Acid-base equilibria anion reaction with water

STRATEGY The solution contains N02 , a base, so we expect the pH to be i higher than that of nitrous acid alone. The K+ ion has no protons to donate j and cannot accept a proton, so it has no measurable effect on the pH of the ] solution. Identify the proton transfer equilibrium and use it to find the pH by means of an equilibrium table. Consider the initial molarity of HN02 (before reaction with water) to be 0.500 mol-L1. Because nitrite ions have also been added to the solution, set their initial molarity equal to the molar-5 ity of added salt (each KN02 formula unit supplies one N02 anion). Then proceed as described in Toolbox 10.1. Because the concentrations of the 1 added ions are much higher than 10-7 mol-L 1, we assume that we can S ignore the contribution to the pH from the autoprotolysis of water. 

Water reacts with HCl, and the organic acid known as formic acid (HCOOH, 1) also reacts with water as a weak acid, as shown. Formic acid is a much weaker acid than HCl. When 1 reacts with water, the conjugate base is the formate anion, 2, and the conjugate acid is the hydronium ion. If 1 is a weaker acid than HCl, the equilibrium for 1 + HgO lies to the left in the reaction shown when compared to the reaction of HCl + H2O in Section 2.1. Note the (aq) term indicates solvation by the solvent water. Note also that the term reaction is used for the acid-base equilibrium. The acid-base equilibria shown for HCl and HCOOH are chemical reactions that generate two products the formate anion (HCOO , 2) and the hydronium ion (from 1) or the hydronium ion and the chloride ion (from HCl). 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

Determine whether the cation is from a strong or weak base, and whether the anion is from a strong or weak acid. Ions derived from weak bases or weak acids react with water and affect the pH of the solution. If both ions react with water, compare the equilibrium constants K and Aib) to determine which reaction goes farthest to completion. 

On the other hand, if HA is an uncharged acid z = — V, e.g. CH3—CO2H), the right-hand side of Eq. (4-10) involves the sum of two reciprocal radii (zha = 0) and a strong influence of the relative permittivity on the ionization equilibrium is expected. Because in acid/base reactions of this charge type, neutral molecules are converted into anions and cations, which attract each other, reaction (4-5) will shift to the right with an increase in relative permittivity of the solvent in which HA is dissolved. Ionization increases when increases. This rule is qualitatively verifiable for water and alcohols as... 

However, the case in which the solubility of a solid can be calculated from the known analytical concentration of added components and from the solubility product alone is very seldom encountered. Ions that have dissolved from a crystalline lattice frequently undergo chemical reactions in solution, and therefore other equilibria in addition to the solubility product have to be considered. The reaction of the salt cation or anion with water to undergo acid-base reactions is very common. Furthermore, complex formation of salt cation and salt anion with each other and with one of the constituents of the solution has to be considered. For example, the solubility of FeS(s) in a sulfide-containing aqueous solution depends on, in addition to the solubility equilibrium, acid-base equilibria of the cation (e.g., Fe + H2O = FeOH + H ) and of the anion (e.g., S + HjO = HS + OH, and HS" + H2O = H2S + OH ), as well as on equilibria describing complex formation (e.g., formation of FeHS" or FeSi ). 

Carboxylate salts are bases because the carboxylate anion RCOO accepts a proton from a base, water in (a) and HCl in (f>). Note that the equilibrium is only partially to the right-hand side for reaction with the weak base water but completely to the right-hand side for the strong acid HCl. 

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. 

As discussed in this chapter, enolate anions are formed when a carbonyl compound containing an a-hydrogen is treated with a base such as hydroxide or an alkoxide. We noted earlier that a-hydrogens normally are considerably less acidic than water or alcohols, so the position of equilibrium in this acid-base reaction greatly favors the reactants rather than enolate products. 

I. Salts of strong acids and strong bases, when dissolved in water, show a neutral reaction, as neither the anion nor the cation combines with hydrogen or hydroxyl ions respectively to form sparingly dissociated products. The dissociation equilibrium of water... 

---