Aqueous solution equilibrium constant

Scales for bases that are too weak to study in aqueous solution employ other solvents but are related to the equilibrium in aqueous solution. These equilibrium constants provide a measure of thermodynamic basicity, but we also need to have some concept of kinetic basicity. For the reactions in Scheme 5.4, for example, it is important to be able to make generalizations about the rates of competing reactions. 

Aagaard Helgeson 1982), provided that solution pH and temperature remain constant However, the solution saturation state is difficult to define because glass cannot be in equilibrium with aqueous solution. Accordingly, the identity of the rate-limiting solid for glass is controversial, but for the moment, we will consider the simplest of the potential candidates. 

The thermodynamic and kinetic characteristics of an intramolecular transimination reaction observed in solutions containing pyridoxal-5-phosphate and ethylenediamine have been investigated (75JA6530). The open-chain structure Schiff bases and the cyclic tautomers such as 54 are in equilibrium in aqueous solution over the pH range 7.5-14, but these equilibria are rather complex owing to the different states of the ionization (protonation) in both tautomers. The ring-chain equilibrium constant (the sum of all cyclic tautomers versus all open-chain tautomers) varies by less than a factor of 4 over the pH range 7-14. At pH 14, KT = 1.2 at pH 10,... 

Equilibrium Constant. To determine the concentrations of I2 and I3 in equilibrium in aqueous solutions we equilibrate aqueous KI solutions (200 mL) with solutions of... 

According to the aforementioned adaptative properties displayed by the PDMAEMA-/-PCL APCNs, the Young modulus has been also recorded as a function of the solution pH. For this purpose, specimens of a polymer conetwork (Entry 2, Table III) were allowed to swell until equilibrium in aqueous solution buffered at pH 4, 7 and 9, respectively, prior mechanical testing. It is worth mentioning that the ionic strength was maintained constant and fixed to 0.05... 

Assuming chemical equilibrium between aqueous solution and solid phase at constant temperature and pressure, concentration of dissolved species could be estimated. In order to estimate the concentration the values of other variables (concentration and activity coefficient of each component in solid phase and activity coefficient of dissolved species in aqueous solution) have to be estimated. 

Most acids are weak acids, which ionize only to a limited extent in water. At equilibrium, an aqueous solution of a weak acid contains a mixture of aqueous acid molecules, hydronium ions, and the corresponding conjugate base. The degree to which a weak acid ionizes depends on the concentration of the acid and the equilibrium constant for the ionization. 

The solubilization of diverse solutes in micelles is most often examined in tenns of partitioning equilibria, where an equilibrium constant K defines the ratio of the mole fraction of solute in the micelle (X and the mole fraction of solute in the aqueous pseudophase. This ratio serves to define the free energy of solubilization -RT In K). 

The strength of a weak acid is measured by its acid dissociation constant, which IS the equilibrium constant for its ionization m aqueous solution... 

According to the Arrhenius definitions an acid ionizes m water to pro duce protons (H" ) and a base produces hydroxide ions (HO ) The strength of an acid is given by its equilibrium constant for ionization m aqueous solution... 

Table 8.9 Selected Equilibrium Constants in Aqueous Solution at Various...

Most reactions involve reactants and products that are dispersed in a solvent. If the amount of solvent is changed, either by diluting or concentrating the solution, the concentrations of ah reactants and products either decrease or increase. The effect of these changes in concentration is not as intuitively obvious as when the concentration of a single reactant or product is changed. As an example, let s consider how dilution affects the equilibrium position for the formation of the aqueous silver-amine complex (reaction 6.28). The equilibrium constant for this reaction is... 

In a simple liquid-liquid extraction the solute is partitioned between two immiscible phases. In most cases one of the phases is aqueous, and the other phase is an organic solvent such as diethyl ether or chloroform. Because the phases are immiscible, they form two layers, with the denser phase on the bottom. The solute is initially present in one phase, but after extraction it is present in both phases. The efficiency of a liquid-liquid extraction is determined by the equilibrium constant for the solute s partitioning between the two phases. Extraction efficiency is also influenced by any secondary reactions involving the solute. Examples of secondary reactions include acid-base and complexation equilibria. 

Aqueous solutions buffered to a pH of 5.2 and containing known total concentrations of Zn + are prepared. A solution containing ammonium pyrrolidinecarbodithioate (APCD) is added along with methyl isobutyl ketone (MIBK). The mixture is shaken briefly and then placed on a rotary shaker table for 30 min. At the end of the extraction period the aqueous and organic phases are separated and the concentration of zinc in the aqueous layer determined by atomic absorption. The concentration of zinc in the organic phase is determined by difference and the equilibrium constant for the extraction calculated. 

It is known that the order of acidity of hydrogen halides (HX, where X = F, Cl, Br, I) in the gas phase can be successfully predicted by quantum chemical considerations, namely, F < Cl < Br < I. However, in aqueous solution, whereas hydrogen chloride, bromide, and iodide completely dissociate in aqueous solutions, hydrogen fluoride shows a small dissociation constant. This phenomenon is explained by studying free energy changes associated with the chemical equilibrium HX + H2O + HjO in the solu-... 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

The exceptions are formaldehyde, which is nearly completely hydrated in aqueous solution, and aldehydes and ketones with highly electronegative substituents, such as trichloroacetaldehyde and hexafluoroacetone. The data given in Table 8.1 illustrate that the equilibrium constant for hydration decreases with increasing alkyl substitution. 

Because of the unfavorable equilibrium constant in aqueous solution and the relatri e facility of the hydrolysis, acetals and ketals are rapidly converted back to aldehydes and ketones in acidic aqueous solution. 

The other C=N systems included in Scheme 8.2 are more stable to aqueous hydrolysis than are the imines. For many of these compounds, the equilibrium constants for formation are high, even in aqueous solution. The additional stability can be attributed to the participation of the atom adjacent to the nitrogen in delocalized bonding. This resonance interaction tends to increase electron density at the sp carbon and reduces its reactivity toward nucleophiles. 

The concentrations of the different intermediates are determined by the equilibrium constants. The observation of immonium ions [Eq. (5)] in strongly acidic solutions by ultraviolet and NMR spectroscopy also Indicates that these equilibria really exist (23,26). The equilibria in aqueous solutions are of synthetic interest and explain the convenient method for the preparation of 2-deuterated ketones and aldehydes by hydrolysis of enamines in heavy water (27). 

Customarily, because the term [HgO] is essentially constant in dilute aqueous solutions, it is incorporated into the equilibrium constant Alto give a new term, K, the acid dissociation constant (where K = [HgO]). Also, the term [HjO ] is often replaced by H, such that... 

The equilibrium constant at room temperature corresponds to pKi, = 4.74 and implies that a 1 molar aqueous solution of NH3 contains only 4.25 mmol 1 of NH4+ (or OH ). Such solutions do not contain the undissociated molecule NH4OH, though weakly bonded hydrates have been isolated at low temperature ... 

The electronic spectrum of a compound arises from its 7r-electron system which, to a first approximation, is unaffected by substitution of an alkyl group for a hydrogen atom. Thus, comparison of the ultraviolet spectrum of a potentially tautomeric compound with the spectra of both alkylated forms often indicates which tautomer predominates. For example, Fig. 1 shows that 4-mercaptopyridine exists predominantly as pyrid-4-thione. In favorable cases, i.e., when the spectra of the two alkylated forms are very different and/or there are appreciable amounts of both forms present at equilibrium, the tautomeric constant can be evaluated. By using this method, it was shown, for example, that 6-hydroxyquinoline exists essentially as such in ethanol but that it is in equilibrium with about 1% of the zwitterion form in aqueous solution (Fig. 2). 

The tautomeric equilibrium 151 152 has been discussed by Cromwell and David.Ultraviolet spectral data indicate that 151 predominates in neutral aqueous solution, and infrared data are in essential agreement. It was further concluded that in acid solution the equilibrium favors 152, but no ionization constants were reported,... 

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