Why does the stability of a base (in an acid base rxn)increase as the size of at

Question

Why does the stability of a base (in an acid base rxn)increase as the size of atom that the Hydrogen is attached toincrease. I undersatand that the electron is found over a largerarea with increased atomic size, but I am having trouble relatingto how decrease in electron density increases the stability of abase. Thanks Why does the stability of a base (in an acid base rxn)increase as the size of atom that the Hydrogen is attached toincrease. I undersatand that the electron is found over a largerarea with increased atomic size, but I am having trouble relatingto how decrease in electron density increases the stability of abase. Thanks

Explanation / Answer

The decreasing strength of the H-Xbond is primarily due to the increase is the size of the X atom aswe move down the periodic table. We conclude that one factor whichinfluences acidity is the strength of the H-X bond: a weaker bondproduces a stronger acid, and vice versa.

In the acids in the other twocategories, the hydrogen atom which ionizes is attached directly toan oxygen atom. Thus, to understand acidity in these molecules, wemust examine what the oxygen atom is in turn bonded to. It is veryinteresting to note that, in examining compounds like R-O-H, whereR is an atom or group of atoms, we can get either acidic or basicproperties. For examples, NaOH is a strong base, whereas HOCl is aweak acid. This means that, when NaOH ionizes in solution, the Na-Olinkage ionizes, whereas when HOCl ionizes in solution, the H-Obond ionizes.

To understand this behavior, wecompare the strength of the simple oxyacids HOI, HOBr, and HOCl.The pKa's for these acids are found experimentally tobe, respectively, 10.6, 8.6, and 7.5. The acid strength for HOXincreases as we move up the periodic table in the halogen group.This means that the H-O bond ionizes more readily when the oxygenatom is bonded to a more electronegative atom.

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