Acid-dissociation constant meaning

We can gain some insight into the meaning of the Brfjnsted relationships by means of the following development. Let us write a Hammett LFER for the acid dissociation constants of a series of acid catalysts, namely, log (KJK ) = p ,uiiO- ... 

Some compounds exhibit pH behavior in which a bell-shaped curve is obtained with maximum instability at the peak [107]. The peak corresponds to the intersection of two sigmoidal curves that are mirror images. The two inflection points imply two acid and base dissociations responsible for the reaction. For a dibasic acid (H2A) for which the monobasic species (HA-) is most reactive, the rate will rise with pH as [HA-] increases. The maximum rate occurs at pH = (pA) + pK2)/2 (the mean of the two acid dissociation constants). Where an acid and base react, the two inflections arise from the two different molecules. The hydrolysis of penicillin G catalyzed by 3,6-bis(di-methylaminomethyl)catechol [108], is a typical example. For a systematic interpretation of pH-degradation profiles, see the review papers by van der Houwen et al. [109] and Connors [110]. 

The equilibrium constant for the isotope-exchange equilibrium can be expressed (6) in terms of the solvent isotope effects on the acid-dissociation constants and of the monocarboxylic acid and dicarboxylic acid monoanion, respectively. It follows that a lower value for the fractionation factor of the hydrogen-bonded proton means that the solvent isotope effect on the acid-dissociation constant will be lower for the dicarboxylic acid monoanion than for the monocarboxylic acid. 

The equilibrium constant is called Ka, the acid dissociation constant. By definition, a weak acid is one that is only partially dissociated in water. This definition means that Ka is small for a weak acid. 

Let s review the meaning of the acid dissociation constant, K.r for the acid HA ... 

A difference plot, also called a Bjerrum plot, is an excellent means to extract metal-ligand formation constants or acid dissociation constants from titration data obtained with electrodes. We will apply the difference plot to an acid-base titration curve. 

To extract acid dissociation constants from an acid-base titration curve, we can construct a difference plot, or Bjerrum plot, which is a graph of the mean fraction of bound protons, H, versus pH. This mean fraction can be measured from the quantities of reagents that were mixed and the measured pH. The theoretical shape of the difference plot is an expression in terms of fractional compositions. Use Excel SOLVER to vary equilibrium constants to obtain the best fit of the theoretical curve to the measured points. This process minimizes the sum of squares [nH(measured) -nH( theoretical) 2. 

The acid-dissociation constant for A1(H20)63+, Ka = 1.4 X 1CT5, is much larger than Kw = 1.0 X 10-14, which means that the water molecules in the... 

When considering the behaviour of acids in solution it is important to consider the so-called acid dissociation constant (denoted Ka)- Put simply this is a means of describing the dissociation of HA into constituent H+ and A , which is represented by the following equation ... 

Acid dissociation constant, acidity constant, or the acid ionization constant, (Kg), is a specific type of equilibrium constant that indicates the extent of dissociation of ions from an acid. The acidity of a substance and its Ka value are directly proportional. A larger Kg (smaller pKg) means a stronger acid. Using the values of Kg, the strength of acids can be determined easily. 

This means that acid dissociation constants are smaller for the deuterated acid than for the undeuterated i.e., for the dissociations... 

Thamsen J, The acidic dissociation constants of glucose, mannitol and sorbitol, as measured by means of the hydrogen electrode and the glass electrode at 0 and 18 °C. Acta Cheni. Scand., 6, 270-284 (1952). 

The pKa or acid dissociation constant is a measure of the strength of an acid in solution. A larger value for the pKa tells us that the analyte dissociates to a lesser extent and is therefore a weak acid. For basic analytes, the reverse applies, in that a higher value for the pKa means a stronger base. 

This is the initial concentration of acid, which means that essentially aU the acid has dissociated. We might have expected this result because the acid is quite dilute and the acid-dissociation constant is fairly high. 

The equilibrium constant, is called the acid dissociation constant. A weak acid is only partly dissociated in water, which means that some undissociated HA remains. Weak bases, B, react with water by abstracting (grabbing) a proton from H2O ... 

The second term in the brackets is negligible compared to the first, so X = [Pv ] = 6.4 X 10 /3.2 X 10 = 2 X 10 M. This is the initial concentration of acid, which means that essentially all the acid has dissociated. We might have expected this result because the acid is quite dilute and the acid-dissociation constant is fairly high. 

We see an example of the inductive effect in alcohols in the fact that an electronegative substituent adjacent to the carbon bearing the —OH group increases the acidity of the alcohol. Compare, for example, the acidities of ethanol and 2,2,2-trifluoroethanol. The acid dissociation constant for 2,2,2-trifluoroethanol is larger than that of ethanol by more than three orders of magnitude, which means that the 2,2,2-trifluoroethoxide ion is considerably more stable than the ethoxide ion. 

This equation shows that the colour of an indicator depends not only on the hydrogen ion concentration, that is the pH, but also on the value of the acid dissociation constant, K. This means that different indicators change colour over different pH ranges. 

In order to determine the values of n formation constants, n + 2 independent concentration measurements are needed. These can then be used to obtain the concentrations of the n species ML, ML2,..., ML and also those of M and L. Two pieces of information are at once available we (should ) know the quantities of M and L (or alternative starting materials) used in the measurement. This means that n additional pieces of information are needed. If it is certain that only one complex, of known empirical formula, is formed, then a measurement of the concentration of the uncomplexed M or L is sufficient to determine the formation constant. This measurement can be made in many ways by polarographic or emf methods (if a suitable reversible electrode exists), by pH measurements (if the acid dissociation constant of HL is known) and by many other techniques, including the whole galaxy of spectroscopic methods. A recent source book for stability constant data (that by Connors, see Further Reading) distinguishes over 30 methods. 

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