Acid-base reactions with ionic compounds

The three types of solids, metals, covalent semiconductors or insulators, and ionic compounds (including oxides) have characteristic surface reactions. In organic catalysis only metals and ionics are considered (Table 6.5), while in CVD all three types of solid surfaces are of interest. Metals absorb hydrogen and nitrogen dissociatively while ionic substrates have redox reactions or acid/base reactions with molecules. Oxidation of gases is often catalyzed by the surface of metal oxides. So is deposition of oxides by oxidation and hydrolysis of metal-containing precursors. When mixed oxides (e.g., perovskites) are deposited care must be taken to ensure a sufficient availability of the separate components. 

Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems. Factors that affect redox kinetics include environmental redox conditions, ionic strength, pH-value, temperature, speciation, and sorption (Tratnyek and Macalady, 2000). Sediment and particulate matter in water bodies may influence greatly the efficacy of abiotic transformations by altering the truly dissolved (i.e., non-sorbed) fraction of the compounds — the only fraction available for reactions (Weber and Wolfe, 1987). Among the possible abiotic transformation pathways, hydrolysis has received the most attention, though only some compound classes are potentially hydrolyzable (e.g., alkyl halides, amides, amines, carbamates, esters, epoxides, and nitriles [Harris, 1990 Peijnenburg, 1991]). Current efforts to incorporate reaction kinetics and pathways for reductive transformations into environmental exposure models are due to the fact that many of them result in reaction products that may be of more concern than the parent compounds (Tratnyek et al., 2003). 

In aqueous solutions of ionic compounds, the ions act independently of each other. Soluble ionic compounds are written as their separate ions. We must be familiar with the solubility rules presented in Chapter 8 and recognize that the following types of compounds are strong electrolytes strong acids in solution, soluble metallic hydroxides, and salts. (Salts, which can be formed as the products of reactions of acids with bases, include all ionic compounds except strong acids and bases and metalhc oxides and hydroxides.) Compounds must be both ionic and soluble to be written in the form of their separate ions. (Section 9.1)... 

On the basis of self-ionic dissociation, these compounds can be prepared by acid-base reactions. Heteropolyhalogen cations are usually prepared by reacting the parent compound with a Lewis acid (equation 51) in which XY = interhalogen and MYm = Lewis acid, for example, hahdes of B, Al, P, As, and Sb, and so on (equations 52 and 53). Such reactions can be performed by direct interaction of the reactants with an excess of the more volatile reactant, which can then be pumped off, after completion of the reaction, leaving behind the pure product. Sometimes it is preferable to perform such reactions in solution, such as in anhydrous hydrogen fluoride (AHF), and pump off the solvent at the end of the reaction. 

The acetylide anions discussed in Chapter 11 are another example of organometallic compounds. These reagents are prepared by an acid-base reaction of an alkyne with a base such as NaNH2 or NaH. We can think of these compounds as organosodium reagents. Because sodium is even more electropositive (less electronegative) than lithium, the C-Na bond of these organosodium compounds is best described as ionic, rather than polar covalent. 

For example, when cyclohexylamine is treated with aqueous HCl, it is protonated, forming an ammonium salt. Because the ammonium salt is ionic, it is soluble in water, but insoluble in organic solvents. A similar acid-base reaction does not occur with other organic compounds like alcohols, which are much less basic. 

In Section 4-9 we introduced classical acid-base reactions. We defined neutralization as the reaction of an acid with a base to form a salt and (in most cases) water. Most salts are ionic compounds that contain a cation other than H+ and an anion other than OH or The common strong acids and common strong bases are listed in the margin. All other common acids may be assumed to be weak. The other common metal hydroxides (bases) are insoluble in water. 

Another simple test that distinguishes acids from bases is the reaction of acids with ionic compounds that contain the carbonate ion,, to form carbon dioxide gas, water, and another compound, as shown in Figure 14.3. A similar reaction, also shown in Figure 14.3, is the source of the destructive action of acidic pollution on marble and limestone sculptures. Bases do not react with carbonates. 

Differentiate between the formula equation, the complete ionic equation, and the net ionic equation. For each reaction in Question 6, write all three balanced equations. What is an acid-base reaction Strong bases are soluble ionic compounds that contain the hydroxide ion. List the strong bases. When a strong base reacts with an acid, what is aiways produced Expiain the terms titration, stoichiometric point, neutralization, and standardization. 

The most general view of acids and bases was advanced by G. N. Lewis. In this model, acids are substances which have an affinity for lone electron pairs, and bases are substances which possess lone electron pairs. Water and ammonia are the most common substances which possess lone electron pairs, and therefore behave as bases in the Lewis scheme. The reaction of silver ion, Ag with cyanide ion, CN , and boron trifluoride, BF3 (an electron-deficient compound), with ammonia, NH3, are two examples of Lewis acid-base reactions. The Lewis acid-base concept is most useful in chemical reactions in nonaqueous solvents. We will not find it useful in our study of ionic equilibria in water. 

The scientific concepts of acids and the complementary compounds, bases , have developed over several centuries with the development of chemistry. Here, it is only necessary to start with the idea associated with Liebig (1838) and Arrhenius (1887), which is still familiar from introductory courses in chemistry. This defines an acid as a compound containing hydrogen that can be replaced by a metal to form a salt. In ionic terms, this amounts to defining an acid as a species that gives hydrogen ions (protons) a base is associated with hydroxyl ions. A typical acid-base reaction can be represented as... 

An acid is a compound that releases hydrogen ions, H+. A substance that contains hydroxide ions, OH , is a base. When an acid is added to an equal amount of base, each hydrogen ion reacts with a hydroxide ion to form a molecule of water. The acid and the base neutralize each other in a neutralization reaction. An ionic compound called a salt is also formed it usually remains in solution. The general equation, using M to represent a metal ion, is... 

Probably the most popular and the most preferred method for the resolution of organic acids or bases is a chiral resolution via diastereomeric salt formation. Ionic salts are easily formed and easily crystallized, and after the separation process, an enantiomerically pure separated compound may be easily isolated, and the resolving agent can be recovered and reused (Figure 1.37). Resolution via diastereomeric salt formation involves the acid-base reaction of a racemate with an enantiomerically pure resolving agent. The resulting two diastereomers have different physical properties e.g., the difference in solubility is used to separate them by crystallization. 

Because many redox reactions involve H, OH , or insoluble ionic compounds, it is easy to confuse a redox reaction with an acid-base or a precipitation reaction. It is important that you remember the defining features of each type of reaction. Precipitation reactions involve the combination of ions in solution to produce an insoluble precipitate, acid-base reactions involve proton (H ) transfer, and redox reactions involve changes in oxidation states. 

Diborane reacts with ammonia to form an ionic compound (there are no other products). The cation and anion each contain one boron atom, (a) Predict the identity and formula of each ion. (b) Give the hybridization of each boron atom, (c) Identify the type of reaction that has occurred (redox, Lewis acid-base, or Bronsted acid-base). 

None of the other reactions so far discussed involve interaction between a pair of charged species. This is but another instance of the electrostatic effect shown by Kirkwood and Westheimer to be responsible for the disparity between the first and second ionization constants of dibasic acids, for the effect of the carboxylate ion on the basicity of an a-amino acid, and for the difference in reactivity of ionic compounds compared with analogous nonionic species in acid- or base-catalyzed reactions. ... 

Neutral solutions also contain a dissolved salt, derived from the neutralization of the acid and the base (a salt is an ionic compound formed in a neutralization reaction and is composed of the cation of an alkali and the anion of an acid). When a solution of carbonic acid (formed when atmospheric carbon dioxide dissolves in water), for example, reacts with an alkaline solution of lime, the two solutions neutralize each other and form a salt, calcium carbonate ... 

Compounds containing carbenium, silyl or phosphonium cations can act as Lewis acids. In addition, phosphorus- and silicon-based hypervalent compounds display a Lewis acid catalytic activity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have shown the ability to catalyze a group of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The solvents can be efficiently recovered after the reaction. Each type of these compounds will be discussed in a separate section. 

A second type of synthetic route to meso-ionic l,3,4-thiadiazol-2-imines (247) is based on the acid-catalyzed reaction of N-thioacylhydrazines (232) with aryl isothiocyanates (Ar-NCS). " This reaction yields the s ts (248) as precursors of the meso-ionic heterocycles (247). An interesting variant upon this route involves the reaction between IV-thioacylhydrazines (232) and acyl isothiocyanates (RCO-NCS). This leads to the meso-ionic heterocycles 247, R = COzEt, CONMej, COMe, COCMe, COAr, and SOjPh. The investigation of these compounds by X-ray photoelectron spectroscopy is a good example of the application of this physical method for the examination of meso-ionic compounds. 

Many different types of reversible reactions exist in chemistry, and for each of these an equilibrium constant can be defined. The basic principles of this chapter apply to all equilibrium constants. The different types of equilibrium are generally denoted using an appropriate subscript. The equilibrium constant for general solution reactions is signified as or K, where the c indicates equilibrium concentrations are used in the law of mass action. When reactions involve gases, partial pressures are often used instead of concentrations, and the equilibrium constant is reported as (p indicates that the constant is based on partial pressures). and are used for equilibria associated with acids and bases, respectively. The equilibrium of water with the hydrogen and hydroxide ions is expressed as K. The equilibrium constant used with the solubility of ionic compounds is K p. Several of these different K expres-... 

For complete neutralization to take place, the proper amounts of acid and base must be present. The salt formed in the above reaction is NaCl. If the water were evaporated after completing the reaction, we would be left with common table salt. Sodium chloride is just one of hundreds of salts that form during neutralization reactions. While we commonly think of salt, NaCl, as a seasoning for food, in chemistry a salt is any ionic compound containing a metal cation and a nonmetal anion (excluding hydroxide and oxygen). Some examples of salts that result from neutralization reactions include potassium chloride (KCl), calcium fluoride (CaF ), ammonium nitrate (NH NOj), and sodium acetate (NaC2H302). 

Salts are a common variety of ionic compound. A salt can form from the reaction between a base and an acid (both of which we cover in Chapter 16). For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride (a salt) and water ... 

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