Acids and Bases in Water

Although water is not an essential participant in all modern acid-base definitions, most laboratory work with acids and bases involves water, as do most environmental, biological, and industrial applications. Recall from our discussion in Chapter 4 that water is a product in all reactions between strong acids and strong bases  

Indeed, as the net ionic equation of this reaction shows, water is the product  

Furthermore, whenever an acid dissociates in water, solvent molecules participate in the reaction  

The earliest and simplest definition of acids and bases that reflects their molecular nature was suggested by Svante Arrhenius, whose work on reaction rate we 

Some typical Arrhenius acids are HCI, HNOj, and HCN, and some typical bases are NaOH, KOH, and Ba(OH 2. Although Arrhenius bases contain discrete OH ions in their structures, Arrhenius acids never contain H ions. On the contrary, these acids contain covalently bonded H atoms that ionize in water. 

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Diphenylcarbazide as adsorption indicator, 358 as colorimetric reagent, 687 Diphenylthiocarbazone see Dithizone Direct reading emission spectrometer 775 Dispensers (liquid) 84 Displacement titrations 278 borate ion with a strong acid, 278 carbonate ion with a strong acid, 278 choice of indicators for, 279, 280 Dissociation (ionisation) constant 23, 31 calculations involving, 34 D. of for a complex ion, (v) 602 for an indicator, (s) 718 of polyprotic acids, 33 values for acids and bases in water, (T) 832 true or thermodynamic, 23 Distribution coefficient 162, 195 and per cent extraction, 165 Distribution ratio 162 Dithiol 693, 695, 697 Dithizone 171, 178... 

TABLE i.l The Strong Acids and Bases in Water Strong acids Strong bases... 

Tables 8.1 and 8.2 give calculated ct,a and (l-a,a) values, respectively, for various acids and bases in water at pH 7. Fig. 8.1 shows schematically the speciation of a given acid (or base) as a function of pH. Some example calculations are given in Illustrative Example 8.1. It should be reemphasized that the neutral and ionic forms of a given neutral acid (base) behave very differently in the environment. Depending on the process considered, either the neutral or ionic species may be the dominant factor in the compound s reactivity, even if the relative amount of that...

The dissociations of other acids and bases in water reveal a pattern. This pattern was first noticed in the late nineteenth century by a Swedish chemist named Svante Arrhenius. (See Figure 10.2.)... 

As a result of the ionization of acids and bases in water, the essential reaction in each case can be shown as... 

Correspondingly, metal amides in liquid ammonia have a basic character. The reaction of ammonium salt with metal amide in liquid ammonia is analogous to the neutralization of acid and base in water. The heats of neutralization in ammonia are even larger than in water. The process of hydrolysis corresponds to ammonolysis in ammonia. This results in ammonobasic compounds, for example, in the infusible precipitate HgNH2Cl from HgCl2. 

Arrhenius theory applies well to solutions of weak acids and bases in water, but fails in the case of strong electrolytes such as ordinary salts. Debye and Hiickel [26] solved this problem assuming complete dissociation, but considering the Coulomb interactions between the ions by a patchwork theory based on both macroscopic electrostatics and statistical mechanics. 

It is important to notice that water appears in these equations as both a proton acceptor and a proton donor. This is an example of the amphoteric (sometimes termed the amphiprotic) nature of water. Although the ionisation of acids and bases in water is best described using the equations above, it is convenient to disregard the water when deriving useful expressions and relationships. 

The reaction of a strong acid with a weak base demonstrates the need for a slightly broader definition of acids and bases. As you learned in the last chapter, much of the behavior of acids and bases in water can be explained by a model that focuses on the hydrogen ion transfer from the acid to the base. This model will also help explain why every acid-base reaction does not result in a neutral solution. 

In the Arrhenius concept of acids and bases, what species are produced by acids and bases in water What is formed from these species in neutralization ... 

The strength of acids and bases in water depends on the extent to which they react with the solvent, water. Acids and bases are classified as strong when the reaction with water is virtually 100% complete and as weak when the reaction with water is much less than 100% complete (perhaps as little as 2-3%). 

The behavior of acids and bases in water is influenced considerably by the fact that water ionizes into hydrogen and hydroxide ions ... 

The equilibrium constant of a reversible dissociation is called the dissociation constant. The term dissociation is also applied to ionization reactions of acids and bases in water for example HCN + H2O H3O+ + CN-which is often regarded as a straightforward dissociation into ions HCN H+ + CN-... 

Most experiments in introductory chemistry courses use hydrophilic solutes with water as the solvent. While there are many chemical reasons for this (such as the solubility of common acids and bases in water), water is also a convenient solvent since it is inexpensive, it is nonflammable, and, if it doesn t have nasty things dissolved in it, it is easy to dispose of by just pouring it down the sink. The transition to organic chemistry traditionally involves a change from hydrophilic chemistry to hydrophobic chemistry in the laboratory. 

A Write stepwise equations for protonation or deprotonation of each of these polyprotic acids and bases in water, (a) C03 (b) H3ASO4 (c) NH2CH2C00 (glycinate ion, a diprotic base). 

Electrophoretic techniques are generally used for separation of charged analytes. Charged analytes move in electrolyte solutions when an electrical field is estabhshed. Separation is obtained if the charged analytes have different m ation velocity. The electrolyte solution is most commonly a mixture of weak acids and bases in water. 

Diffusion controlled rates are expected for most proton transfers between certain acids and bases in water. As long as the pfC of the conjugate acid of the proton acceptor B" is greater than that of the donor HA by two or more units, the reaction is normally found to be diffusion controlled, and the rate becomes independent of the donor strength. When the pKa of the conjugate acid of the acceptor drops below that of the donor, the forward reaction is endothermic, and hence the reverse reaction becomes diffusion controlled (proton transfer from BH to A ). We examine further the dynamics of proton transfers below. 

What has come to be known as the HLB (hydrophilic lipophilic balance) is essentially proportional to ( — log g A ), albeit with minor modifications. The neutral case (A = 1) is arbitrarily assigned an HLB value of 7 by analogy with the pH values of acids and bases in water. This convention leads to... 

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