Some Bronsted acids and bases

The activity of any pure substance in its standard state is defined to be unity. 

The relative activity of a solute is related to its molality by equation 7.7 where 7 is the activity coefficient of the solute, and m and m° are the molality and standard state molality, respectively. Since the latter is defined as being unity, equation 7.7 reduces to equation 7.8. 

Although aU thermodynamic expressions dealing with aqueous solutions should strictly be expressed in terms of activities, inorganic chemists, and students in particular, may be dealing with situations in which two criteria are true  

If these criteria hold, then we can approximate the activity of the solute to its concentration, the latter being measured, most often, in molarity. We use this approximation throughout the book, but it is crucial to keep in mind the limitations of this approach. 

The smaller the value of pW, the stronger the acid. The larger the value of K, the stronger the base. 

For a solute in a solution, the definition of its standard state is referred to a situation of infinite dilution it is the state (a hypothetical one) at standard molality m°), Ibar pressure, and exhibiting infinitely dilute solution behaviour. In the standard state, interactions between solute molecules or ions are negligible. 

When the concentration of a solute is greater than about 0.1 moldm , interactions between the solute molecules or ions are significant, and the effective and real concentrations are no longer equal. It becomes necessary to define a new quantity called the activity, which is a measure of concentration but takes into accoxmt the interactions between the solution species. The relative activity, a, of a component i is dimensionless and is defined by eq. 7.6 where is the chemical potential of component i, yL° is the standard chemical potential of i, R is the molar gas constant, and T is the temperature in kelvin.  

Equilibrium 6.9 describes the dissociation of MeC02H in aqueous solution it is a weak acid with = 1.75 x 10 at 298 K. 

Acids 6.2 and 6.3 undergo stepwise dissociation in aqueous solution, and equations 6.10 and 6.11 describe the steps for oxalic acid. 

Carboxylic acids examples of mono-, di- and polybasic acids 

In organic compounds, acidity is quite often associated with the presence of a carboxylic acid group (CO2H) and it is relatively easy to determine the number of ionizable hydrogen 

Each dissociation step has an associated equilibrium constant (acid dissociation constant), and it is general for polybasic acids that Aia(l) K 2), and so on it is more difficult to remove H from an anion than from a neutral species. Values of equilibrium constants may be temperature-dependent, and the inclusion of the temperature to which 

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Some of the earliest chemical studies of nonhydrocarbons, which resulted in the isolation of numerous strong acids and bases, were conducted at the University of Texas under the leadership of Lochte and Littman (I). This work used the classical extraction of petroleum with mineral acids and caustic followed by chemical identification (boiling point, refractive index, derivatives, etc.). Since only strong Bronsted acids and bases can be isolated by aqueous solvents, their identifications were limited to low-molecular-weight species aqueous solvents also do not isolate weak acids or bases (low pK values). 

The recent explosion in the development of asymmetric strategies for organic synthesis has fostered investigations into the discovery of methods for enantioselective and diastereoselective Diels-Alder reactions. Some early forays into this field focused on the use of chiral auxiliaries covalently attached to one of the reaction partners however, nearly all recent investigations have centered on developing chiral catalysts. The multitude of new catalysts spans the range of Lewis acids and Bronsted acids and bases as well as metal-based and organic molecules. 

Note that we do not ever need to refer to hydrolysis to explain acid-base behavior in water. It only confuses the issue. All the Bronsted acids and bases follow identical patterns of proton transfer. There is no chemical reason to call some of these examples dissociation and some hydrolysis ... 

Johannes Nicolaus Bronsted (1879-1947). Danish chemist. In addition to his theory of acids and bases, Bronsted worked on thermodynamics and the separation of mercury into its isotopes. In some books, Bronsted acids and bases are called Bronsted-Lowry acids and bases. Thomas Martin Lowry (1874-1936). English chemist. Bronsted and Lowry developed essentially the same acid-base theory independently in 1923. 

General acid/base catalysis is less significant in natural fresh waters, although probably of some importance in special situations. This phenomenon can be described fairly well via the Bronsted law (relating rate constants to pKa and/or pKb of general acids and bases). Maximum rates of general acid/base catalysis can be deduced from a compound s specific acid/base hydrolysis behavior, and actual rates can be determined from relatively simple laboratory experiments (34). 

Bronsted, J. N. (1923). Some remarks on the concept of acids and bases. Recueil des Travaux Chimiques des Pays-Bas 42 718-728. 

In contrast to some related reviews, which use reaction class or electrophiles as organizational elements, this chapter is divided into three main sections according to catalyst class (i) Bronsted acid catalysis by phosphoric acid and phosphoramide derivatives, (ii) N—H hydrogen bond catalysis by organic base and ammonium systems, and (iii) combined acid catalysis including Bronsted-acid-assisted Bronsted acid, Lewis-acid-assisted Bronsted acid, and Lewis-acid-assisted Br0nsted acid systems (Figure 5.1). 

You no doubt noticed that some of the bases in Table 16-1 don t contain a hydroxide ion, which means that the Arrhenius definition of acids and bases can t apply. When chemists realized that several substances behaved like bases but didn t contain a hydroxide ion, they reluctantly acknowledged that another determination method was needed. Independently proposed by Johannes Bronsted and Thomas Lowry in 1923 and therefore named cifter both of them, the Bronsted-Lowry method for determining acids and bases accounts for those pesky non-hydroxide-containing bases. 

Acid and Base Catalysis. Opportunities are now available for checking some of the theories of acids and bases which involve both the manner of readjustment within the molecule and the function of the catalyst. An acid dissociates according to the Bronsted theory to give a proton and a base. The rate of a given change will be faster for a proton than for a deuton but there will be little differ-... 

In recent years several more general concepts of acids and bases have been introduced. They are useful for some purposes, such as the discussion of non-aqueous solutions. One of these concepts, due to the Danish chemist J, N. Bronsted, is that an acid is any molecular or ionic species which can give up a proton (which is a proton donor), and a base is any one which can take up a proton (which is a proton ac ceptqr). Thus NH + is called an acid, since it can give up a proton ... 

Table 2.11 lists the principal types of solid base catalysts. We should remember, however, that base catalyst is a relative definition and thus the materials listed in Table 2.11 do not necessarily function as a base in all cases. Some of these materials may act as an acid if the reactants are strongly basic. The terms, acid and base, should be used according to the function. The materials may be called solid base catalysts only if acting as a base toward the reactants by abstraction of a proton (Bronsted base) or by donation of an electron pair (Lewis base) to form anionic intermediates that undergo catalytic cycles. 

A major problem with Arrhenius s acid-base theory is that some substances, like ammonia, produce basic solutions and react with acids, but do not contain hydroxide ions. In 1923 Johannes Bronsted, a Danish chemist, and Thomas Lowry, an English chemist, independently proposed a new way to define acids and bases. An acid donates hydrogen ions (also called a proton donor) a base accepts hydrogen ions (also called a proton acceptor). These definitions not only explain all the acids and bases covered by Arrhenius s theory, they also explain the basicity of ammonia and ions such as carbonate, and phosphate, P04 ... 

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