Acid-base reactions stages

The second stage is a Brpnsted acid-base reaction and is fast... 

The cement-forming reaction is a special case of an acid-base reaction so that concepts of acid, base and salt are central to the topic. In AB cement theory, we are concerned with the nature of the acid-base reaction and how the acidity and basicity of the reactants are affected by their constitution. Thus, it is appropriate at this stage to discuss the various definitions and theories available. 

In order to understand why the activation energies differ between the two pathways, Mui et al. examined the transition state geometries [279]. They found that as electron density is donated from the amine lone pair to the down silicon atom upon adsorption into the precursor state, the up Si atom in the dimer becomes electron rich. At this stage, the dative bonded precursor can be described as a quaternary ammonium ion. The N—H dissociation pathway can thus be interpreted as the transfer of a proton from the ammonium ion to the electron-rich up Si atom through a Lewis acid-base reaction. In the transition state for this proton transfer, the N—H and Si—H... 

Actually, Equation (4-11) does not describe adequately the acid-base reaction (4-10) as we usually determine it experimentally. Reaction (4-10) can be considered to take place in two stages the first is the proton transfer or ion-pair generation reaction, and the second is the formation of free ions. These two stages for an acid can be expressed as... 

Inorganic reactions can be divided into four categories, namely redox, acid/ base, precipitation and complexation. By considering complexation reactions as the first stage towards precipitation, and further by considering complexation reactions as a type of acid/base reaction, it is possible to reduce this number to two fundamental categories for inorganic reactions. 

If it is accepted that reactions catalyzed by acids or bases involve at some stage a slow acid-base reaction, then the Bronsted relation (cf. Sec. II.4) assumes a reasonable aspect. The constants used to express the strengths of the catalyzing species are usually defined with reference to an equilibrium with some standard acid-base system such as the solvent, but they could in principle be defined in terms of the (hypothetical) protolytic equilibrium between the catalyst and the substrate. The Bronsted relation then amounts to a parallelism between the rates and equilibrium constants of a series of similar reactions. The general form of the relation can in fact be inferred without any reference to a molecular interpretation. Suppose that we have any acid-base equilibrium... 

An acid-base reaction can be used to enhance separation by taking advantage of the difference in the pK values of the components to be separated (Duprat and Gau, 1991). For example, in the close boiling 3-/4-picolines mixture, addition of trifluoroacetic acid in stoichiometric deficiency results in preferential complexa-tion of 4-picoline with a selectivity of about 2 in formamide as the solvent. Because the pyridinium salts are nonvolatile, the vapor phase is enriched with respect to 3-picoline. A near complete separation can be expected by repetition of this enrichment by countercurrent staging. 3-Picoline will leave the column as the distillate at the top, and 4-picoline will leave as a liquid phase complex with the acid as the bottoms product. 4-Picoline can be regenerated from the complex by adding a stronger base. 

Amorphons or poorly crystallized silica contained in aggregate reacts with sodium or potassium hydroxide in aqneons solntion, and, as it is commonly known, in concrete pore solution, a significant amonnt of these hydroxides can be present. To this reactive silica belong the most freqnently opal, tridymite with defected stractnre [77] or siliceous glass. The two-stage process occurs the first stage is the typical acid-base reaction, in which the acidic silanol groups react with sodium or potassium hydroxide ... 

This melt is the most investigated among chloride based melts. Some oxoacidity studies were performed by Shapoval et al. Main purposes of these works were to investigate oxygen electrode reversibility and to obtain equilibria constants for acid-base reactions including oxo-compounds of Cr , Mo and W " at 700 C. Equilibrium constants of acid-base interactions for Cr03 and P03 have been determined. The titration of the former substance proeeeds in two stages ... 

Many of the reactions that occur in organic chemistry are either acid-base reactions themselves or they involve an acid-base reaction at some stage. 

Primary amines react with benzenesulfonyl chloride to form A/ substituted benzenesulf-onamides. These, in turn, undergo acid-base reactions with the excess potassium hydroxide to form water-soluble potassium salts. (These reactions take place because the hydrogen attached to nitrogen is made addic by the strongly electron-withdrawing — SO2— group.) At this stage our test tube contains a clear solution. Acidification of... 

These four mechanisms concur that alkaline flooding enhances the recovery of acidic oil by two-stage processes. In the first and common stage of these alkaline recovery mechanisms, surface active salts are formed by the in situ acid-base reaction between the alkali contained in the floodwater and the organic acid present in the residual oil. The surfactants can adsorb onto the oil-water interface to lower the interfacial tension and thus promote emul-sificati6n under the action of surface driven forces (spontaneous) and/or under the action of shear driven forces (external and in-... 

The various acid-base reactions discussed in Sections 21.2-21.4 are generally very fast processes. Important exceptions, however, are the later stages of hydrolysis of ions (Section 21.2.3). The kinetics of these reactions are complicated. Thus, in the case of thorium(iv), not only the size of the aggregates but also their rate of formation depend very much upon the anions present (Fig. 21.2). In chloride solutions, the aggregates are not only larger than in perchlorate solutions, but they also continue to grow during extended periods of time [35]. 

The acid-base equilibria in the scheelite (CaW04) solutions in the chloride mixture at lOOOK were studied by Combes and Tremillon. CaO solubility in molten KCl-NaCl is about of 0.084 mol%. Similar parameter for scheelite equals to 10 mole kg". Equilibrium constants for acid-base reactions with WO3 participation are determined, too. The data show that the neutralization of WO3 is one-stage process ... 

Hydroxylation of the metal cation may be obtained through an acid-base reaction (neutralization, thermolysis, etc.) or through an oxidation-reduction reaction. The charge-pH diagram (Figure 1.6) shows that reduction and oxidation of cations as 0x0 and aquo species, respectively, allows them to reach the stability domain of hydroxo forms. Hydroxylation is the initiation stage of the process and the hydro-xylated complex is the precursor of the condensation products. 

Phenolic Resins. PhenoHc resins [9003-35 ] (qv) are thermosets prepared by the reaction of phenol with formaldehyde, through either the base-cataly2ed one-stage or the acid-cataly2ed two-stage process. The Hquid intermediate may be used as an adhesive and bonding resin for plywood, particle board, ftberboard, insulation, and cores for laminates. The physical properties for typical phenoHc laminates made with wood are Hsted in Table 1. 

---