Standard states definitions

Statidard entropy is the absolute entropy of a substance at 1 atm. (Tables of standard entropy values typically are the values at 25°C because so many processes are carried out at room temperature—although temperature is not part of the standard state definition and therefore must be specified.) Table 18.2 lists standard entropies of a few elements and compounds. Appendix 2 provides a more extensive listing. The units of entropy are J/K mol. We use joules rather than... 

Table 12.1 Most Commonly Used Standard State Definitions...

It is strictly for convenience that certain conventions have been adopted in the choice of a standard-state fugacity. These conventions, in turn, result from two important considerations (a) the necessity for an unambiguous thermodynamic treatment of noncondensable components in liquid solutions, and (b) the relation between activity coefficients given by the Gibbs-Duhem equation. The first of these considerations leads to a normalization for activity coefficients for nonoondensable components which is different from that used for condensable components, and the second leads to the definition and use of adjusted or pressure-independent activity coefficients. These considerations and their consequences are discussed in the following paragraphs. 

The standard states of Ag and of Ag (aq) have the conventional definitions, but there is an ambiguity in the definition of the standard state of e. Suppose that a reference electrode R is positioned above a solution of AgN03, which in turn is in contact with an Ag electrode. The Ag electrode and R are connected by a wire. Per Faraday, the processes are... 

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. 

For biochemical reactions in which hydrogen ions (H ) are consumed or produced, the usual definition of the standard state is awkward. Standard state for the ion is 1 M, which corresponds to pH 0. At this pH, nearly all enzymes would be denatured, and biological reactions could not occur. It makes more sense to use free energies and equilibrium constants determined at pH 7. Biochemists have thus adopted a modified standard state, designated with prime ( ) symbols, as in AG°, AH°, and so on. For values determined... 

The standard rates the offending noise according to its nature 5dB(A) is added where the noise has a definite continuous note and a further 5 dB(A) added for noise of an intermittent nature. The number of occasions that happen in an 8-hour period is then plotted on a graph and the correction for intermittency is derived. When these calculations have been performed, the noise level is compared to the background level. The standard states that where the noise exceeds the background by 5 dB or more, the nuisance is to be classed as marginal, and where the background is exceeded by lOdB(A) or more, complaints are to be expected. 

A chemical reaction can occur only if — AG > 0, i.e. if — AG is positive in addition the value a = is by definition the maximum activity for a condensed component where the pure phase is taken as standard state, thus A/i is always negative. This discussion will be restricted to gases where p 1 taking p = 1 atm (101 325 kN/m ) as the standard state for the gas, X, it is evident that A/ix is always a negative quantity or zero. 

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 standard state given by the unsymmetric convention for normalization has one very important advantage it avoids all arbitrariness about/2°, which is an experimentally accessible quantity the definition off2° given by Eq. (37) assures that the activity coefficient of component 2 is unambiguously defined as well as unambiguously normalized. There is no fundamental arbitrariness about f2° because Hl2p(M) can be determined from experimental measurements. 

With our definition of activity as the ratio of the fugacity of the component in the solution to that in the standard state, we find that... 

The definition of the strong electrolyte standard state gives a2 = a+a so that... 

Most thermochemical data are reported for 25°C (more precisely, for 298.15 I<). Temperature is not part of the definition of standard states we can have a standard state at any temperature 298.15 K is simply the most common temperature used in tables of data. All reaction enthalpies used in this text are for 298.15 K unless another temperature is indicated. 

As remarked previously, in some texts you will sec 1 atm used in the definition of standard states. 

We connected our earlier definition of activity to a standard state of 1.0 bar or 1.0 M or a mole fraction of unity. None of these make much sense for electrons, but we may define electron... 

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. 

By definition, the standard enthalpy of formation of all elements in their standard states, under standard conditions, is zero. 

Strictly, this definition only applies to substances in equilibrium at the standard state. 

Strictly, this definition only applies to substances in equilibrium at the standard state. For the high temperatures achieved in combustion, cp should always be regarded as a function of temperature. Some values are shown in Table 2.1. Values for the molar heats of formation for several substances are listed in Table 2.2. More extensive listings have been compiled and are available in reference books. The student should recognize that the values have been determined from measurements and from the application of Equation (2.20). Some examples should show the utility and interpretation of the heat of formation. 

If an arbitrary standard state is marked with, a formal definition of a Raoultian standard state for component A of a solution is... 

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