Constants, dissociation

The strength of an acid is measured by the value of its dissociation constant, strong acids, e.g. HCl, HNO3. being substantially fully ionized in solution and weak acids predominately unionized. 

Hammen equation A correlation between the structure and reactivity in the side chain derivatives of aromatic compounds. Its derivation follows from many comparisons between rate constants for various reactions and the equilibrium constants for other reactions, or other functions of molecules which can be measured (e g. the i.r. carbonyl group stretching frequency). For example the dissociation constants of a series of para substituted (O2N —, MeO —, Cl —, etc.) benzoic acids correlate with the rate constant k for the alkaline hydrolysis of para substituted benzyl chlorides. If log Kq is plotted against log k, the data fall on a straight line. Similar results are obtained for meta substituted derivatives but not for orthosubstituted derivatives. 

Michaelis constant An experimentally determined parameter inversely indicative of the affinity of an enzyme for its substrate. For a constant enzyme concentration, the Michaelis constant is that substrate concentration at which the rate of reaction is half its maximum rate. In general, the Michaelis constant is equivalent to the dissociation constant of the enzyme-substrate complex. 

The dissociation constant of ethanoic (acetic) acid in liquid ammonia is greater than it is in water. Suggest a reason for the difference. 

The amount of carbonic acid present, undissociated or dissociated, is only about 1 of the total concentration of dissolved carbon dioxide. Carbonic acid, in l especi of its dissociation into hydrogen and hydrogencarbonate ions, is actually a stronger acid than acetic acid the dissociation constant is ... 

But a solution of carbon dioxide in water behaves as a very weak acid since the effective dissociation constant K is given by ... 

Hydrogen peroxide in aqueous solution is a weak dibasic aeid the dissociation constant for H2O2 — -1- HO2 is 2.4 x 10 ... 

The dissociation constant, K., and standard free energy change, AG°, for this reaction are related by the equation. 

Determination of the dissociation constants of acids and bases from the change of absorption spectra with pH. The spectrochemical method is particularly valuable for very weak bases, such as aromatic hydrocarbons and carbonyl compounds which require high concentrations of strong mineral acid in order to be converted into the conjugate acid to a measurable extent. 

To evaluate the dissociation constants it would be necessary to measure the equDibrium constant for the reaction in Scheme 4. The dissociation of thiazolecarboxylic acids has been studied principally by Erlenmeyer et al. (47, 48). It seems that no systematic and reliable determination of the acidity dissociation constants have been realized until now. 

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

The acid dissociation constant has the same form m Brpnsted-Lowry as m the Arrhenius approach but is expressed m the concentration of H30" rather than The concentration terms [H30" ] and [H" ] are considered equivalent quantities m equilibrium constant expressions... 

The C—H bonds of hydrocarbons show little tendency to ionize and alkanes alkenes and alkynes are all very weak acids The acid dissociation constant for methane for exam pie IS too small to be measured directly but is estimated to be about 10 ° (pK 60)... 

Acid dissociation constant (Section 1 12) Equilibrium constant for dissociation of an acid... 

Dissociation Constants for Selected Br0nsted Acids (Table 4 2, p 135)... 

Table 1 6 VSEPR and Molecular Geometry Table 1 7 Dissociation Constants (pK ) of Acids Table 2 5 Oxidation Numbers in Compounds with More Than One Carbon...

The values listed in Tables 8.7 and 8.8 are the negative (decadic) logarithms of the acidic dissociation constant, i.e., — logj, For the general proton-transfer reaction... 

Indicator Chemical name Dissociation constants and colors of free indicator species Colors of metal-indicator complexes Applications... 

This table lists the and pi (pH at the isoelectric point) values of a-amino acids commonly found in proteins along with their abbreviations. The dissociation constants refer to aqueous solutions at 25°C. 

The equilibrium constant for this reaction is called an acid dissociation constant, K-, and is written as... 

Monoprotic weak acids, such as acetic acid, have only a single acidic proton and a single acid dissociation constant. Some acids, such as phosphoric acid, can donate more than one proton and are called polyprotic weak acids. Polyprotic acids are described by a series of acid dissociation steps, each characterized by it own acid dissociation constant. Phosphoric acid, for example, has three acid dissociation reactions and acid dissociation constants. 

The decrease in the acid dissociation constant from K i to tells us that each successive proton is harder to remove. Consequently, H3PO4 is a stronger acid than H2P04, and H2P04 is a stronger acid than HP 04 . 

Weak bases only partially accept protons from the solvent and are characterized by a base dissociation constant, kj,. For example, the base dissociation reaction and base dissociation constant for the acetate ion are... 

Polyprotic bases, like polyprotic acids, also have more than one base dissociation reaction and base dissociation constant. 

A species that can serve as both a proton donor and a proton acceptor is called amphiprotic. Whether an amphiprotic species behaves as an acid or as a base depends on the equilibrium constants for the two competing reactions. For bicarbonate, the acid dissociation constant for reaction 6.8... 

This relationship between and Kb simplifies the tabulation of acid and base dissociation constants. Acid dissociation constants for a variety of weak acids are listed in Appendix 3B. The corresponding values of Kb for their conjugate weak bases are determined using equation 6.14. 

The reverse of reaction 6.15 is called a dissociation reaction and is characterized by a dissociation constant, Ka, which is the reciprocal of K. ... 

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