Protonic acid defined

Acid strength may be defined as the tendency to give up a proton and base strength as the tendency to accept a proton. Acid-base reactions occur because acids are not equally strong. If an acid, say HCI, is placed in contact with the conjugate base of a weaker acid, say acetate ion, the proton will be transferred because the HCI has a greater tendency to lose its proton than acetic acid. That is, the equilibrium... 

From this discussion it can be seen that there is no ideal acid-base theory for AB cements and a pragmatic approach has to be adopted. Since the matrix is a salt, an AB cement can be defined quite simply as the product of the reaction of a powder and liquid component to yield a salt-like gel. The Bronsted-Lowry theory suffices to define all the bases and the protonic acids, and the Lewis theory to define the aprotic acids. The subject of acid-base balance in aluminosilicate glasses is covered by the Lux-Flood theory. 

Various groups of workers have attempted to determine the k, by the use of common-ion salts so as to repress the dissociation of the ion-pairs at the growing end. In this context the two types of initiator pose different, but related, problems. If the initiator is a protonic acid AH, as in most of the stopped-flow experiments, the greatest part of the AH does not participate in the initiation (see Section 4.3.2). In these systems, therefore, conjugate ions A2H can be formed from the added salt Bs+A" and therefore the effective [A ] < [Bs+A ]0. In the usual plots [Equation (23) below] of kp (defined in Section 2.5.3) versus [Bs+A ]01/2 this uncertainty will not affect the intercept k, at [Bs+A ]0 1/2 = 0, but it will make the slope, from which the kp can be found, smaller, so that the resulting kp will be an underestimate. 

Later on, Pearson [75] introduced the concept of hard and soft acid and bases (HSABs) hard acids (defined as small-sized, highly positively charged, and not easily polarizable electron acceptor) prefer to associate with hard bases (i.e., substances that hold their electrons tightly as a consequence of large electronegativities, low polarizabilities, and difficnlty of oxidation of their donor atoms) and soft acids prefer to associate with soft bases, giving thermodynamically more stable complexes. According to this theory, the proton is a hard acid, whereas metal cations may have different hardnesses. 

Titration method (consisting in the stndy of the interaction of indicator dyes with solids from solntions) is a techniqne for both qnalitative and qnantitative characterization of solid snrfaces. If a basic indicator B is nsed, the proton acidity of the snrface is expressed by the Hammett acidity fnnction. Similarly, the basicity can be defined when an acid is converted by its conjngated base. This allows defining acidity and basicity in the same scale. 

But. wlial is an acid Earlier, the acidic traits were used to define an acid. But with the modem understanding of the atom, a different definition is used. You will remember thaL the nucleus of an atom contains positively charged protons. Acids in solution liberate protons as ions (H ). And so we say that an acid is a substance that will give up — or "donate — protons to another substance. Acids are proton donors. The foremost acids used in industry are sulfuric acid (HjSOJ, nitric acid (HNOa), and hydrochloric acid (HC1). 

The first clear definition of acidity can be attributed to Arrhenius, who between 1880 and 1890 elaborated the theory of ionic dissociation in water to explain the variation in strength of different acids.3 Based on electrolytic experiments such as conductance measurements, he defined acids as substances that dissociate in water and yield the hydrogen ion whereas bases dissociate to yield hydroxide ions. In 1923, J. N. Brpnsted generalized this concept to other solvents.4 He defined an acid as a species that can donate a proton and defined a base as a species that can accept it. This... 

The total protonation energy, defined as the difference, AE, between the total energy of the non-protonated molecule and that of its conjugated acid was analyzed via theoretical quantum chemical calculations. It was considered that this energy could be split into three additive contributions, electronic (Ae), electrostatic (AZ elst) and polarization (A p0lar). An increase was observed on going from carbon to silicon that was attributed mainly to the difference in the respective polarizabilities of carbon and silicon atoms.34,35... 

Early workers in the field of H20-D20 systems recognized that it was possible to define different acidity constants for the isotopic hydrogen ions, that for an isotopically mixed ion it was necessary to distinguish between proton acidity and deuteron acidity and that it... 

In our discussion of acid catalysis we have up to now restricted our attention to protonic or Bronsted acids. However, the Bronsted acid, defined as a proton donor, calls into existence the Bronsted base, which can act as proton acceptor. But such a base accepts protons by virtue of the fact that it can offer the proton an unshared pair of electrons for bonding, e.g.,... 

The meaning to be attached to pH values obtained in different solvents and under different conditions needs interpretation. For example, solutions of pH 5 in the three solvents water, ethanol, and acetonitrile do not denote the same acidity (acidity defined as a measure of the tendency for protons to be donated to basic substances). In view of what is known about transfer activity coefficients (Section 4-1), an acetonitrile solution of pH 5 is far more acidic (higher absolute activity) than an ethanol solution of pH 5 and, in turn, one in ethanol is more acidic than one in water. The significant point is that a solution of, say, pH 5 in a given solvent has an acidity 10 times the acidity of another solution of pH 6 in that same solvent. A single universal scale of pH for all solvents does not exist instead there is a different scale for every solvent of differing composition. To denote pH values in nonaqueous solvents, the... 

Almost simultaneously with the papers of Brpnsted (26) and Lowry (25) on proton acids, another viewpoint on acids was formulated by G. N. Lewis (44). Lewis defined a basic molecule as one that has an elec-... 

Rather than introduce new words into an already confusing terminology, Bell (64) is of the opinion that the words acid and base should be confined to proton acids, and the Sidgwick classification of molecules as electron donor and electron acceptor (which is essentially equivalent to the Lewis classification of acids and bases) be employed, together with the categories of nucleophilic and electrophilic reagents as defined by Ingold (65). 

As mentioned before, ajc refers to the ionization fraction of the species that has lost X protons from the most protonated acid species / defines the equivalence points, the point at the lowest pH being / =0.)... 

The related concepts of acidity and basicity are fundamental to studies of chemical behaviour. The Bronsted acidity of a protonic acid HA is usually expressed in terms of pKa, as defined by the negative of the logarithm of the equilibrium constant for reaction 1. 

The final acid-base concept we consider was developed by Gilbert N. Lewis, whose contribution to understanding the importance of valence electron pairs in molecular bonding we discussed in Chapter 9. Whereas the Brpnsted-Lowry concept focuses on the proton in defining a species as an acid or a base, the Lewis concept highlights the role of the electron pair. The Lewis acid-base definition holds that... 

Figure 5 compares the activity of the silica-supported SAC-13 with unsupported Nafion particles. Both catalysts show similar product selectivity at identical conditions. As expected, the butene turnover frequency, defined as the butene conversion rate per acid site, is enhanced fourfold when using the supported catalyst. Similar enhancements on supported Nafion have been reported for other reactions as well [10,11,24]. Since hydrated Nafion can conduct protons, acid sites that are hidden within the polymer may not be available for reaction, but may still be measured by the aqueous titration method. 

The medium effect on the proton was defined by eqn. 3.3.9. Medium effects on a molecular or ionic weak acid and conjugate base... 

In 1923 Bronsted and Lowry each developed an acid-base theory based on the central role of the proton. They defined an acid as a proton donor and a base as a proton acceptor. Thus, an acid-base reaction is one in which proton transfer occurs, i.e.. 

Systems defined by the Arrhenius description, solvent system, Lux-Flood and proton acid-base definitions... 

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