Exothermic process ionic bonds

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... 

The chief driving force for the formation of the salt is the last step, in which the separated ions come together to form a crystal held together by ionic bonds. When a cation and anion form an ionic bond, it is an exothermic process. Energy is released. 

In the case of chemisorption this is the most exothermic process and the strong molecule substrate interaction results in an anchoring of the headgroup at a certain smface site via a chemical bond. This bond can be covalent, covalent with a polar part or purely ionic. As a result of the exothermic interaction between the headgroup and the substrate, the molecules try to occupy each available surface site. Molecules that are already at the surface are pushed together during this process. Therefore, even for chemisorbed species, a certain surface mobility has to be anticipated before the molecules finally anchor. Otherwise the evolution of ordered structures could not be explained. 

This lattice energy is 787 kJ/mol and is more than sufficient to make the overall process for formation of sodium chloride from the elements exothermic. Eorces between oppositely charged particles are called electrostatic, or Coulombic, and constitute an ionic bond when they are attractive. 

Processes in which the atoms or ions of an element experience an increase in oxidation state are oxidation processes. The combustion of metallic sodium in an atmosphere of chlorine gas is shown in Figure 1.3. The sodium ions and chloride ions produced during this strongly exothermic reaction form a cubic crystal lattice in which sodium cations form ionic bonds to chloride anions. The chemical equation for this reaction is written as follows. 

The adsorption of gas molecules on the interior surfaces of zeolite voids is an ionic interaction with a characteristic potential energy called the heat of adsorption. The molecular adsorption process results in an exothermic attachment of the gas molecules to the surface of the voids, and is characterized by a high order of specificity. Zeolites exhibit a high affinity for certain gases or vapors. Because of their "effective" anionic frameworks and mobile cations, the physical bonds for adsorbed molecules having permanent electric moments (N2, NH-j, H20) are much enhanced compared with nonpolar molecules such as argon or methane. 

Theoretical density functional calculations on the possibility of addition of imidazolium salts to electron-rich palladium centers predicted an exothermic enthalpy for such a process [36]. These results suggested that, under appropriate reaction conditions and with the use of a proper carbene precursor, this reaction should present a feasible synthetic path to carbene/palladium complexes. Only recently, the addition of the C(2)-H bond of an imidazolium salt, in the form of an ionic liquid, to a Pd(0)/NHC complex with the formation of a stable Pd-H bond has been reported [41]. These complexes bear three carbenes per metal center, the fourth coordination position being occupied by hydrogen. The isolation of these complexes has proven that the beneficial role of ionic liquids as solvent can lead to the formation of catalytically active palladium-carbene complexes (see Scheme 7). 

The first equilibrium shows an enthalpy change of —3.6 kJ/mol (1 M) and —6.3 kJ/mol (2 M), whereas for the second, the AH value is 17.1 kJ/mol. Although the overall effect of an increase in temperature would be to increase the ionic association because the second equilibrium is strongly endothermic, the exothermic nature of the first step is rather surprising because it involves the loss of two DMF molecules from the first solvation sphere of the lithium ion and the formation of only one Li —N03 bond. Furthermore, in the second step, the substitution of one molecule of DMF by one nitrate ion in the Li first solvation sphere has an enthalpy cost of 17.1 kJ/mol, which demonstrates that, as expected, the desolvation of the lithium ion is an endothermic process, which is partially compensated by the likely exothermic nature of the ion-pair formation. 

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