Enthalpies electron attachment

The sign of the RT term is opposite to that in the corresponding equation which converts ionisation energies into enthalpies. In a thermochemical analysis involving ionisation enthalpies, electron attachment enthalpies are sure to occur also, unless electrons are to appear in the overall... 

Lattice enthalpy > Z(ionization enthalpy) + (electron attachment... 

The crystal of CaO assumes the NaCl structure, the closest approach between positive and negative ions being 239 pm. The enthalpy of atomization of calcium metal is 193 kJ mol that of dioxygen is 248 kJ moT1 (of oxygen atoms). The two electron attachment enthalpies of oxygen are -148 and +838 kJ mol respectively. The first and second successive ionization enthalpies of calcium are 596 and 1156 kJ mol respectively. Use the data to calculate a value for the standard enthalpy of formation of calcium oxide and compare the result with the experimental value of -635 kJ mol... 

It also includes the enthalpy of ionization of the hydrogen atom (equal to, but opposite in sign to, the electron attachment enthalpy of the gaseous proton), the enthalpy of atomization of dihydrogen and the enthalpy of hydration of the proton. The enthalpy of formation of the cation is estimated by use of the equation ... 

The difference in the two values for enthalpy of dissociation of 56 + 42 = 98 kJ mol-1 is equivalent to a difference in pKa of 17 units in favour of HCI being the stronger acid. The factors that contribute to reducing the value for HCN are its high H—CN dissociation enthalpy, its relatively large negative electron attachment enthalpy, and its relatively less negative value for the enthalpy of hydration of the ion. 

Two issues associated with thermodynamic standard states bear some further attention. The first is associated with the enthalpy of ions. Ion heats of formation may be defined based on the heats of ionization of neutral molecules (or electron attachments thereto). For example, one might consider a reaction like... 

The first of these involves the removal of two electrons from zinc atoms in the gas phase, and A/f° for the process will be equal to the sum of the first and second ionisation enthalpies of Zn(g), designated /, and /2 respectively. For the second process, AH° will be equal to twice the electron attachment enthalpy of H+(g) (which is the same as —21, where / is the ionisation enthalpy of atomic hydrogen). Obviously, we could use ionisation and electron attachment energies (instead of enthalpies), without correction for the RT terms which cancel out when we sum up the enthalpy changes for all the steps in the analysis. Thus for the process ... 

AH° is equal to the sum of the appropriate ionisation and electron attachment energies and the corresponding enthalpies need not be explicitly considered. Throughout this book, ionisation and electron attachment energies may, where appropriate, masquerade as enthalpies on the understanding that the RT corrections which ought to have been made ultimately cancel. 

The atomisation enthalpy of elemental sodium Afl%tom, the first ionisation energy of atomic sodium Iu the dissociation enthalpy D of gaseous chlorine, the electron attachment energy Ex of atomic chlorine and the enthalpy of formation A//)1 of crystalline sodium chloride can all be taken from standard tabulations of experimental data. An experimental lattice energy UL is thus given by ... 

It is generally accepted that the electron affinity is positive (electron gain is exothermic) if the energy release accompanies the electron attachment. The standard enthalpy of electron gain, AegTP, at a temperature T, is related to the electron affinity,... 

ENTHALPIES OF ELECTRON ATTACHMENT (ELECTRON AFFINITIES) OF ATOMS... 

The threshold energy for the dissociative electron attachment process e" + SPg(g) = SF "(g) + F(g) has been measured directly by means of mass spectrometry ( .- ). In addition, mass spectrometric studies (5) have been reported on various electron transfer reactions which provide Independent estimates of the enthalpy for the above process. Presented below is a summary of the results derived from these measurements. Also included in the summary are results obtained from an electron-impact study ( ) of the process e + SFgCl(g) = SFg (g) + Cl(g). We assume that the fragment-ions are formed in each process with no excess or kinetic energies. As a result, the derived electron affinities represent lower limits to the true value. Other reported values of EA(SFg) include >1.465 eV (9), 3.2 eV (5) and 3.66 0.04 eV (10). The first two results are based on charge-transfer studies (5-9) while the latter value represents a direct measurement of EA using the magnetron technique (10). 

This is particularly apparent in the sections on introductory inorganic chemistry, where the underlying electron configuration of the elements of the Periodic Table not only determines the Long Form of the Periodic Table, but also determines the physical properties of the elements, atom size, ionisation energies and electron affinities (electron attachment enthalpies), and the chemical properties, characteristic or group oxidation numbers, variable valence and the formation of ionic... 

Chapter 3 briefly describes how the Physical Properties of the Elements are related to the electron configuration of the elements and hence to their positions in the Periodic Table, namely, their size, ionisation potential and electron attachment enthalpies. 

The negative value of the enthalpy change for Equation 8 when n = 0 is defined (7) as the electron affinity (EA) of the oxidized species when the oxidized and reduced species are in their ground rotational, vibrational and electronic states (0 K). At any temperature for any value of n (0, positive, or negative), the thermodynamic state functions for l uation 8 are given by aX (X = G, H, or S), and the thermochemistry of electron attachment can be defined in the ion convention ("stationary electron convention") (7). The relationship between EA and aG is given by Equation 9. A similar relationship applies for adiabatic ionization energies. 

A positive value of A (often in electron-volts) indicates that heat is given out. Often the molar enthalpy is given for this process of electron attachment (AH). Here the units are joules per mole (J moH), and, by the usual convention, a negative value indicates that energy is released. 

D.M.P. Mingos (1998) Essential Trends in Inorganic Chemistry, Oxford University Press, Oxford - This text includes detailed discussions of trends in ionization energies and electron attachment enthalpies within the periodic table. 

Relativistic effects (see Box 13.2) have a profound influence on the ability of gold to exist in the —1 oxidation state. The enthalpy of attachment of the first electron to Au (equation 23.161) is -223kJmor, a value that lies between those for iodine (-295 kJ moF ) and sulfur (-201kJmoF ). 

The standard electrode potential convention is a different approach from that normally taken with thermodynamic quantities related to chemical change in electron number for gas-phase molecules and ions. Ionization potentials and electron affinities are referenced to the electrostatic zero potential energy of the infinitely separated electron in a field-free vacuum." The electron itself is conventionally treated as an ideal gas (the thermal electron convention) or as a subatomic particle with no heat capacity or entropy (the ion convention). lonization/electron attachment enthalpies under the two conventions differ by 1.48kcalmoU at 298 K," while the corresponding free energies under the two conventions are only equal at 0 K and 297 K. ... 

Electron affinity (EA) For a neutral atom or molecule, equal to the energy difference between the enthalpy of formation of a neutral species and the enthalpy of formation of the negative ion of the same structure. The EA is defined as the negative of the 0 K enthalpy change for the electron attachment reaction ... 

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