Acids, strength

Acids are classified as strong or weak, depending upon their degree of ionization in water. A weak acid ionizes in water reversibly to form HjO ions. A weak acid is a weak electrolyte, and its aqueous solution does not conduct electricity well. The dissociation reaction occurs to a very small extent usually, fewer than 1 percent of the HA molecules are ionized. The ionization of a weak acid is shown as follows  

Kg refers to the acid dissociation constant which is the measure of an acid s strength. Some references call the acid ionization constant. 

Acid solutions are known from many types of foods. According to the famous Bronsted-Lowry definition an acid is a proton donor (in terms of ions), while a base is a proton receiver (water may act as an acid as well as act as a base). 

In an acid-base reaction a ET ion is transferred from the acid to the base following the principle below where the acid is written in general as HA while the corresponding base is written as A  

Syie Base Coiresponderende acid Corresponderende base 

An equilibrium expression for the general acid-base reaction may be expressed in a similar way as in chapter 3 and chapter 4  

The strength of an acid is defined by the equilibrium position of its dissociation (ionization) reaction  

A strong acid is one for which this equilibrium lies far to the right. This means that almost all the original HA is dissociated (ionized) at equilibrium [see Fig. 14.4(a)]. There is an important connection between the strength of an acid and that of its conjugate base. 

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Strength of conjugate base A much weaker A much stronger 

The relationship of acid strength and conjugate base strength for the reaction 

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The common strong acids are sulfuric acid [H2S04(ag)], hydrochloric acid [HCl(a )], nitric acid [HNOs) )], and perchloric acid [HC104(a )]. Sulfuric acid is actually a diprotic acid, an acid having two acidic protons. The acid H2SO4 is a strong acid, virtually 100% dissociated (ionized) in water  

We have seen that when an acid dissolves in water, a proton is transferred from the acid to water  

In this reaction a new acid, H30 (called the conjugate acid), and a new base, A (the conjugate base), are formed. The conjugate acid and base can react with one another, 

Note that the products in the forward reaction are the reactants in the reverse reaction. We usually represent the situation in which the reaction can occur in both directions by double arrows  

An acid that completely dissociates to produce H ions in solution 

An acid that dissociates to a slight extent in aqueous solution 

While the necessity of having both and acid and a base present in order to have an acid-base reaction is axiomatic, it is surprising how often this concept is neglected. However, if these conditions are met, then a wide variety of organic compounds can donate protons to appropriate bases (they are deprotonated) and a wide variety of compounds can accept protons from appropriate acids (they are protonated). 

The same considerations are true for Lewis acids and bases. Pure boron trifluoride does not act as a Lewis acid because there is nothing present capable of donating electrons to it. If diethyl ether is added, boron trifluoride etherate, a stable Lewis acid-base complex, is produced. Obviously the electron pairs on the oxygen atom of diethyl ether can be donated to boron trifluoride. When a Lewis base is present, then boron trifluoride functions very effectively as a Lewis acid. 

Long before we ever took a chemistry course we all had some knowledge of the relative strengths of acids and bases. If you asked any of your friends if they 

But die idea is clear The strength of an acid can be quantitated by knowing how well it transfers a proton to some standard base. Moreover die extent to which it transfers a proton to the standard base could be compared with die extent to which other acids transfer a proton to die same standard base. Thus, by using a single base, we could not only measure the strength of an acid but also compare acids quantitatively. 

To make it simple, let us choose a molecule as die standard base. The acid strength will be a measure of how well the acid transfers a proton to that molecule acting as a base, and we can quantitate the acid strength by measuring the amount of die acid diat is ionized in die presence of that base. Thus die ratio of the amount of acid which has transferred a proton to the base compared to the amount of acid which has not transferred a proton to the base is a direct measure of die strength of diat acid. Water has been chosen as the standard base molecule (in 

AIMS To understand what acid strength means. To understand the relationship between acid strength and the strength of the conjugate base. 

This situation represents a competition for the ion between H2O (in the forward reaction) and A (in the reverse reaction). If H2O wins this competition—that is, if H2O has a very high attraction for compared to A —then the solution will contain mostly HsO and A . We describe this situation by saying that the H2O molecule is a much stronger base (more attraction for H ) than A . In this case the forward reaction predominates  

Audrieth and Moeller (61) have applied the Lewis concept to poly acids, and Hill and Audrieth (62) have shown that fluoride acts as a strong anionic base and catalyzes the high-temperature depolymerization of the polymetaphosphate and polyphosphate in the fused state. 

Bjerrum (63) attempts to resolve the difficulties in nomenclature by reserving the word acid for proton acids and calling other acids antibases. 

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). 

In media of high dielectric constant, the assumption that /a and /b depend only on charge type is plausible since A and B differ only by a proton and similar specific effects would not change the value of the ratio /a//b. In practice, since the proton would be solvated, we deal with the transfer of a proton from one acid to the base of another acid and have a double acid-base equilibrium 

Proton Acids in Water When A2 is the solvent water, we have 

Although the Brpnsted-Lowry model helps explain acid strength, the model does not provide a quantitative way to express the strength of an acid or to compare the strengths of various acids. The equilibrium constant expression provides the quantitative measure of acid strength. 

Observing and inferring The electrical conductivities of solutions of weak acids, such as acetic acid, are related to the degree of ionization of the acid. 

Materials glacial acetic acid distilled water 10-mL graduated cylinder dropping pipette 50-mL beaker 24-well micro plate conductivity tester with battery stirring rod 

Use a 10-mL graduated cylinder to measure 3 mL of glacial acetic acid. Use a dropping pipette to transfer the 3 mL of glacial acetic acid into well A1 of a 24-well micro plate. 

Lower the electrodes of a conductivity tester into the glacial acetic acid in well A1. Record your results. 

Graphical representation of the behavior of acids of different strengths in aqueous solution, (a) A strong acid, (b) A weak acid. 

The various ways of describing the strength of an acid are summarized in Table 7.1. 

Most acids are oxyacids, in which the acidic proton is attached to an oxygen atom. The strong acids mentioned above, except hydrochloric acid, are typical examples. Many common weak acids, such as phosphoric acid (H3P04), nitrous acid (HN02), and hypochlorous acid (HOC1), are also 

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Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

If, for a given acid, we wish to increase the acid strength, then we choose a solvent which has a greater affinity for protons than has water. If we add ammonia to a solution of hydrogen chloride in water, the essential equilibrium is... 

If the formulae of the acids are written as shown on the right, it becomes apparent that acid strength increases as the number oj oxygen atoms not involved in O—bonding increases. 

Since the hydrogen-element bond energy decreases from sulphur to tellurium they are stronger acids than hydrogen sulphide in aqueous solution but are still classified as weak acids—similar change in acid strength is observed for Group Vll hydrides. 

In dilute aqueous solution hydrogen fluoride is a weak acid but the acid strength increases with the concentration of hydrogen fluoride. 

The significance of the possible diprotonation of water under extremely acidic conditions directly affects the question of acid strength achievable in superacidic systems. The leveling effect mentioned above limits the acidity of any system to that of its conjugate acid. Thus, in... 

For the methyl-substituted compounds (322) the increase in AG and AHf values relative to the unsubstituted thiazole is interpreted as being mainly due to polar effects. Electron-donating methyl groups are expected to stabilize the thiazolium ion, that is to decrease its acid strength. From Table 1-51 it may be seen that there is an increase in AG and AH by about 1 kcal mole for each methyl group. Similar effects have been observed for picolines and lutidines (325). 

HS04 and H3PO4 are very similar in acid strength Both are much weaker than H2SO4 which IS a strong acid... 

There is a striking difference in the acidity of cyclopentadiene compared with cycloheptatriene Cycloheptatriene has a pK of 36 which makes it 10 times weaker m acid strength than cyclopentadiene... 

Table 14 2 repeats some approximate data presented earlier m Table 1 7 for the acid strengths of representative hydrocarbons and reference compounds... 

Acids that are better proton donors than the solvent are leveled to the acid strength of the protonated solvent bases that are better proton acceptors than the solvent are leveled to the base strength of the deprotonated solvent. 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

Perfluorinated carboxylic acids are corrosive liquids or solids. The acids are completely ionized in water. The acids are of commercial significance because of their unusual acid strength, chemical stabiUty, high surface activity, and salt solubiUty characteristics. The perfluoroaLkyl acids with six carbons or less are hquids the higher analogues are soHds (Table 1). 

The most important appHcation of metal alkoxides in reactions of the Friedel-Crafts type is that of aluminum phenoxide as a catalyst in phenol alkylation (205). Phenol is sufficientiy acidic to react with aluminum with the formation of (CgH O)2Al. Aluminum phenoxide, when dissolved in phenol, greatiy increases the acidic strength. It is beheved that, similar to alkoxoacids (206) an aluminum phenoxoacid is formed, which is a strong conjugate acid of the type HAl(OCgH )4. This acid is then the catalyticaHy active species (see Alkoxides, metal). 

Organic Reactions. Nitric acid is used extensively ia iadustry to nitrate aHphatic and aromatic compounds (21). In many iastances nitration requires the use of sulfuric acid as a dehydrating agent or catalyst the extent of nitration achieved depends on the concentration of nitric and sulfuric acids used. This is of iadustrial importance ia the manufacture of nitrobenzene and dinitrotoluene, which are iatermediates ia the manufacture of polyurethanes. Trinitrotoluene (TNT) is an explosive. Various isomers of mononitrotoluene are used to make optical brighteners, herbicides (qv), and iasecticides. Such nitrations are generally attributed to the presence of the nitronium ion, NO2, the concentration of which iacreases with acid strength (see Nitration). 

The combination of oxidi2ing effect, acidic strength, and high solubiHty of salts makes perchloric acid a valuable analytical reagent. It is often employed in studies where the absence of complex ions must be ensured. The value of wet ashing techniques, in which perchloric acid is used to destroy organics prior to elemental analysis for the determination of trace metals in organics, has been well estabHshed (see Trace and residue analysis). 

Acids react with alkyl hydroperoxides in two different ways, depending on the hydroperoxide stmcture and the acid strength (45). 

The bottoms from the stripper (40—60 wt % acid) are sent to an acid reconcentration unit for upgrading to the proper acid strength and recycling to the reactor. Because of the associated high energy requirements, reconcentration of the diluted sulfuric acid is a cosdy operation. However, a propylene gas stripping process, which utilizes only a small amount of added water for hydrolysis, has been described (63). In this modification, the equiUbrium quantity of isopropyl alcohol is stripped so that acid is recycled without reconcentration. Kquilibrium is attained rapidly at 50°C and isopropyl alcohol is removed from the hydrolysis mixture. Similarly, the weak sulfuric acid process minimizes the reconcentration of the acid and its associated corrosion and pollution problems. 

Mechanistically the rate-determining step is nucleophilic attack involving the hydroxide ion and the more positive siUcon atom in the Si—H bond. This attack has been related to the Lewis acid strength of the corresponding silane, ie, to the abiUty to act as an acceptor for a given attacking base. Similar inductive and steric effects apply for acid hydrolysis of organosilanes (106). 

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