Standard affinity

Potentiometry is used in the determination of various physicochemical quantities and for quantitative analysis based on measurements of the EMF of galvanic cells. By means of the potentiometric method it is possible to determine activity coefficients, pH values, dissociation constants and solubility products, the standard affinities of chemical reactions, in simple cases transport numbers, etc. In analytical chemistry, potentiometry is used for titrations or for direct determination of ion activities. 

The unitary affinity of a reaction in the standard state (298 K, 101.3 kPa, and unit activities) is normally called the standard affinity A0. 

The standard affinity of this reaction at the standard temperature and pressure is expressed by Eq. 6.8 ... 

With = 0, pq = 0, and - -1.3 kJ -mol-1 found in chemical handbooks, we then obtain the standard affinity equal to A0 = -/% = 1.3 kJ - mol-1. If the gas phase is an ideal gas, the activities are equal to the molar fraction of gaseous substances. Further, if solid I2is a pure substance, the activity of I2 is unity. The equilibrium constant K in the standard state will then be expressed by the molar fractions of gaseous constituents as shown in Eq. 6.9 ... 

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.]. 

The reaction exergy is usually assumed to consist of a chemical part at the standard state (7 °, p°, and unit activity) and a physical part associated with the physical state of the reaction. The chemical part AE°hemT p is equivalent to the standard affinity A0 of the reaction, and the physical part AEphy is due to the change in temperature, pressure, and concentration of the reactants and products between the standard state and the state at which the reaction proceeds ... 

In the case of solid substances the reference species is often set at the most stable solid compounds in lithospheric rocks. For example, metallic iron is most stable in the form of its oxides. The standard chemical exergy of metallic iron can then be obtained from the standard affinity Aaf of the formation of iron oxide, Fe +0.75O2 = 0.5Fe2O3 A° = e e + 0.75s 2 - 0.5 pe2Oj and = 0 hence e°c = A° -0.75e° . Table 10.3 shows the standard molar chemical exergy of a few substances relative to the solid reference species in the lithosphere at the standard temperature and pressure. 

On comparing the definition (7.27) of the equilibrium constant with that of the standard affinity in table 7.2 we see immediately that... 

When dealing with a reaction which is a linear combination of known reactions, it is advantageous to work with affinities rather than equilibrium constants since the standard affinity is obtained simply by adding together the corresponding standard affinities of the separate reactions in the same linear combination. 

Equation (7.80) holds whether a system is ideal or not, for the standard affinity of a reaction in a non-ideal system is the same as that in the corresponding reference system. 

In this chapter we shall consider the application of tabulated values of affinities, heats and entropies of reaction to the calculation of equilibrium constants. As we have pointed out already it is much more convenient to consider standard affinities of reaction than equilibrium constants. This is because standard affinities can be added and subtracted in just the same way as stoichiometric equations, so that the standard affinity of a reaction not included in the table is easily calculated. This means, as we shall see, that the only reactions which need to be included are those relating to the formation of compounds from their elements. 

The standard affinity of formation Af, the standard heat of formation and the standard entropy change of formation, are then defined as the standard affinity, standard heat and standard entropy of the formation reaction of the compound i. At 25 °C these three quantities are related, cf, (7.54), by... 

On the other hand, the standard affinity of formation of gaseous atomic hydrogen according to the reaction... 

The standard affinities of formation of inorganic compounds are usually positive, although we find in the table negative values for compounds such as ozone, NO and NO 2, which are known to be rather unstable. Similarly is negative for elements in physical states which are unstable under the standard conditions. This is so, for example, for monoclinic sulphur and gaseous chlorine atoms. 

The significance and use of standard affinities may be illustrated by considering the dissociation of chlorine molecules according to the equation... 

Thus the large negative value of the standard affinity means that the mole fraction of Cl atoms in equilibrium with Cl2 molecules at 25 °C and 1 atm. is extremely minute, and for practical purposes may be taken as zero. 

The standard affinity is given in the table as 22,769 cal./mole so that... 

Because of the large positive value of the standard affinity this reaction is practically complete. 

These examples demonstrate the very important relation between the standard affinity and the equilibrium position of a reaction. By... 

Calculation of the Standard Affinity for a Reaction which does not appear in the Table. 

The standard affinity of formation of each of these compounds is... 

This is the stoichiometric equation for the reaction under consideration. The standard affinity for this reaction is therefore given by ... 

The examples discussed in 3 and 4 show clearly the importance of tables of standard affinities and standard heats of formation, since from them we can calculate the thermodynamic behaviour of an almost unlimited number of reactions. 

We find, taking the standard affinities of formation of the molecules involved from table 8.1, and proceeding as in the last paragraph, that... 

In discussing the stability of compounds we note that a large positive standard affinity of formation means that the compound will not decompose spontaneously into its elements under the standard conditions since the synthesis reaction is practically complete. This does not prove however that the compound will not decompose to form a more stable compound. 

As an example of this we find that the standard affinity of formation of hydrogen peroxide at 25° C and 1 atm. is 31,470 cal./mole, so that under these conditions hydrogen peroxide will not decompose spontaneously to hydrogen and oxygen. However, hydrogen peroxide does decompose almost completely to form water and oxygen since the standard affinity of the reaction... 

As a general example we may examine the oxidation of acetone. Formic and acetic acids can be formed according to the equation (CH3)2C0(Z) + 02( ) ->HC00H(Z) + CH3C00H(Z), for which the standard affinity is 139,300 cal. 

Table of Standard Affinities of Formation, Heats of Formation and Standard Entropies. 

---