Why strong acids are strong?

i suppose that you know something about inductive effect, resonance and charge density.

anion with lower charge density are more stable, in terms of protonation; relatively, the equilibrium tends to shift to ionic-species side.
for example, HCl ionizes to give chloride ion, in which the charge density of the anion is low and more stable. thus equilibrium shifts to ionic side, i.e. the acid is strong.

for hydrogensulfate ion, the negative charge on oxygen is attracted by the other three oxygen, which have same (high) electronegativity. the strong inductive effect reduces the charge density of anionic-oxygen, stabilizing the anion. thus sulfuric acid is strong acid.

nitrate has strong inductive effect by an N and 2 more O. apart from this, the negative charge (read: lone pair) can delocalize through pi-bond system by resonance. this greatly reduce the charge density on a particular oxygen atom ---- in fact in this case, all three oxygen atoms carry same negative charge. and what follows is same as above.


inorganic acid often contain more electronegative elements, or more oxygen atoms; the halide ions themselves are stable (perhaps except fluoride ion).

in contrast, organic acids rely on delocalization of carboxylate group, and often contains electron-donating alkyl groups. solubility and polarity of conjugate base may also affect.


2010-11-23 19:08:19 補充：
note that inductive effect in not present in chloride. it's stable only because it has low charge density.

the ultimate goal of such stabilization of anions, is to reduce the negative charge density, such that tendency of re-protonation is lower.