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Lewis Acid/Base Chemistry in formation of Sulfuric Acid

  1. May 14, 2014 #1


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    1. The problem statement, all variables and given/known data

    How does sulfur trioxide react with water to form sulfuric acid

    2. Relevant equations

    This is clearly a Lewis acid/base problem.

    3. The attempt at a solution

    Okay, I tried drawing sulfur trioxide out with three double bonds (as it is traditionally drawn). I know the oxygens are nucleophilic and the hydrogens on the water are electrophilic. Nucleophile attacks electrophile but that gives the central sulfur two lone pairs of electrons. This clearly can't be right because I have never seen a sulfuric acid Lewis structure with two lone pairs.

    I went to Wikipedia and it says the sulfur trioxide molecule has 2 dative bonds and one double bond. This seems to make a little bit more sense. I had the datively bonded oxygens attack the hydrogens.

    My question is how does the extra oxygen get appended onto the sulfur? I guess the sulfur, being electrophilic, attacks the nucleophilic oxygen too?

    Also, on a side note, would this reaction be an example of why sulfur isn't hypervalent - i.e. the above reaction wouldn't make sense if sulfur were hypervalent?
    Last edited: May 14, 2014
  2. jcsd
  3. May 14, 2014 #2
    I'm just going to shout out my n00b knowledge: a proton shift apparently occurs (zumdahl, chemistry). Hope that helps!
  4. May 14, 2014 #3


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    Yes, that's clear, but what is the mechanism? This is my proposed "mechanism" if we stick with tradition - i.e. sulfur trioxide and sulfuric acid are both hypervalent. I end up with two lone pairs on the sulfur (??).

  5. May 16, 2014 #4
    Your set up implies the belief that there is one reaction mechanism. This is clearly wrong. There seem to be three homogenous cases: bimolecular gasseous, bimolecular aqueous, and trimolecular. You didn't specify temperature or pressure (concentration). I wasn't able to find any clear answer to your question, regardless of the context. It seems to be clear that SO3.H2O complex exists in the atmosphere, based on the literature. Another paper used MD DFT to model the bimolecular reaction between SO3 and H2O in solution, where they claim concerted reaction at both the S and the O (with the -O-H of the water). Another paper claims that the reaction occurs between two water molecules and the SO3 (I don't recall if this was in solution or gasseous at low pressure). In many real world situations, the reaction is inhomogenous (occurring on a surface). I doubt if THREE reaction centers H→O, H→O, O→S is a likely mechanism. Nor is some O(-) ion going to be floating around. So, neither of your proposed mechanisms seem reasonable to me. (HSO4(-) exists in the atmosphere, hint hint.).
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