Ionic compounds energy effects

In cases where the solvation energies are large, as for example when ionic compounds dissolve in water, these hydrophobic effects, based on adverse changes in entropy, are swamped. Dissolving such compounds can be readily accomplished due to the very large energies released when the ions become hydrated. 

Results have shown that the properties of solids can usually be modeled effectively if the interactions are expressed in terms of those between just pairs of atoms. The resulting potential expressions are termed pair potentials. The number and form of the pair potentials varies with the system chosen, and metals require a different set of potentials than semiconductors or molecules bound by van der Waals forces. To illustrate this consider the method employed with nominally ionic compounds, typically used to calculate the properties of perfect crystals and defect formation energies in these materials. 

Jprgensen57 has referred to this tendency of fluoride ions to favor further coordination by a fourth fluoride (the same is true for hydrides) as symbiosis." Although other factors can work to oppose the symbiotic tendency, it has widespread effect in inorganic chemistry and helps to explain the tendency for compounds to be symmetrically substituted rather than to have mixed substituents. We have seen (Chapter 5) that the electrostatic stabilization of C—F bonds (ionic resonance energy) will be maximized in CF4, and similar arguments can be made for maximizing hard-hard or soft-soft interactions. 

It is a remarkable thing for an ionic compound to dissolve in water. You probably learned at some point that opposite charges attract each other. The energy cost of separating positively charged cations from negatively charged anions is immense. Dissolution occurs only because water interacts very effectively with ions. We will explore this phenomenon more fully in chapter 8. For now, however, you just need to accept that when ionic compounds dissolve in water, they (mostly) separate into ions that freely and independently move around in the solution. Since these ions are free to move around in the solution, the solution conducts electricity. 

Effects of Reactants The nature of the reactants can play a part in the rate at which a reaction takes place. Organic chemists often reflux (a careful method of boiling flammable compounds) their reactions for a number of hours to get covalently bonded compounds to react. Reactions involving aqueous ionic compounds will react instantly. For example, if the colorless solutions Pb(N03)2(aq) and KI(aq) are mixed, a yellow solid precipitates instantly. Covalently bonded substances have strong bonds that take time and energy to break in a controlled manner. Ionic compounds dissolved in solution have freed up their ions and they remain available to react with other substances in solution. 

Dielectric continuum models such as the Generalized Born Solvent Accessible Surface Area (GB/SA) model are, in conjunction with force fields, excellent tools for fast and reliable calculations of hydration energies and solvent effects on, e.g., conformational equilibria and ligand-receptor interactions. The performance for neutral solutes is very good, whereas calculations on ionic compounds are currently more problematic. A solution to these problems most probably requires force fields that include polarization effects. For optimal accuracy of calculations using a dielectric continuum model, it is a clear advantage if the model is parameterized for the particular force field used. 

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