Acid-base strengths acids weak bases

Protogenic solvents are acidic in nature and readily donate protons. Anhydrous acids such as hydrogen fluoride and sulphuric acid fall in this category because of their strength and ability to donate protons they enhance the strength of weak bases. 

As for acids, the strength of a base depends on the solvent a base that is strong in water may be weak in another solvent and vice versa. The common strong bases in aqueous solution are listed in Table J.l. 

In the treatment of weak acids, we found that the percentage deprotonated gave an indication of acid strength. Similarly, when we describe the strengths of weak bases, it is useful to know the percentage protonated, the percentage of base molecules that have been protonated ... 

Neutralization of a weak acid-weak base solution is a complex type of acid-base solution. If the weak acid and weak base have the same strength, the solution is neutral. If the strength of an acid and the strength of a base are not equal, the solution will be either acidic or basic, depending on the strength of either. 

A thermochemical method for comparing pKa values of weak acids in DMSO has been described by Arnett et al. (1973). A linear relationship is observed between AGf and AHf for 30 weak acids in DMSO over a range of 20 pKa units. Thus AHf, the enthalpy of ionization in DMSO, is a useful guide to relative acid/base strength (Arnett and Scorrano, 1976). The relationship implies, of course, that AS° is linearly related to AHf. Correlations have been drawn between acidities in DMSO and intrinsic gas-phase acidities (Arnett et al., 1975 Bordwell et al., 1975). 

More detailed comparisons are possible, but another factor, called the inherent acid-base strength, must also be kept in mind in these comparisons. An acid or a base may be either hard or soft and at the same time be either strong or weak. The strength of the acid or base may be more important than the hard-soft characteristics both must be considered at the same time. If two soft bases are in competition for the same acid, the one with more inherent base strength may be favored unless there is considerable difference in softness. As an example, consider the following reaction. Two hard-soft... 

Many substances which behave as acids in hydroxylic solvents exhibit basic properties in sulphuric acid. Thus most carboxylic acids are strong bases, forming the ion RCOOHJ, though reaction is incomplete for strong acids such as di- and tri-chloroacetic acids, which are thus weak bases in this solvent. Nitro-compounds, sulphones, and sulphonic acids also behave as weak bases, and it is in fact difficult to find substances which are soluble in sulphuric acid without detectable ionization. Since cryoscopic measurements lead only to the total number of solute particles, it is not possible to obtain quantitative measurements of base strength over a wide range, especially since there are complications caused by the self-dissociation of the solvent, and interionic effects, though small, must be taken into account. 

The large sensitivity of the rate constants to base strength for weakly basic catalysts indicates that the catalyst resembles its conjugate acid in the transition state, i.e. there is a large amount of, or complete, proton transfer to the catalyst in the transition state. For strongly basic catalysts the small sensitivity of the rate constants upon base strength suggests that the catalyst resembles its free unprotonated basic form in the transition state. 

You may be wondering what type of indicator is used to determine the equivalence point of weak-acid/weak-base titrations. The surprising answer is "none at all. The pH at the equivalence point of a weak-acid/ weak-base titration could be acidic, basic, or neutral, depending on the relative acid-base strengths. Because the pH value does not change dramatically as the equivalence point is approached, it is not practical to carry out weak-acid/weak-base titrations. 

Acid-base strength is not the seime as concentration. Concentration refers to the amount of acid or base that you initially have. You can have a concentrated solution of a weak acid, or a dilute solution of a strong acid, or a concentrated solution of a strong acid or. .. well, I m sure you get the ide L... 

The hydrogen bond, A-H. .. B, forms between a weak acid AH and a weak base, B. The M-H bond is a weak base, so it can form hydrogen bonds with N-H. . . H-M or O-H. . . H-M structures, commonly known as dihydrogen bonds (DHBs), and involving proton-hydride attractions. The bond strengths do not differ much from conventional HBs, and the H. .. H distance is typically 1.8 A, much shorter than the sum of their van der Waals radii (2.4 A). The N-H or O-H acid approaches side-on to the M-H base because the proton has to get close to the pair of electrons in the M-H bond that constitute the weak base. ... 

We can turn the above arguments around and compare the strengths of weak bases, A , such as acetate, by the extent to which they can retrieve a proton from water. A base, A , removes a proton from water to form hydroxide ion and the conjugate acid, HA. The base dissociation constant, for the reaction is analogous to the acid dissociation constant, AT. 

From practical and theoretical points of view concerning binary metal oxides, it is interesting to And oxide combinations having well defined and mnable acid or basic properties. On a catalytic oxide surface, the acid or basic sites can be either too strong causing some irreversible adsorption of the substrate species or the sites can be too weak to activate the substrate species. Therefore, the possibility to regulate the acid-base strength, besides the acid site amount of the oxide surfaces, appears a necessary tool for catalytic purposes. 

All other things being equal, the strength of a weak acid increases if it is placed in a solvent that is more basic than water, whereas the strength of a weak base increases if it is placed in a solvent that is more acidic than water. In some cases, however, the opposite effect is observed. For example, the pKb for ammonia is 4.76 in water and 6.40 in the more acidic glacial acetic acid. In contradiction to our expectations, ammonia is a weaker base in the more acidic solvent. A full description of the solvent s effect on a weak acid s piQ or on the pKb of a weak base is beyond the scope of this text. You should be aware, however, that titrations that are not feasible in water may be feasible in a different solvent. 

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