Weak base dissociation constants

The equilibrium constant expression is called the weak base dissociation constant, K[, and has the form ... 

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... 

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. 

Br0nsted-Lowery acids are H+ donors and bases are H+ acceptors. Strong acids dissociate completely in water. Weak acids only partially dissociate, establishing an equilibrium system. Weak acid and base dissociation constants (Ka and Kb) describe these equilibrium systems. Water is amphoteric, acting as both an acid or a base. We describe water s equilibrium by the Kw expression. A pH value is a way of representing a solution s acidity. Some salts and oxides have acid-base properties. A Lewis acid is an electron pair acceptor while a Lewis base is an electron pair donor. 

Table 8.3 lists the base dissociation constants for several weak bases at 25°C. Nitrogen-containing compounds are Bronsted-Lowry bases, because the lone pair of electrons on a nitrogen atom can bond with H+ from water. The steps for solving problems that involve weak bases are similar to the steps you learned for solving problems that involve weak acids. 

A similar situation exists for bases. Strong bases such as KOH, NaOH, and Ca(OH)2 dissociate completely in water. Weak bases don t dissociate completely in water, and their strength is measured by the base dissociation constant,... 

The equilibrium reaction of any base B with water is characterized by an equilibrium equation similar in form to that for the dissociation of a weak acid. The equilibrium constant K, is called the base-dissociation constant ... 

As usual, [H20] is omitted from the equilibrium constant expression. Table 15.4 lists some typical weak bases and gives their Kb values. (The term base-protonation constant might be a more descriptive name for Kb, but the term base-dissociation constant is still widely used.)... 

The base-dissociation constant, Kb, is the equilibrium constant for the addition of a proton to a weak base by water to form its conjugate acid and an 0H ion. 

For ammonia (the most common weak base), the equilibrium reaction and base-dissociation constant are ... 

Dis.sociation of a weak acid or base Dissociation constant, K.orKt, CH3COOH + HjO H3O+ + CH3COO K, =... 

Aliphatic amines generally have base dissociation constants on the order of 10 and can thus be titrated directly with a solution of a strong acid. In contrast, aromatic amines such as aniline and its derivatives are usually too weak for titration in aqueous medium ( Ib 10 °). The same is true for cyclic amines with aromatic character, such as pyridine and its derivatives. Many saturated cyclic amines, such as piperidine, tend to resemble aliphatic amines in their acid/base behavior and thus can be titrated in aqueous media. 

Base dissociation constant, /([, The equilibrium constant for the reaction of a weak base with water. 

Acids will dissociate into hydrogen ions, i.e., protons, and their matching anions. Bases will dissociate into a cation and a hydroxyl anion. If the acid or base is a strong acid or strong base, then the dissociation is complete. Weak acids and weak bases dissociate only partially. How much is defined by the dissociation constant of the acid or base. When expressed as the negative logarithm of the proton concentration in equivalents/liter, it is called the pK of the acid or base. Therefore, in a solution of a weak acid, three participants are present proton, matching anion, and the undissociated parent weak acid. Similarly, a solution of a weak base contains a cation, a hydroxyl anion, and the undissociated parent base. How much of each is defined by the pH of the solution and the pK of the respective acid or base The Henderson-Hasselbalch equation defines the relationship ... 

The base-dissociation constant, Kb, is the equilibrium constant for the addition of a proton to a weak base by water to form its conjugate acid and an OH- ion. In these reactions, it is water that is dissociating as a result of reaction with the base weak base(ag) + H20(/) conjugate acid(aqr) + OH- (aq)... 

Ammonia, amines, and anions of weak acids behave as weak bases in a process associated with a base-dissociation constant, K. The reaction of HA with H2O added to the reaction of A" with H2O gives the reaction for the autoionization of wata thus. Kg x Kb = Ky,. 

As with the relation between pK and K, we know that pA" , the negative logarithm of the base-dissociation constant, decreases with increasing K, (that is, increasing base strength). In aqueous solution, the two large classes of weak bases are (1) ammonia and the amines and (2) the anions of weak acids. 

The extent to which a weak base accepts a proton from water to form OH is expressed by a base-dissociation constant, K. Bronsted-Lowry bases include NH3 and amines and the anions of weak acids. All produce basic solutions by accepting from water, which yields OH and thus makes [HaO" ] < [OH ]. A solution of HA is acidic because [HA] [A ], so [HsO ] > [OH j. A solution of A is basic because [A ] >> [HA], so [OH ] > [H3O ]. By multiplying the expressions for Kg of HA and Kb of A , we obtain K. This relationship allows us to calculate either Kg of BH, the cationic conjugate acid of a molecular weak base B, or Kb of A , the anionic conjugate base of a molecular weak acid HA. 

The properties of primary importance in acid-base chemistry are the acid dissociation constant and the base dissociation constant Ky,. Consider first the dissociation of a weak organic acid HA ... 

As with K,y and K , the subscript b in Ky denotes that the equilibrium constant refers to a particular type of reaction, namely the ionization of a weak base in water. The constant Ky, the base-dissociation constant, always refers to the equilibrium in which a base reacts with H2O to form the corresponding conjugate acid and OH. ... 

SECTION 16.7 Weak bases include NH3, amines, and the anions of weak acids. The extent to which a weak base reacts with water to generate the corresponding conjugate acid and OH is measured by the base-dissociation constant, Ki,. This is the equilibrium constant for the reaction B(aq) + HjOU) HB (aq) + OH" (aq), where B is the hase. 

The relative strengths of Bronsted-Lowry bases can be measured based on their K, base dissociation constants. A strong base, such as NaOH or Mg(OH)2, has a Kb > I and will dissociate 100% in aqueous solution. The definition of K for the generic weak base A is given by Equation (14.9) ... 

Acid-base indicators Amphoteric oxides, hydroxides Common strong acids and bases Dissociation constants (complexions), Ionization constants for weak acids, K, Ionization constants for weak bases, Aj, Names and formulas of common ions Solubility product constants,... 

For weak acids and bases, the situation is more complicated. Here, the acid and base dissociation constants (ATa(b) or pATa(b) values) have to be considered. For such media there is no complete dissociation, but its degree is given by the equilibrium, Eq. 3. In the following, for reasons of simplification HaO" is replaced by HT and [i] stands for the activity of a component i ... 

Base (Arrhenius definition) a substance that produces hydroxide ions, OH , when it dissolves in water. (4.4 and 16.1) (Br0nsted-Lowry definition) the species (molecule or ion) that accepts a proton in a proton-transfer reaction. (4.4 and 16.2) Base-ionization (or base-dissociation) constant (A ) the equilibrium constant for the ionization of a weak base. Thus K), forNHsis 1.8 X 10 (173)... 

The following equilibrium equation for a typical weak base, B, is used to derive the generalized expression for the base dissociation constant... 

As we did with the acid dissociation expression, the concentration of water is omitted from the equilibrium expression, which gives the base dissociation constant, Ky, (or base ionization constant). Thus, for a weak base such as methylamine, the Ky, is written... 

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