Acid-base equilibria determining major species

Most acids and bases are weak. A solution of a weak acid contains the acid and water as major species, and a solution of a weak base contains the base and water as major species. Proton-transfer equilibria determine the concentrations of hydronium ions and hydroxide ions in these solutions. To determine the concentrations at equilibrium, we must apply the general equilibrium strategy to these types of solutions. 

The titration reaction is lSIH3(a ij) -I-H3 0 (a q) NH4 (a q) + H2 0(/) At the stoichiometric point, all the ammonia molecules have been converted to ammonium ions, so the major species present are NH and H2 O. The pH of the solution is thus determined by the acid-base equilibrium of... 

The major species in an aqueous solution determine which categories of equilibria are important for that solution. Each major species present in the solution must be examined in light of these general categories. Are any of the major species weak acids or weak bases Are there ions present that combine to form an insoluble salt Do any of the major species participate in more than one equilibrium Any chemical reaction can approach equilibrium from either direction. Consequently, there are six different t q)es of aqueous equilibria in which major species are reactants ... 

Four anthocyanin species exist in equilibrium under acidic conditions at 25°C/ according to the scheme in Figure 4.3.3. The equilibrium constant values determine the major species and therefore the color of the solution. If the deprotonation equilibrium constant, K, is higher than the hydration constant, Kj, the equilibrium is displaced toward the colored quinonoidal base (A), and if Kj, > the equilibrium shifts toward the hemiacetalic or pseudobase form (B) that is in equilibrium with the chalcone species (C), both colorless." - Therefore, the structure of an anthocyanin is strongly dependent on the solution pH, and as a consequence so is its color stability, which is highly related to the deprotonation and hydration equilibrium reaction constant values (K and Kj,). 

Plan The major species in the solution will be NH4, Cl, and NH3. Of these, the Cl" ion is a spectator (it is the conjugate base of a strong acid). Thus, the NH4 -NH3 conjugate acid-base pair will determine the pH of the buffer. The equilibrium relationship between NH4 and NH3 is given by the base-dissociation reaction for NH3 ... 

In the preceding two acid-base reactions, water was the base (proton acceptor). But what if we have a base other than water as the proton acceptor How can we determine which are the major species present at equilibrium That is, how can we determine whether the position of equilibrium lies toward the left or toward the right ... 

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