Connections to Acids and Bases

The opening paragraph of this chapter emphasized that the connection between structure and properties is what chemistry is all about We have just seen one such con nection From the Lewis structure of a molecule we can use electronegativity to tell us about the polarity of bonds and combine that with VSEPR to predict whether the mol ecule has a dipole moment In the next several sections we 11 see a connection between structure and chemical reactivity as we review acids and bases... 

Understanding the acidity and/or basicity of acids and bases in macrocyclic systems has an important connection to biological problems. It should help us to understand enzymes. In many proteins, drastically varying pKa values have been found for functional groups in the active site for example, the... 

Tables 13.4 and 13.5 contain a summary of typical stability values for a number of polymers and elastomers against typical chemical agents. As expected, condensation polymers generally exhibit good stability to nonpolar liquids while they are generally only (relatively) moderately or unstable toward polar agents and acids and bases. This is because of the polarity of the connective condensation linkages within the polymer backbone. By comparison, vinyl type of polymers exhibit moderate to good stability toward both polar and...

Lewis Theory of Acids and Bases. According to Lewis, acids are electron-pair acceptors (EPA) and bases electron-pair donors (EPD) connected through the equilibrium (fig 3.2). 

This question of equilibration of the protonation and deprotonation processes leads to another fundamental problem in the case of excited state reactions between which states can a protolytic equilibrium be at all established A molecule has only one ground state, so there can be no ambiguity about the thermal protolytic equilibrium which connects of course the ground states of the acid and base forms. However, there are many excited states of both these forms, excited states which can differ greatly in electron distribution (e.g. mr and 7T7T states) or even in multiplicity (e.g. singlet and triplet states). 

We ve already discussed the common-ion effect in connection with the dissociation of weak acids and bases (Section 16.2). To see how a common ion affects the position of a solubility equilibrium, let s look again at the solubility of MgF2 ... 

In this connection a most interesting fact comes to our attention, namely, that the heat produced by the neutralization of one equivalent of any strong base with one equivalent of any strong acid is always the same, namely, 13,700 calories. That the heat effect is the same is in itself a strong indication that the reaction is in each case the same, and this fact, then, is in entire accord with our conception of the reaction of neutralization. In the following table are given some of the measured values of the heat of neutralization of acids and bases, both weak and strong. 

This section will integrate material from this chapter with material from the previous chapter. In Chapter 14, we looked at salts composed of strong and weak acids and bases. We saw that these salts have characteristic behaviors in solution. So far in this chapter, we have not considered the pH of the solutions. We re going to connect these two ideas to consider the solubilities of salts in non-neutral solutions. 

Such an equilibrium system is termed a conjugate (or corresponding) acid-base system. A and B are termed a conjugate acid-base pair. It is important to realize that the symbol H+ in this definition represents the bare proton (unsolvated hydrogen ion), and hence the new definition is in no way connected to any solvent. The equation expresses a hypothetical scheme for defining the acid and base - it can be regarded as a half reaction which takes place only if the proton, released by the acid, is taken up by another base. 

Place 10 ml of urine in each of two 20-ml glass bottles fitted with plastic s ew-caps (McCartney bottles), and to one bottle add 1 to 2ml of M sulphuric acid and to the other add sufficient sodium bicarbonate to saturate the solution. Label the bottles A and B (acids and bases) respectively. Fill each bottle with a mixture of chloroform and isopropyl alcohol (9 1), screw on the caps, shake for 5 minutes, and centiifiige for 5 to 10 minutes. Aspirate the top aqueous phase using a Pasteur pipette connected to a water-operated vacuum pump, filter the chloroform extracts throu phase-separating paper to remove residual water, and collect the filtrates in 10-ml conical test-tubes ... 

Alternative approaches exist to explaine the mechanism of chemical interaction. It is accepted that water is formed as a result of mechanochemical interaction, solid reagents are dissolved in water, and the reaction proceeds via the dissolved state (hydrothermal-like process). On the other hand, it is assumed that the initial stages of the process involve the interaction between acid and base centers present at the surfaces in contact, and the next stages are connected with the process of calcium cations insertion into aluminum hydroxide lattice. 

Errors in Measurements with Indicators.—Three chief sources of error in connection with pll determinations by means of indicators may be mentioned. In the first place, if the test solution is not buffered, eg., solutions of very weak acids or Imsos or of neutral salts of strong acids and bases, the addition of the indicator may produce an appreciable change of pH this source of error may be minimized by employing small amounts of indicator which have been previously adjusted, as a result of preliminary experiments, to have approximately the same pH as the test solution. Such indicator solutions are said to be isohydric vith the test solution. 

Neutralization Curves.—The variation of the pH of a solution of acid or base during the course of neutralization, and especially in the vicinity of the equivalence-point, i.e., when equivalent amounts of acid and base are present, is of great practical importance in connection with analytical and other problems. It is, of course, feasible to measure the pH experimentally at various points of the neutralization process, but a theoretical study of the subject is possible and the results are of considerable interest. For this purpose it is convenient to consider the behavior of different types of acid, viz., strong and weak, with different bases, viz., strong and weak. For the present the discussion will be restricted to neutralization involving a conventional acid and base in aqueous solution, but it will be shown that the results can be extended to all forms of acids and bases in aqueous as well as non-aqueous solvents. 

Acid and Basic.—As the amino acids contain both an amino group and a carboxyl group they react both as acids and as bases. In this double r61e they exhibit very interesting properties. This same double character of acid and base has also been referred to in connection with the hydroxy acids. In this case the basic character is due to the presence of an alcoholic hydroxyl group and is consequently weak. With the amino acids, however, the basic character is due-to the presence of the much more basic amino group so that the double acid and basic character of the compounds is more pronounced. As acids the amino acids yield esters and acid chlorides. 

Structural and dynamical studies have been carried out on chromophores of aromatic amino acids and bases [27,30,33,36]. These aromatic biomolecules possess strongly UV absorbing short lived r r state. Short lifetime of this state is established to be caused by an optically dark state which connects it to the So ground state via two successive CIs as illustrated in the diagram of Fig. 1. Extensive studies have also been carried out on DNA base pairs validating such a... 

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