Enthalpy change electron affinity

Negative ions also have two unique thennodynainic quantities associated with them the electron affinity, EA, defined as the negative of the enthalpy change for addition of an electron to a molecule at 0 K [117. 121. 122]... 

Typical elements in Groups V. VI and VII would be expected to achieve a noble gas configuration more easily by gaining electrons rather than losing them. Electron affinity is a measure of the energy change when an atom accepts an extra electron. It is difficult to measure directly and this has only been achieved in a few cases more often it is obtained from enthalpy cycle calculations (p. 74). 

Electron affinity and hydration energy decrease with increasing atomic number of the halogen and in spite of the slight fall in bond dissociation enthalpy from chlorine to iodine the enthalpy changes in the reactions... 

All the energy changes for the cycle are found among the data, except for the electron affinity of O, which E A2 We need to subtract 2.48 kJ/mol from the enthalpy of vaporization to convert it into an energy of vaporization. Then we set up the sum of the individual energy changes and solve for E A2. ... 

All of the previous discussion applies, with minor changes, to the second important concept we wish to address the adiabatic electron affinity, Eea. For any molecule AB (mono-, di-, or polyatomic), ea( AB) is the minimum energy required to remove an electron from the isolated anion at 0 K. In other words, /i ea(AB) is the standard enthalpy of reaction 4.7 at T = 0. 

The equilibrium concentration of the ions A- and B- participating in the equlibrium can be directly observed by mass spectrometry. Thus, the free-energy change can be derived from the equilibrium constant, since the concentrations of the neutral species are known in advance. Similarly, by measuring the temperature dependence of the equilibrium constants, the associated enthalpy and entropy can be obtained from van t Hoff plots. By measuring a series of interconnecting equlibria, an appropriate scale can be established. The primary standard in such work has frequently been SO2 whose electron affinity is well established by electron photodetachment36. 

Ervin et al. [27] have determined the electron affinity of the acetylide radical, HC = C-, to be equal to 2.969 + 0.010 eV and the enthalpy of the acid dissociation of acetylene in the gas phase to be equal to 377.8 + 0.6 Kcal mol Use these data, together with the ionization potential of the hydrogen atom, 13.595 eV, to calculate the enthalpy for the dissociation of the CH bond in acetylene. The ionization potentials are properly applied at 0 K, but a good approximation is to assume that they are equal to enthalpy changes at 298.15 K, the temperature at which the enthalpy of the acid dissociation was measured. 

The value of the electron affinity is the heat given out on the addition of an electron, so this implies that the enthalpy change for the addition of an electron to the S (g) anion is... 

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... 

This chapter provides further evidence for the importance of the electron affinity of the oxidizing half reaction in the intercalation of graphite by fluo-rospecies. Moreover, the simple thermodynamic model that accounts for the oxidative intercalation of graphite by fluoroanions is confirmed by reversal of the intercalation in reactions of known enthalpy change. Threshold values of electron affinity (E) for the onset of intercalation and for first-stage intercalation by MFg have also been assessed. 

The sign of the enthalpy change indicates that this is an exothermic process however, the electron affinity of fluorine is assigned a value of +328 kJ/mol. Thus we can think of electron affinity as the energy that must be supplied to remove an electron from a negative ion. For the removal of an electron from a fluoride ion, we have... 

Two features of electron affinity to remember are (1) The electron affinity of an element is equal to the enthalpy change that accompanies the ionization process of its anion, and (2) a large positive electron affinity means that the negative ion is very stable (that is, the atom has a great tendency to accept an electron), just as a high ionization energy of an atom means that the atom is very stable. 

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. 

---