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First, let's consider the acidity of the halogen acids--HF, HCl, HBr, and HI-- collectively abbreviated HX as shown in , where X represents the halogen. From the data in the figure below, you can see that the major factor affecting the acidity of halogen acids is the strength of the H-X bond. Intuitively, a larger electronegativity difference should lead to a stronger acid due to the polarization of electrons away from hydrogen. However, the trend in bond strength is enough to overrule that competing trend in electronegativity. The smaller the halogen, the closer in size it is to the proton and the greater the orbital overlap; so HF is most the strongly bonded and weakly acidic of the halogen acids.

Figure %: Factors affecting the strengths of halogen acids

Generalizing that result, we can say that when the an H-A bond is strong, the acid is weak. Experimental confirmation of this postulate comes from the oxyacid series of compounds. An oxyacid is a molecule of the form AOn(OH)m, where A is a non-metal. Pauling and Ricci derived the following approximate equation for oxyacid acidity from experimental observations:

pKa = 8 - 9f + 4n

The variable f is the formal charge on A when all oxygens are singly bound to A. The variable n represents the number of O atoms bound to A that are not bound to an H. A general trend, summarized in the Pauling-Ricci rule above, is that the more electron withdrawing (more electropositive) the non- metal center, the stronger the acid due to a weakening of the O-H bond.

In summary, we note the following trend regarding acidity: hydrogens are more weakly bound to more electronegative groups, and this produces stronger acids. By using the relationship Kw = Ka * Kb, you should be able to figure out that we need only to discuss the acidity of compounds to describe basicity. From the above discussion, we can deduce that bases with weaker conjugate acids are more basic than those with stronger conjugate acids. Therefore, bases that form stronger bonds to H will have larger Kb's and are stronger bases.

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