Basicity constant, expression

In dilute solutions, the water is almost pure and its activity can be set equal to 1. With this approximation, we obtain the basicity constant, Kb. If we make the further approximation of replacing the activities of the solute species by the numerical values of their molar concentrations, we can write the basicity constant expression for ammonia as... 

Basicity constant, Kh (Section 24.3) A measure of base strength. For any base B, the basicity constant is given by the expression... 

To express the relative strengths of an acid and its conjugate base (a conjugate acid-base pair ), we consider the special case of the ammonia proton transfer equilibrium, reaction C, for which the basicity constant was given earlier (Kb = [NH4+l[OH ]/ NH3]). Now let s consider the proton transfer equilibrium of ammonia s conjugate acid, NH4+, in water ... 

In this expression, Ka is the acidity constant of a weak acid and Kh is the basicity constant of the conjugate base of that acid. The acid and base must form a conjugate acid-base pair (such as CH C00H/CH3C02 or NH4+/NH3). We can express Eq. 1 la in another way by taking logarithms of both sides of the equation ... 

For each of the following weak acids, write the proton transfer equilibrium equation and the expression for the equilibrium constant Kv Identify the conjugate base, write the appropriate proton transfer equation, and write the expression for the basicity constant Kb. (a) HC102 (b) HCN ... 

As with the expression in Equation (6.6), this equilibrium constant can be simplified by incorporating the water term into K, thereby yielding a new constant which we will call Kh, the basicity constant ... 

In particular, when two tautomers HA and AH form the common cation HAH+, the tautomeric equilibrium constants KT can be expressed through the basicity constants KB and KA of these tautomers KT = KA/KB. Since... 

These are qualitative conclusions. We ought to be able to use the acidity and basicity constants—which are measures of acid and base strength— to express them quantitatively. 

In these expressions, Ka is the acidity constant and Kb is the basicity constant of a given conjugate pair of an acid and a base (such as CH3C00H/CH3C02 or NH4+/NH3) in water. 

Step 4 Substitution of the equilibrium molarities into the expression for the basicity constant (Kh = 3.6 X 10-4) yields... 

With a weak acid, the calculations are a bit more complex. We begin by computing the initial pH, using the method of Example 1, pH = 2.79. At the equivalence point, we have 50.0 mL of 0.060 M sodium acetate. Since acetate is the base conjugate to a weak acid, the solution is basic and we must compute the pH. From eq (13-12) for acetate ion, Kb = KyKa =5.62 x 10 10, and the equilibrium constant expression is... 

For example, let s write the equilibrium constant expression for the basic ionization of ammonia in water. The equation for the chemical equilibrium reaction is ... 

The above Kb for the equilibrium amine + H+ ammonium ion, bears a simple relation to the more customary but less general basicity constant Kb of the equilibrium amine -f- H20 ammonium ion -f- OH-, according to the expression Kh = Kb. [H20]/Kw = 55.5 io14 Kh while Kb" = 55.5 Kb is always quoted. This has no influence on the above ratio. 

If the first equilibrium lies farther to the right than the second, the solution is acidic. If the second equilibrium is more favorable, the solution is basic. These two equilibrium-constant expressions show that the relative concentrations of the hydro-nium and hydroxide ions depend not only on the magnitudes of and but also on the ratio between the concentrations of the acid and its conjugate base. 

Write the equilibrium-constant expressions and obtain numerical values for each constant in (a) the basic dissociation of ethylamine, C2H5NH2. (b) the acidic dissociation of hydrogen cyanide, HCN. 

We recognize that both NaCH3COO and NaCN are salts of strong bases and weak acids. The anions in such salts hydrolyze to give basic solutions. As we have done before, we first write the appropriate chemical equation and equihbrium constant expression. Then we complete the reaction summary, substimte the algebraic representations of equilibrium concentrations into the equilibrium constant expression, and solve for the unknown concentration(s). 

The basic substance (B) that originally accepted the proton becomes the conjugate acid (BH+) of that substance, since the conjugate acid could conceivably donate a proton to an even stronger base than the original substance. The concentration-based ionization constant expression corresponding to equation... 

Kxcii is the acidity constant of the acid and K-Raae is the basicity constant of the base. For a corresponding acid-base system, AAcid is the reciprocal of ABase- In equations (4) and (5) [aH ] denotes the proton activity, and ca and cb are respectively the conventional activities of the acid and the base and are set equal to the concentration c in very dilute solutions. The hydrogen ion activity on the contrary is expressed in absolute units and at first glance it seems rather arbitrary to make the distinction. We shall see, however, that (4) and (5) give us practical and serviceable expressions for the acidity and basicity constants which now permit comparisons between different solvents. 

B. Expressing Relative Basicities of Amines The Basicity Constant... 

Relative basicities are expressed using the basicity constant, Kb, which is defined as the concentrations of the ionized amine products in water (ammonium salt and hydroxide) divided by the concentration of the un-ionized amine. Larger Kb s mean greater basicity. pKb is the negative logarithm of Kb the smaller the pKb the stronger the base. 

Section 22.4 Basicity of amines is expressed either as a basicity constant K, (p/fb) of the amine or as a dissociation constant (pA a) of its conjugate acid. 

Because this reaction produces OH ions, the sodium acetate solution will be basic. The equilibrium constant for this hydrolysis reaction is the base ionization constant expression for CHaCOO, so we write (see p. 615)... 

Section 22.4 Basicity of amines is expressed either as a basicity constant of... 

In the Bronsted-Lowry theory all substances are considered, to some extent, ainphoteric. Theoretically, the strength of individual acids can be expressed through the equilibrium constant of process (I), K, called the "acidity constant" its reciprocal is termed the "basicity constant," /Tt,. Since neither acidity nor basicity of a system (system 1) manifest themselves without a proton acceptor or a proton donor (system 2), the individual constants and cannot be measured directly. One can, however, measure the equilibrium constant of the protolysis reaction (II) ... 

The strength of Lewis acid-base interactions cannot be expressed in terms similar to the acidity and basicity constants. K. and of the Bronsted-Lowry theory. Consequently an equilibrium constant resembling the protolytic constant of Bronsted acid-base couples, Eq. (1), cannot be specified. Because of the broad variety of Lewis acid-base interactions there would be as many acid strength scales as there are interacting bases. 

The half-reactions in (c), (e), and (f) are obtained by combining the solubility constant expressions in (d) and (g) with the basic Nernst expressions (a) and (b), equations (12-6) and (12-7). One example should make the method clear. Let us derive the e line. 

A (a) Write a chemical equation showing the ionization of water, (b) Write the equilibrium constant expression for this equation, (c) What is the special symbol used for this equUibrium constant (d) What is the relationship between [H" ] and [OH ] in pure water (e) How can this relationship be used to define the terms acidic and basic ... 

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