AH electron affinities

Were we to simply add the ionization energy of sodium (496 kJ/mol) and the electron affin ity of chlorine (—349 kJ/mol) we would conclude that the overall process is endothermic with AH° = +147 kJ/mol The energy liberated by adding an electron to chlorine is msuf ficient to override the energy required to remove an electron from sodium This analysis however fails to consider the force of attraction between the oppositely charged ions Na" and Cl which exceeds 500 kJ/mol and is more than sufficient to make the overall process exothermic Attractive forces between oppositely charged particles are termed electrostatic, or coulombic, attractions and are what we mean by an ionic bond between two atoms... 

AH and AS to various notional subprocesses such as bond dissociation energies, ionization energies, electron affinities, heats and entropies of hydration, etc., which themselves have empirically observed values that are difficult to compute ab initio. 

Because of this, the thermochemistry of many physical processes can be described in different ways. Thus, the ionization energy of neutral A is the same as the electron affinity of A, the proton affinity of B is also the gas-phase acidity of BH, and the gas-phase acidity of AH is the same as the proton affinity of A. ... 

The early agreement between calculated and experimental heats for fluorides was fortuitous because the high value given to D(F2) was compensated by the large electron affinity value [A/f F ) = lD(Ft) -.EA(F)]. The drop in value of AH°(F >) over the years (see Table I) vitiates some of the more elaborate lattice-energy calculations and Ka-pustinskii s semiempirical method seems adequate (138), but see reference (126). 

Here, and in the following thermochemical schemes, AHs denotes solvation energy, D homolytic bond dissociation energy, I ionization potential, A electron affinity. 

Note that the standard enthalpy of this reaction, Aacid77°(AH), is equal to the proton affinity of the anion, PA(A ). As shown in figure 4.5, this quantity can be related to PA(A) by using the adiabatic ionization energy of AH and the adiabatic electron affinity of A. The result is also expressed by equation 4.28 (derived from equations 4.4 and4.9), where A = (TT g - o)ah+ ( 298 o)ah and A = ( 298 o )a- - ( 298— o )a These thermal corrections are often smaller than the usual experimental uncertainties of proton affinity data (ca. 4 kJ mol-1). 

Figure 4.5 Thermochemical cycle (T = 298.15 K), showing how the proton affinities of A and A- are related. Fj(AH) is the adiabatic ionization energy of AH, and fea(A) is the adiabatic electron affinity of A. A, A, and X are thermal corrections (see text).

Figure 9.5. Schematic of the Sn2 reaction coordinate according to the VBCM model. The energy gaps ER and Ep are identified with sum of the ionization potential and electron affinities of the appropriate species. The avoided crossing occurs at a fraction of ER determined by the reaction enthalpy, AH, and the expected steepnesses of the descending curves. The activation energy for the reaction is E = fER — B, where is the energy of the avoided crossing.

On going from one acid to another, IP(H) of course is constant and D(H—AR ) might be expected to be fairly closely correlated with AH°ion, particularly when the molecules HAR have similar electronic structures. Hence for any isoelectronic set of AH molecules the heat of ionization would be expected to be linearly related to the electron affinity of the... 

It has the usual energy units of kJmol-1, and differs in sign from the electron affinity for example, the electron affinity of the H atom is 0.75 eV, but the electron attachment energy of H(g) is -73 kJ mol-1. In order to convert electron attachment energies from A U° quantities at 0 K to AH" quantities at 298 K, it is necessary to make a correction ... 

In this chapter we focus largely on enthalpies of formation of neutral molecules. However, the enthalpies of formation, AH , of ions are also of interest. They can be obtained by combining calculated ionization potentials, electron affinities, or proton affinities ... 

For the gaseous reaction K + F - K4" + F, AH was calculated to be 91 kJ under conditions where the cations and anions were prevented by electrostatic separation from combining. K s ionization energy is 4.34 eV. What is F s electron affinity ... 

This order would vary slightly if other halide differences were used rather than A(MF2—MI2). (In some other systems, MXn, however, the order can be made to vary greatly depending on which halide differences are taken (see later).) Since the atomic replacement reaction (MI2- -2F = MF2+2I) differs only from the ionic replacement reaction (MI2+2F = MF2+2I ) by a constant which is the difference between the electron affinities of F and I, the order which we obtain here is the same order which would be obtained had we originally used AH of association of neutral atoms. 

Use bond energies (Table 13.6), values of electron affinities (Table 12.8), and the ionization energy of hydrogen (1312 kj/mol) to estimate AH for each of the following reactions. 

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