I am confused about the molecular basis by which nonmetal oxides react with water to form acidic compounds. For example, I understand that sulfur trioxide reacts with water to form sulfuric acid as shown in the following reaction:

SO3 (g) + H2O (l) --> H2SO4 (aq)

I recognize that the polarized nature of the O-H bond in sulfuric acid makes it thermodynamically favorable for H2O to abstract a proton and subsequently form an acidic solution with the formation of H3O+. However, I fail to recognize how reactions like this with a nonmetal oxide proceeds to form an acidic oxide. Namely, I fail to understand the role of the covalent bond in the nonmetal oxide for the mechanism of this reaction. I recognize that metal oxides dissolve in water to form the highly polarizing, high charge density ion O2-, which polarizes the O-H bond in H2O to form hydroxide. Can someone give a similar explanation for nonmetal oxides and their dominant acidic nature in water?


Well, in terms of this reaction, sulfur in $\ce{SO3}$ acts as a Lewis acid for an incoming $\ce{OH-}$ nucleophile. The reaction mechanism seems to be any old nucleophilic addition reaction:

  • $\ce{H2O <=> H+ + OH-}$ : (Water can disassociate into its ions)
  • $\ce{SO3 + OH- -> SO2(OH)O-}$ : (I couldn't decide on whether this is the right way to write this intermediate, but I'll be happy to add a diagram for clarity)
  • $\ce{SO2(OH)O- + H+ -> SO2(OH)2}$ : (This is the final step of the mechanism to yield $\ce{H2SO4}$)
  • $\begingroup$ So, is it that the polar covalent nature of bonds in nonmetal oxides (such as CO2 and N2O5) allow for the molecule to act as a lewis acid and subsequently form an acid (H2CO3 and HNO3)? Then why do metal oxides which are even more polarized not act as lewis acids and form acids as well? One more question--why do some nonmetal oxide molecules such as carbon monoxide not form acids? $\endgroup$
    – Ethiopius
    Nov 9 '18 at 22:59
  • $\begingroup$ See it like this - non-metal oxides are acid anhydrides, and so adding non-metal oxides to water will form the subsequent acid. Metal-oxides don't act as Lewis acids because of the fact that the metals don't have any 'empty' orbitals for electron pairs to be donated to. Technically speaking, adding carbon monoxide to water will produce carbon dioxide - an acidic gas - which can then proceed to form carbonic acid as mentioned. $\endgroup$ Nov 10 '18 at 12:24
  • $\begingroup$ @KlanStevens The dissociation constant for water is very low, would the $\ce{H+}$ and $\ce{OH-}$ from water be enough to turn $\ce{SO3}$ into sulfuric acid with appreciable concentration? Or would the amount of sulfuric acid produced be very low? $\endgroup$
    – Eyy boss
    Nov 4 '20 at 15:50

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