition, and standard potential thus refers to H2O in equilibrium with an atmosphere of pure O2 at T = 25 °C and P = bar. Applying the Nernst and Faraday relations to equation 8.176 and transforming natural logarithms into base 10 logarithms, we obtain... 

Figure 30(a) concerns the EE mechanism for the reaction O + 2 e = R. The solid curve represents the standard free energy profile pertaining to the standard potential E° of the redox couple O/R. In this case, the energy levels of the initial and the final state are equal by definition. Well... 

Oxidation-Reduction Indicators.—A reversible oxidation-reduction indicator is a substance or, more correctly, an oxidation-reduction system, exhibiting different colors in the oxidized and reduced states, generally colored and colorless, respectively. Mixtures of the two states in different proportions, and hence corresponding to different oxidation-reduction potentials, will have different colors, or depths of color every color thus corresponds to a definite potential which depends on the standard potential of the system, and frequently on the hydrogen ion concentration of the solution. If a small amount of an indicator is placed in another oxidation-reduction system, the former, acting as a potential mediator, will come to an equilibrium in which its oxidation-reduction potential is the same as that of the system under examination. The potential of the given indicator can be estimated from its color in the solution, and hence the potential of the system under examination will have the same value. 

The standard state is the hypothetical ideal solution of molality 1 molkg (or the relative activity of H 3 O+,aH3O+ = 1) at standard pressure. The standard pressure is 1 bar (earlier 1 atm = 1.01325 bar however, the shift is only 0.00026 V at the potential scale). By definition, the potential of this electrode is zero. Although the standard potential should not depend on the material of the metal, the SHE exclusively contains a platinum wire or a platinum sheet covered with platinum black (platinized platinum). Owing to the spontaneous dissociation (dissociative chemisorption) of H2 at Pt... 

This chapter is concerned with the definition and determination of standard potentials. Such standard potentials are of use in obtaining Gibbs free energies of electrochemical reactions under definite standard conditions, and for obtaining activity coefficients of solutes, as will be discussed in detail below. However, in order to deal with such potentials it is first necessary to discuss the matter of standard states. 

Standard potential values are usually those of ideal unimolal solutions at a pressure of 1 atm (ignoring the deviations of fugacity and activity from pressure and concentration, respectively). A pressure of 1 bar = 10 Pa was recommended as the standard value to be used in place of 1 atm = 101 325 Pa (the difference corresponds to a 0.34-mV shift of potential). If a component of the gas phase participates in the equilibrium, its partial pressure is taken as the standard value if not, the standard pressure should be that of the inert gas over the solution or melt. In a certain case, a standard potential can be established in a system with nonunity activities, if the combination of the latter substituted in the Nemst equation equals unity. For any sohd component of redox systems, the chemical potential does not change in the course of the reaction, and it remains in its standard state. In contrast to the common thermodynamic definition of the standard state, the temperature is ignored, because the potential of the standard hydrogen (protium) electrode is taken to be zero at any temperature in aqueous and protic media. The zero temperature coefficient of the SHE corresponds to the conventional assumption of... 

Five oxidation states of At have been definitely established (-1, 0, +1, V, VII) and one other (III) has been postulated. The standard oxidation potentials connecting these states in 0.1 M acid solution are E°fV) ... 

The actual state, and absolute amount, of intrinsic energy existing in a body, or system of bodies, are things which lie quite outside the range of pure thermodynamics. This indefiniteness has, however, not the slightest influence on the stringency of the definition, since we can proceed as in the definition of electrostatic potential, and choose any convenient standard state of the body, and use the term intrinsic energy with reference to this standard state. 

The advantages to using MSDSs for chemical inventories are that MSDSs are publicly available and required by law. There is no extra cost or labour needed to supply them. The limitations to using MSDSs as a source of inventory data are that currently, in the United States, a standardized reporting format is not mandatory. MSDSs are not verified or audited and MSDS information can be incomplete and/or inaccurate. In addition, OSHA s definition of hazard does not include the broader scope of hazard used in green chemistry. Therefore, chemicals that are potential hazards from the green chemistry perspective but are not defined by OSHA as hazards, are not required to be identified on a MSDS. 

It is very often necessary to characterize the redox properties of a given system with unknown activity coefficients in a state far from standard conditions. For this purpose, formal (solution with unit concentrations of all the species appearing in the Nernst equation its value depends on the overall composition of the solution. If the solution also contains additional species that do not appear in the Nernst equation (indifferent electrolyte, buffer components, etc.), their concentrations must be precisely specified in the formal potential data. The formal potential, denoted as E0, is best characterized by an expression in parentheses, giving both the half-cell reaction and the composition of the medium, for example E0,(Zn2+ + 2e = Zn, 10-3M H2S04). 

Although we cannot determine its absolute value, the chemical potential of acomponent of a solution has a value that is independent of the choice of concentration scale and standard state. The standard chemical potential, the activity, and the activity coefficient have values that do depend on the choice of concentration scale and standard state. To complete the definitions we have given, we must define the standard states we wish to use. 

The activity of a perfect gas, as for any substance, is unitary, by definition, at standard state. Moreover, for a perfect gas, activity is (numerically) equivalent to pressure, at all pressures. Let us consider the relationship existing, with T held constant, between the chemical potential of component i in gaseous phase g at 1 bar (/a, 17 ) and at pressure P... 

Stanski, electrodeposition, 1301,1303 Standard hydrogen electrode, 1108 see also hydrogen electrode potential, definition, 840, 1060, 1061 Steady state, 1147, 1212 current, 1248... 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

Values of E° by definition refer to conditions under which all species are in their standard states at 298 K. For non-standard conditions the electrode potential, E, of a redox reaction is given by the familiar Nernst expression (equation 24), where... 

In summary, a reference state or standard state must be defined for each component in the system. The definition may be quite arbitrary and may be defined for convenience for any thermodynamic system, but the two states cannot be defined independently. When the reference state is defined, the standard state is determined conversely, when the standard state is defined, the reference state is determined. There are certain conventions that have been developed through experience but, for any particular problem, it is not necessary to hold to these conventions. These conventions are discussed in the following sections. The general practice is to define the reference state. This state is then a physically realizable state and is the one to which experimental measurements are referred. The standard state may or may not be physically realizable, and in some cases it is convenient to speak of the standard state for the chemical potential, for the enthalpy, for the entropy,... 

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