Simple Dissociation Reactions

To illustrate the main ideas involved, we will calculate the pH of 1.0 M HCl. We first list the major species H+, Cl-, and H20. Since we want to cal culate the pH, we will focus on those major species that can furnish H+. Obviously, we must consider H+ from the dissociation of HCl. However, H20 also furnishes H+ by autoionization, which is often represented by the simple dissociation reaction... 

The actual course of the partial ionization reaction of Ph3AsX2 has been shown to depend on the nature of X . When X = Br the ionization equilibrium is a simple dissociation reaction ... 

We next consider the solubility of a pure salt, strong electrolyte, in an aqueous solution saturated arith respect to that salt. The simple dissociation reaction can be represented as ... 

For simple dissociation reactions the mass action law reads... 

The recovery of hydrogen from ammonia involves the simple dissociation reaction ... 

Define a reaction coordinate X(r), which in general is a function of some (or even many) of the atomic positions in the systems. Examples include a torsional angle in a molecule for a conformational transition, a bond distance for a simple dissociation reaction, or, a function of many solvent atomic positions in the case of an electron transfer reaction. Keep in mind that in many cases the simple classical force fields described earlier in this chapter are inadequate for describing the proper dynamics of the system along the coordinate X. A quantum description at some level is likely needed... 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

For a simple displacement reaction involving atoms or radicals, such as A + BC -> AB + C, where the reaction is written in the exothermic direction, the activation energy is 5.5% of the dissociation energy of the bond which is broken. For the reverse endothermic reaction, the standard energy change of reaction must be added to this quantity in order to obtain the activation energy. 

In the preceding chapter, thermodynamic aspects of macrocycle complexation were treated in some detail. In this chapter, kinetic aspects are discussed. Of course, kinetic and thermodynamic factors are interrelated. Thus, in terms of a simple complexation reaction of the type given below (charges not shown), the stability constant (/CML) may be expressed directly as the ratio of the second-order formation constant (kf) to the first-order dissociation rate constant (kd) ... 

Consider first the case of a simple combination-dissociation reaction, which for definiteness we shall take to be the passivation of an acceptor A... 

Ten years ago Rorabacher (13) observed the substitution rate constants for aquonickel(II) ion with different amines (Table II). There is a decrease in the rate constants by a factor of 14 in going from ammonia to dimethylamine. If nickel-(II) substitution reactions are dissociative, then why is the effect this large Is this a steric effect with some associative contribution or is it an outer-sphere effect There has been surprisingly little investigation of the nature of the entering ligand so far as its bulk or its nucleophilicity is concerned even for what have been generally considered as simple substitution reactions. 

If the products n, of this reaction are known, Eq. (1.11) can be solved for the flame temperature. For a reacting lean system whose product temperature is less than 1250 K, the products are the normal stable species C02, H20, N2, and 02, whose molar quantities can be determined from simple mass balances. However, most combustion systems reach temperatures appreciably greater than 1250K, and dissociation of the stable species occurs. Since the dissociation reactions are quite endothermic, a small percentage of dissociation can lower the flame temperature substantially. The stable products from a C—H—O reaction system can dissociate by any of the following reactions ... 

Reactions within and between partners may occur below the dissociation thresholds for simple dissociations. 

The filtration conditions (i.e., the rate and time of filtration) should be established in preliminary experiments. The two major considerations are (1) filter washing must be sufficient to minimise the nonspecific retention of radiologand by the filter and (2) the time course of filtration should be sufficiently short that dissociation of the radioligand from its receptor is avoided. In these studies, it is important to have a preliminary estimate of the dissociation rate, i.e., k i in equation23. For a simple exponential reaction, the half-time of dissociation (t./,) is related to the dissociation rate constant by t./, = 0.693/k i... 

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