Gas standard state for

However, the standard state for gases is the ideal gas state at the standard-state pressure, for which ° = P°. Therefore... 

Since concentration variations have measurable effects on the cell voltage, a measured voltage cannot be interpreted unless the cell concentrations are specified. Because of this, chemists introduce the idea of standard-state. The standard state for gases is taken as a pressure of one atmosphere at 25°C the standard state for ions is taken as a concentration of 1 M and the standard state of pure substances is taken as the pure substances themselves as they exist at 25°C. The half-cell potential associated with a halfreaction taking place between substances in their standard states is called ° (the superscript zero means standard state). We can rewrite equation (37) to include the specifications of the standard states ... 

You may wonder how a reaction, such as combustion of methane, can occur at 25°. The fact is that the reaction can be carried out at any desired temperature. The important thing is that the AH° value we are talking about here is the heat liberated or absorbed when you start with the reactants at 25° and finish with the products at 25°. As long as AH0 is defined this way, it does not matter at what temperature the reaction actually occurs. Standard states for gases are 1 atm partial pressure. Standard states for liquids or solids usually are the pure liquid or solid at 1 atm external pressure. 

Table A5.6 is obtained from the extensive tabulation by W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, Second Edition, Prentice-Hall, Inc., Engelwood Cliffs, N.J. (1952). His tabulation was published at the time when p = 1 atm was the standard state for gases. His results should be corrected to the p = 1 bar standard state for comparison with other modern standard state thermodynamic data. The correction is given by (RT/F) In (1.01325), which has a value of 0.0003 volts at T = 298.15 K. The correction is small and probably negligible for all except the most precise work. (E° values measured to better than 1(T3 volts are unusual.)...

Standard-State conventions for chemical elements and dissolved solutes are summarized in Table si. 1. Note that the Standard states for gases and for solutes are hypothetical, ideal states and not actual states. For gases, this choice of Standard State is useful because the ideal gas represents a good limiting approximation to the real behavior of gases and possesses equations of state that are mathematically tractable in applications. For solutes, the choice of a hypothetical Standard State is of value because the alternative choice, consisting simply of the pure solute at unit mole fraction, is not very relevant to a solution component whose concentration must always remain small. Moreover, by... 

When pure solids or liquids, which do not form any solution, participate in reaction beside gases, the standard state for gases is chosen the same as describe in the preceding chapter, while the standard state of the condensed component is defined by their state at 1 atm pressure and at the temperature of the jafstemr Activity of liquid and solid substances varies only very slightly wit]j... 

Property values in the standard state are denoted by the degree symbol (°). For example, C°P is the standard-state heat capacity. Since the standard state for gases is the ideal-gas state, C% for gases is identical with Cj , and the data of Table 4.1 apply to the standard state for gases. All conditions for a standard state are fixed except temperature, which is always the temperature of the system. Standard-state properties are therefore functions of temperature only. 

The standard state of a substance is a reference state that allows us to obtain relative values of such thermodynamic quantities as free energy, activity, enthalpy, and entropy. All substances are assigned unit activity in their standard state. For gases, the standard state has the properties of an ideal gas, but at one atmosphere pressure. It is thus said to be a hypothetical state. For pure liquids and solvents, the standard states are real states and are the pure substances at a specified temperature and pressure. For solutes In dilute solution, the standard state is a hypothetical state that has the properties of an infinitely dilute solute, but at unit concentration (molarity, molality, or mole fraction). The standard state of a solid is a real state and is the pure solid in its most stable crystalline form. 

The definition of the standard state for gases has been changed to 1 bar, a slightly lower pressure than the 1 atm standard on which the data in this book are based (1 atm = 101.3 kPa = 1.013 bar). For most purposes, this makes very little difference in the standard enthalpy values. 

Standard states for gases. When species i is a gas at the equilibrium conditions, the standard state is usually taken to be the pure ideal gas at the equilibrium temperature T and P, = 1 bar. (Caution in older literature, the standard pressure was usually taken as 1 atm = 1.0133 bar.) Then, the standard-state fugacity becomes... 

Standard state for gases (g) It is defined as the pure substance in the ideal-gas state at 1 bar. 

The standard state for gases, denoted by (g), is specified as the pure gas in the ideal-gas state, at the temperature of the system and atP° = i bar. Accordingly, the fugadty is calculated using the ideal-gas... 

The reference point for all enthalpy expressions is called the standard molar enthalpy of formation (AHf) which is defined as the heat change that results when 1 mole of a compound in its standard state is formed from its elements in their standard states. The standard state of a liquid or solid substance is its most thermodynamically stable pure form at 1 bar pressure. The standard state for gases is similar, except that standard state gases are assumed to obey the ideal gas law exactly. The standard state for solutes dissolved in solution will be discussed in Chapter 10. In the notation AHf, the superscript represents standard-state conditions (1 bar), and the subscript f stands for formation. Although the standard state does not specify a temperature, we will assume, unless otherwise stated, AH° values are measured at 25°C. 

But keep in mind that fl does not have units and, consequently the units of P must correspond to those of the standard state. In this book, for the standard state for gases we generally assume that 1 atm = 1 bar. 

In these equations, the sign Aj denotes the difference between the sum of the specified thermodynamic quantity for the products and that for the reactants, the superscript ° denoting that the species are in their thermodynamic standard state, for gases 1 bar. These quantities can be derived from thermal measurements that is, measurements of heats evolved for particular reactions, molar specific heats and latent heats. Such quantities can be determined (and then tabulated) for relatively stable species. When —A H° RT, the value of the first term on the right-hand-side of (1.21) is likely to dominate the second term, and K(T) for such strongly exothermic reactions can be huge, meaning that /(T) KiT). 

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