Acid-base reaction indicators

Draw the products of each acid-base reaction. Indicate whether equilibrium favors the starting materials or the products. 

Look closely at the acid-base reaction in Figure 2.5, and note how it is shown. Dimethyl ether, the Lewis base, donates an electron pair to a vacant valence orbital of the boron atom in BF3, a Lewis acid. The direction of electron-pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown using curved arrows in Section 2.5. A cuived arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. We ll use this curved-arrow notation throughout the remainder of this text to indicate electron flow during reactions. 

Acid-base indicator, 403-404q colors, 392-393 equivalence point and, 84 Acid-base reactions, 96-97q, 402q amino acids, 622-625 Brensted-Lowry model, 353-354 buffer systems, 383-391 equations for, 82-84 Lewis acid in, 410 Lewis base in, 410 types, 81-82... 

Now by taking one more step we can view acid-base reaction in a broader sense. Suppose we mix aqueous solutions of ammonium chloride, NH4CI, and sodium acetate, CH3COONa. A sniff indicates ammonia has been formed. Reaction occurs,... 

When we mix two solutions the result is often simply a new solution that contains both solutes. However, in some cases the solutes can react with each other. For instance, when we mix a colorless aqueous solution of silver nitrate with a clear yellow aqueous solution of potassium chromate, a red solid forms, indicating that a chemical reaction has occurred (Fig. 1.1). This section and the next two introduce three of the main types of chemical reactions precipitation reactions, acid-base reactions, and redox reactions, all of which are discussed in more depth in later chapters. (The fourth type of reaction discussed in this text, Lewis acid-base reactions, is introduced in Section 10.2.) Because many chemical reactions take place in solution, particularly in water, in this section we begin by considering the nature of aqueous solutions. 

In a comparable solution of benzoic acid the freezing point is depressed only twice the predicted amount, indicating only a normal acid-base reaction. Further, a sulfuric acid solution of methyl mesitoate when poured into water gave mesitoic... 

C21-0093. Some pure liquid interhalogen compounds are good electrical conductors, indicating that they contain cations and anions. Show a Lewis acid-base reaction between two bromine trifluoride molecules that would generate ionic species. 

A final point needs to be made. Theory has indicated that AB cements should be amorphous. However, a degree of crystallization does sometimes occur, its extent varying from cement to cement, and this often misled early workers in the field who used X-ray diffraction as a principal method of study. Although this technique readily identifies crystalline phases, it cannot by its nature detect amorphous material, which may form the bulk of the matrix. Thus, in early work too much emphasis was given to crystalline structures and too little to amorphous ones. As we shall see, the formation of crystalUtes, far from being evidence of cement formation, is often the reverse, complete crystallinity being associated with a non-cementitious product of an acid-base reaction. 

Indicators are chemical dyes that change color with a change of pH. Litmus paper and phenolphthalein are two common indicators used in acid-base reactions. They are chosen because they change color at or very near solution neutrality. Litmus paper is red in acidic solutions and blue in basic solutions. Phenolphthalein is colorless in acidic solutions and turns red in basic solutions. 

In a titration, the equivalence point is the point at which chemically equivalent amounts of reactants have reacted, whereas the end point is the point at which an indicator changes color and a titration should be stopped. So, a chemist needs to be careful when choosing an indicator in an acid-base reaction, to be certain that the pH at which the indicator changes color is close to the pH at the equivalence point of the titration. 

In each case we indicate whether the compound is more soluble in acid than in water. We write the net ionic equation for the reaction in which the solid dissolves in acid. Substances are more soluble in acid if either (1) an acid-base reaction occurs [as in (b-d)] or (2) a gas is produced, since escape of the gas from the reaction mixture causes the reaction to shift to the right. 

The chromophore in hydrangeas is delphinidin (X), which is a member of the anthrocyanidin class of compounds. Compound X reminds us of phenol (VII), indicating that delphinidin is also a weak acid. In fact, all pH indicators are weak acids or weak bases, and the ability to change colour is a visible manifestation of the indicator s ability to undergo reversible changes in structure. In the laboratory, only a tiny amount of the pH indicator is added to the titration solution, so it is really just a probe of the solution pH. It does not participate in the acid-base reaction, except insofar as its own structure changes with the solution pH. 

The redox and proton transfer reactions undergone by the flavin prosthetic group are summarized in Scheme 5.2. The vertical reactions are oxidations by Q regenerating P. From the standard potential values (V vs. SCE) of the four flavin redox couples that are involved in Scheme 5.2 and those of the mediators (Table 5.1), all four oxidation steps may be regarded as irreversible. The horizontal reactions are deprotonations by the bases present in the buffer. From the pA values of the various flavin acid-base couples indicated in Scheme 5.2 (over or below the horizontal arrows), reactions H2 and H4 may be regarded as irreversible and reactions HI and... 

For an indicator to be useful in detecting the end-point of an acid-base reaction, the pH range of the indicator must fit into the vertical part of the titration curve for that reaction. The vertical part of the titration curve will include the pH of the salt solution at the end-point. 

Other uses for 1,3,5-TNB include use as a vulcanizing agent in the processing of natural rubber and as an indicator in acid- base reactions in the pH range of 12.0-14.0 (HSDB 1994). 

In a comparable solution of benzoic acid the freezing point is depressed only twice the predicted amount, indicating only a normal acid-base reaction. Further, a sulfuric acid solution of methyl mesitoate when poured into water gave mesitoic acid, while a similar solution of methyl benzoate similarly treated did not.534 The AacI mechanism is also found when acetates of phenols or of primary alcohols are hydrolyzed in concentrated (more than 90%) H2SO4 (the mechanism under the more usual dilute acid conditions is the normal Aac2).535... 

Surface reactions are most frequently observed for systems in which the reaction takes place very quickly. The faster the reaction, the thinner the layer of the solution round the dropping electrode in which the equilibrium is perturbed and the chemical reaction takes place. The thinner the reaction layer, the greater are the specific effects of the electrode surface on the reaction. Accordingly, one is more likely to obtain values of rate constants similar to values obtained by other methods for relatively slow than for fast reactions. Because acid-base equilibria are often rapidly established, it is not surprising that most waves affected by acid-base equilibria indicate a surface reaction, whereas other slower chemical reactions result in waves corresponding to volume reactions. 

Sometimes the acid-base reaction involved is more complex than that indicated in scheme (19). The two most frequently observed complications are participation of proton-donors other than hydroxonium ion and dissociation of two or more protons. Participation of various proton-donors is demonstrated by the dependence of the height of the kinetically controlled wave on the nature and concentration of the buffer with the usual type of buffers, a pH-dependence of wave f in the shape of a deformed dissociation curve is obtained. For polybasic acids several po-larographic dissociation curves are observed at various pK -values under certain conditions the slopes of these curves may differ. 

Hardness and softness as chemical concepts were presaged in the literature as early as 1952, in a paper by Mulliken [138], but did not become widely used till they were popularized by Pearson in 1963 [139]. In the simplest terms, the hardness of a species, atom, ion or molecule, is a qualitative indication of how polarizable it is, i.e. how much its electron cloud is distorted in an electric field. The adjectives hard and soft were said to have been suggested by D.H. Busch [140], but they appear in Mulliken s paper [138], p. 819, where they characterize the response to spatial separation of the energy of acid-base complexes. The analogy with the conventional use of these words to denote resistance to deformation by mechanical force is clear, and independent extension, by more than one chemist, to the concept of electronic resistance, is no surprise. The hard/soft concept proved useful, particularly in rationalizing acid-base chemistry [141]. Thus a proton, which cannot be distorted in an electric field since it has no electron cloud (we ignore the possibility of nuclear distortion) is a very hard acid, and tends to react with hard bases. Examples of soft bases are those in which sulfur electron pairs provide the basicity, since sulfur is a big fluffy atom, and such bases tend to react with soft acids. Perhaps because it was originally qualitative, the hard-soft acid-base (HSAB) idea met with skepticism from at least one quarter Dewar (of semiempirical fame) dismissed it as a mystical distinction between different kinds of acids and bases [142]. For a brief review of Pearson s contributions to the concept, which has been extended beyond strict conventional acid-base reactions, see [143],... 

Beside consecutive-competing reactions, instantaneous (generally acid-base) reactions are also used as an indicator of segregation, especially in multijet tubular reactors. Ottino (102) deduced the relationship t (t) between nwarpedn and real time from the comparison between experimental conversion X(t) along the axis of a tube and the theoretical expression X(tw). ay was then calculated by (7-10) and the efficiency eff(t) by ... 

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