Jumat, 09 Oktober 2009

The pK Scale

In our discussion of conductance, we introduced the term acidity constant, Ka, as a measure of the concentration of hydronium ions produced by the dissociation of a compound when it was dissolved in water. In the case of acetic acid, the acidity constant is given by the expression

Mathematical manipulation yields an alternative form of this equation:

The important point about this equation is the relationship between pKa and the more familiar pH; pH is a measure of the acidity of a solution, i.e. the concentration of hydronium ions, while pKa is a measure of the inherent acidity of a compound, i.e. the tendency of a compound to transfer a proton to a base. These parameters vary in the same way: the lower the pH, the higher the hydronium ion concentration; the lower the pKa the more acidic the compound. We will use the pKa scale to gain insight into the variables that determine the acidity of a compound as well as to develop a system for predicting approximate equilibrium constants for a variety of reactions.

Acid-Base Relationships

Consider the generalized representation of an acid-base reaction shown in Equation 1.

In this reaction, an acid, H-A1, transfers a proton to a base, -:A2, forming a new acid, H-A2 and a new base, -:A1. The species -:A1 is called the conjugate base of H-A1; H-A1 and -:A1 constitute a conjugate acid-base pair. The same terms apply to H-A2 and -:A2.

When we talk about acid strength, we're talking about the tendency of a compound to transfer a proton to a base. This tendency is related to the relative stability of the acid: compounds with high acid strength are very reactive; they have low stability in comparison to another acid of lower acid strength.

By the same token, if an acid is very reactive, its conjugate base is very non-reactive, i.e. very stable.

In any acid-base equilibrium, the equilibrium will favor the less reactive, more stable compounds. In other words, the equilibrium constant will be greater than 1 whenever the products of the acid-base reaction are more stable than the reactants. In other words, there will always be more of the weaker acid present in the mixture at equilibrium. This means that if the pKa of H-A2 is greater than that of H-A1, the equilibrium constant for Equation 1 will be greater than 1.

Table 1 lists approximate pKa values of a representative group of acids. All the values have been rounded to the nearest whole number. The proton that is transferred is shown in red.

Table 1

pKa Values of Selected "Acids"

Compound
Structure
pKa
Conjugate Base
ethane
50
ethene
44
ammonia
38
hydrogen
35
toluene
35
ethyne
25
acetone
19
water
16
methanol
16
phenol
10
acetylacetone
9
hydrogen cyanide
9
acetic acid
5
hydrogen fluoride
3
hydronium ion
-2
hydrogen chloride
-7
hydrogen bromide
-9
sulfuric acid
-9
hydrogen iodide
-10

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