Acid-base equilibria weak bases

An inflection point in a pH-rate profile suggests a change in the nature of the reaction caused by a change in the pH of the medium. The usual reason for this behavior is an acid-base equilibrium of a reactant. Here we consider the simplest such system, in which the substrate is a monobasic acid (or monoacidic base). It is pertinent to consider the mathematical nature of the acid-base equilibrium. Let HS represent a weak acid. (The charge type is irrelevant.) The acid dissociation constant, = [H ][S ]/[HS], is taken to be appropriate to the conditions (temperature, ionic strength, solvent) of the kinetic experiments. The fractions of solute in the conjugate acid and base forms are given by... 

In Chapter 13 we dealt with the equilibrium established when a single solute, either a weak acid or a weak base, is added to water. This chapter focuses on the equilibrium established when two different solutes are mixed in water solution. These solutes may be—... 

Consider now the salt of a strong acid and a weak base class (3). Here the initial high concentration of cations M + will be reduced by combination with the hydroxide ions of water to form the little-dissociated base MOH until the equilibrium ... 

Case 2. Salt of a strong acid and a weak base. The hydrolytic equilibrium is represented by ... 

Our goal in this chapter is to help you continue learning about acid-base equilibrium systems and, in particular, buffers and titrations. If you are a little unsure about equilibria and especially weak acid-base equilibria, review Chapters 14 and 15. You will also learn to apply the basic concepts of equilibria to solubility and complex ions. Two things to remember (1) The basic concepts of equilibria apply to all the various types of equilibria, and (2) Practice, Practice, Practice. 

Earlier, we noted that the conjugate base of a weak acid is a weak base, and the conjugate acid of a weak base is a weak acid. We also derived an exceedingly important relation between the equilibrium constants for a conjugate acid-base pair Ks- Kb = Kk. 

Recall that water can be considered a weak acid (or a weak base) because it dissociates into a proton and a hydroxide ion, according to the equilibrium... 

As a rule, hydrogen ion is involved not only in the pH-dependency of the reaction term (Thiele modulus) but also as the actively participating species involved in the acid-base equilibrium of all the substrates, reaction intermediates, products, and even the gel matrix. Furthermore, enzymatic reactions are always carried out in the presence of the mobile buffer. By mobile we mean a weak acid or a weak base that can move in and out of the reaction layer, as opposed to the fixed buffer represented by the gel (and by the protein) itself. Thus, we have to include the normalized diffusion-reaction equations for hydrogen ion and for the buffer. 

Because of the presence of the nucleophilic oxygen and electrophilic proton, alcohols can act both as weak acids and as weak bases when dissolved in water (Following fig.). However, the equilibrium in both cases is virtually completely weighted to the unionised form. 

An equation for the observed capacity factor (kobs) can be derived if it is assumed that the acid-base equilibrium kinetics are fast enough to be considered as instantaneous. This corresponds to the observation of a single sharp peak for the ionizable species in the chromatogram. In that case it may be assumed [340,342,343] that for the weak acid HA... 

Having a conceptual understanding of the effect is a good starting point, but we still need to be able to understand the quantitative relationships between the different components in the equilibrium mixture. In this section, we will see how to deal with the common-ion effect in acid-base equilibrium problems. You will find that these problems are very similar to the weak acid problems earlier in the chapter. 

If the conjugate-base-dissociation mechanism is the correct one, the lower substitution rates observed with nitrite, acetate, and azide may be considered to arise because these weakly basic anions are less effective than methoxide in generating conjugate base CB in the preliminary acid base equilibrium. 

In contrast to strong bases, a weak base ionizes only partially in dilute aqueous solution to form the conjugate acid of the base and hydroxide ion. The weak base methylamine (CH3NH2) reacts with water to produce an equilibrium mixture of CH3NH2 molecules, CH3NH3+ ions, and OH ions. 

Consider a typical weak acid, formic acid (HCOOH), and its conjugate base, formate ion (HCOO ). The latter can be obtained by dissolving a salt such as sodium formate (NaHCOO) in water. The acid-base equilibrium established between... 

Outline the procedure for the exact treatment of acid-base equilibrium and use it to find the pH of a very dilute solution of a weak acid or base (Section 15.8, Problems 69-70). 

We are often interested in the changes in composition that occur while a solution of a weak acid or a weak base is being titrated. These changes can be visualized by plotting the relative equilibrium concentration uq of the weak acid as well as the relative equilibrium concentration of the conjugate base aj as functions of the pH of the solution. 

Apply acid-base equilibrium concepts to salts of strong bases and weak acids... 

Small weak acids and bases such as propionic acid, ammonia and carbon dioxide are very permeant and cross cell membranes very quickly. Equilibrium with the concentration of the acid or base in the medium occurs in a few seconds. The addition of a weak acid or a weak base to the medium becomes a threat to the... 

The reaction occurs only because ammonium butyrate, being the salt of a weak acid and a weak base, is in equilibrium with a significant amount of ammonia and butyric acid. The actual dehydration step is probably the result of nucleophilic additon of ammonia to the carboxyl group of butyric acid itself. 

Buffering in aqueous solution is based on a weak acid-base equilibrium ... 

While NO is a free radical with a doublet ground state, and therefore involved in several radical reactions, HNO has a singlet ground state (47,48). It is a weak acid with an accepted pi a of 11.4 (37,49). Interestingly enough, its anion, NO , has a triplet ground state (50,51), the same as the isoelectronic O2 molecule. Therefore, loss of a proton from HNO is not a simple acid/ base equilibrium but a spin-forbidden slow deprotonation (Table I, Eq. (8)) (37) ... 

---