Acid-dissociation constant oxyacids

Arrhenius concept Bronsted-Lowry model hydronium ion conjugate base conjugate acid conjugate acid-base pair acid dissociation constant Section 14.2 strong acid weak acid diprotic acid oxyacids organic acids carboxyl group monoprotic acids amphoteric substance autoionization... 

According to the Bronsted—Lowry deHnition (Section 2.7), an cicid is any substance that donates H. Altiioi h we usually think of oxyacids (H2SOJ, HNOj) or halogen acid.s (HC1, HBr) in this context, any compound containing a hydrc en atom can be an add under the ri t circumstances. By measuring dissociation constants of dilFerent adds and expressii the results as pX, values, an acidity order can be established. Recall from Section 2.8 that a low pK corresponds to a strong add, and a high pK corresponds to a weak acid. 

By analogy with other oxyacids, such as phosphoric, one would expect the acidity of individual silanol groups to increase with the connectivity of the group to which they are attached the accepted values of the few known dissociation constants support this idea. (For orthosilicic acid, pKi 9.5, pK2 12.6 (7, 15) for silanol groups on silica gel, pK s7 (16).) However, this Ts more than compensated by the fact that in the course of polymerisation the total number of silanol groups is greatly reduced, so that despite the pK values just quoted the overall result is a reduction in acidity, and a consequent rise in pH. 

Many detailed discussions have been published on the effects of substituents on the strengths of organic acids and bases/ We have seen that caution is necessary in interpreting small differences in dissociation constants in terms of molecular models, and this chapter will deal only with a few of the more striking effects, with special reference to some which are of interest in reaction kinetics. We shall consider first the strengths of hydrides and oxyacids of different elements. 

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