Standard affinity of formation

The formation reaction of a chemical compound is defined as the reaction in which the compound is produced from its elements, these elements being taken in their normal physical state under the specified conditions. Thus if we consider reactions at 298 16 °K and at 1 atm. pressure, chlorine, hydrogen and oxygen are taken in the gas state mercury and bromine in the liquid state while carbon is normally taken in the well-defined j8-graphite state, and sulphur in the rhombic crystalHne state. It is also necessary to specify the physical condition of the compound which is formed, although this need not necessarily be the stable state under the conditions considered. 

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 

The standard values for the formation of an element in the stable physical state are, by definition, zero. For example the reaction for the formation of hydrogen is 

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. 

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

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... 

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

In the following table are collected together the standard affinities of formation of some of the more important chemical substances, together with the heat content changes accompanying their formation, at 25° C and 1 atm. pressure. 

It should be noted that the standard affinity of formation is numerically the same as the standard free energy of the compound, but has the opposite sign. 

Standard Affinities of Formation, Standard Heats of Formation, and Standard Entropies of Chemical Substances (in calories)... 

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 ... 

Adiabatic detachment energy [7]. Abbreviations used rcoy (Em) = covalent radius of element E in a trivalent compound BE(E-E) = bond enthalpy of a single E-E bond D°298(E2) = dissociation enthalpy of the E2 molecule at standard conditions IE = ionization enthalpy EA = electron affinity AHf°(E2 g) = standard enthalpy of formation of the gaseous E2 molecule. 

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. 

The types of values reported in the database standard enthalpies of formation at 298.15 K and 0 K, bond dissociation energies or enthalpies (D) at any temperature, standard enthalpy of phase transition—fusion, vaporization, or sublimation—at 298.15 K, standard entropy at 298.15 K, standard heat capacity at 298.15 K, standard enthalpy differences between T and 298.15 K, proton affinity, ionization energy, appearance energy, and electron affinity. The absence of a check mark indicates that the data are not provided. However, that does not necessarily mean that they cannot be calculated from other quantities tabulated in the database. 

The lattice enthalpy U at 298.20 K is obtainable by use of the Born—Haber cycle or from theoretical calculations, and q is generally known from experiment. Data used for the derivation of the heat of hydration of pairs of alkali and halide ions using the Born—Haber procedure to obtain lattice enthalpies are shown in Table 3. The various thermochemical values at 298.2° K [standard heat of formation of the crystalline alkali halides AHf°, heat of atomization of halogens D, heat of atomization of alkali metals L, enthalpies of solution (infinite dilution) of the crystalline alkali halides q] were taken from the compilations of Rossini et al. (28) and of Pitzer and Brewer (29), with the exception of values of AHf° for LiF and NaF and q for LiF (31, 32, 33). The ionization potentials of the alkali metal atoms I were taken from Moore (34) and the electron affinities of the halogen atoms E are the results of Berry and Reimann (35)4. 

The results obtained by using Equation 1 are summarized in Table I. The heats of formation given for the isolated anions were computed from the standard heat of formation of the corresponding potassium salt by means of the Kapustinskii approximation. The value thus derived from F (using a radius of 1.33 A.) falls within 5 kcal./mole of that from the heat of formation and measured electron affinity of F. The value for BF4 differs by 19 kcal./mole from that obtained in a detailed lattice-energy calculation by Altschuller (I) his value is very close to that derived by Kapustinskii and Yatsimirskii (11) by modifying the Kapustinskii equation. 

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 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. 

Match each of the following energy changes with one of the processes given ionization energy, electron affinity, bond dissociation energy, and standard enthalpy of formation. 

I.r. spectra of some lanthanide disulphides have been measured and the presence of —S—S groups has been demonstrated. Values for the electron affinities of Se , and Te have been calculated as part of a study of the properties of alkaline-earth sulphides, selenides, and tellurides. Thermodynamic data for the sulphides of Fe, Co, and Ni have been related to the pH of their precipitation. The standard enthalpy of formation for the non-stoicheiometric sulphide Fe,, S has been deduced. Thermodynamic data for some lead chal-cogenides have been determined. ... 

Experimental values for the standard enthalpy of formation of this ion, as reported in Table 7, are in fair agreement. They originate in (1) the proton affinity of acetylene " obtained in SIFT experiments and (2) the adiabatic ionization energy of the vinyl radical combined with its standard enthalpy of formation. ... 

Table 24.2 Mean absolute deviations (MAD) from experiment for standard enthalpies of formation (Af/7 29g), ionization potentials (IP), electron affinities (EA), proton affinities (PA), equilibrium bond lengths irf), and harmonic vibrational frequencies (co ) computed with approximate functionals using the 6-311 ++G(3df,3pd) basis set. The fully nonempirical functionals in this table are HF, LSDA, PW91, PBE, and TPSS...

In practice, it is easier to measure standard enthalpies of formation than to measure some of the other steps. The electron affinic)- is the... 

The Born-Haber cycle can be used to calculate any missing energy term, not just the lattice enthalpy. Using the data below, calculate the first electron affinity of chlorine. Standard enthalpy of formation of rubidium chloride -431 kJ mol Lattice enthalpy of rubidium chloride +695 kJ mol ... 

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