Polar molecules, reactions with ions ionic

If the reactants are ionic with opposite charges, the rate constant can be greater than 1010 L mol -1 s-1 due to the favorable attractive forces. For example, the rate constant for the reaction of H+ with OH- in aqueous solutions at 25°C is 10n L mol-1 s-1. On the other hand, the electrostatic repulsion between ions of like sign can significantly slow their reaction. Similarly, if the reactants are polar molecules, electrostatic forces between them and the solvent may come into play. 

In sufficiently dilute solutions, the activity of a nonelectrolyte is directly proportional to its concentration, whereas, according to the DEBYE-HtiCKEL equation (1), the activity of ions in electrolyte solutions is an exponential function of the ionic strength. This proportionality factor, which is different for each substance and ion, changes also with the solvent. The state of a solute (ion or nonelectrolyte molecule) is different in each solvent. A reaction with the solvent takes place, this change usually being termed solvation (or hydration in aqueous medium). The term solvation has no stoichiometric significance, but rather indicates a physical process (polarization). 

Any substance whose aqueous solution contains ions is called an electrolyte. Any substance that forms a solution containing no ions is a nonelectrolyte. Electrolytes that are present in solution entirely as ions are strong electrolytes, whereas those that are present partly as ions and partly as molecules are weak electrolytes. Ionic compounds dissociate into ions when they dissolve, and they are strong electrolytes. The solubility of ionic substances is made possible by solvation, the interaction of ions with polar solvent molecules. Most molecular compounds are nonelectrolytes, although some are weak electrolytes, and a few are strong electrolytes. When representing the ionization of a weak electrolyte in solution, half-arrows in both directions are used, indicating that the forward and reverse reactions can achieve a chemical balance called a chemical equilibrium. 

With the aid of the so-called ion-pair chromatography, it is possible to selectively retain polar ionic compounds. According to this mechanism, charged, polar sample molecules form salts with oppositely charged reaction partners (ions) containing hydrophobic substituents. Because of their nonpolar character, the ion pair formed can interact in a selective way with RP phases. Applications for ion-pair chromatography in reversed-phase thin-layer chromatography include the separation of alkaloids (132,133), antibiotics (134), carboxylic acids (135-137), pharmaceuticals (138), porphyrins (139) and sulfonic acids (140). 

The important theoretical expressions for fcc above have been derived from a model of hard-sphere molecules in a continuous medium. Intermolecular forces between reactant molecules have been neglected. When the reactants are ionic or polar, there will be long-range Coulombic interactions between them. For reactions between ions, we stated in Chapter 2 (Section 2.5.3) an expression for the value of the rate constant at low concentrations, and noted some reactions between oppositely-charged ions that have rate constants in approximate agreement with it. We also noted that for several such reactions the effect of added inert ions follows approximately the Debye-Hiickel limiting law. 

Electronic structure methods are aimed at solving the Schrodinger equation for a single or a few molecules, infinitely removed from all other molecules. Physically this corresponds to the situation occurring in the gas phase under low pressure (vacuum). Experimentally, however, the majority of chemical reactions are carried out in solution. Biologically relevant processes also occur in solution, aqueous systems with rather specific pH and ionic conditions. Most reactions are both qualitatively and quantitatively different under gas and solution phase conditions, especially those involving ions or polar species. Molecular properties are also sensitive to the environment. 

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