Formation reaction standard affinity

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 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. 

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... 

The standard affinity of each of these reactions can be evaluated from a knowledge of the affinities of formation of the compounds involved. If these are known as a function of temperature, then the standard affinity can also be obtained at various temperatures. Thus the standard affinities of formation (in calories) of the hydrocarbons involved in the above reactions are, in the temperature range 300 -1000 at 1 atm. pressure ... 

From these standard affinities of formation we can derive quite readily the standard affinities for reactions (1) to (9). The detailed arithmetic is left as an exercise to familiarize the reader with the general method. The resulting expressions are ... 

The enthalpy of reaction 2.45 cannot be determined directly. As shown in figure 2.5, it is calculated by using several experimental quantities the standard enthalpy of formation of the solid alkoxide, the standard sublimation enthalpy and the ionization energy of lithium, and the standard enthalpy of formation and the adiabatic electron affinity of gaseous methoxy radical (equation 2.47). 

When comparing literature data for the quantities addressed in this section, it is therefore essential to check if those data are consistent, that is, if they are based on the same value for the anchor. On the other hand, note that proton affinity, basicity, and acidity values do not depend on whether we follow the electron convention, the ion convention, or the electron FD convention. This is clearly evidenced by reactions 4.25 and 4.27, which do not involve the electron as a reactant or product species. However, it is also obvious that the values of the standard enthalpies of formation of AH+ and A-, calculated from PA(A) and A acid-7/0 (AB), respectively, will vary with the convention used to derive the standard enthalpy of formation of the proton. 

In these calculations, the electron affinity of the methyl radical has been taken1 as 27 kcal.mole-1. The other enthalpy terms are all well-known quantities the enthalpies of hydration of individual ions have been assigned as done by Valis ev (see ref. 2) and the enthalpy of hydration of the gaseous methyl anion has been taken as that of the bromide ion. It can be seen from Table 1 that not only is the formation of the methyl anion energetically very unfavoured in the gas phase, but it is also endothermic to the extent of 54 kcal.mole-1 in aqueous solution. A check on this final result can be made by consideration of the standard entropy change for the reaction... 

The thermodynamics of this reaction can be calculated using standard enthalpy and entropy of formation data in M.W. Chase, Jr., MST-JANAF Thermochemical Tables, fourth edition. Journal of Physical and Chemical Reference Data, Mono aph 9, 1998 (see also the NIST Website at http //nist.gov). For a metal having weaker affinity for oxygen, such as in mercury(II) oxide, both enthalpy and entropy favor this reaction. For a metal having somewhat stronger affinity, as in copper(ll) oxide, enthalpy disfavors the reaction but is overwhelmed by entropy. 

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