Relative acidic strength

  1. Which is more acidic H2S or H2O?And why?
     
  2. jcsd
  3. How acidity is defined?

    Which acid-base theory you should consider here? :smile:
     
  4. According to here:
    evans.harvard.edu/pdf/evans_pka_table.pdf

    H2S is more acid.
     
  5. chemisttree

    chemisttree 3,723
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    We should remember that to argue that something is more acidic because it produces more hydronium ions in solution or that it is due to a lower pKa, while absolutely true, do not answer the OP's question of "WHY".
     
  6. Of course, but if you tell me it's not a homework, I can answer that question too :smile:
     
  7. If you use the Bronsted-Lowry definition, you can say that the conjugate bases of both molecules are OH- and SH-. As oxygen is more electronegative than sulfur, it will attract electrons more than sulfur does. This means that oxygen will be charged more than sulfur, and it will want to receive a proton more than sulfur. Hence, OH- (hydroxide) is a stronger base than SH- (hydrsulfide), which implies that H2S is a stronger acid than H2O. This also holds with the Lewis definition of an acid.
     
  8. I don't understand this explanation: since oxygen is more electronegative than sulfur, it should retain the extra electron more strongly than sulfur! The reason H2S is more acid than H2O is in the different bond enthalpies of S-H and O-H.
     
  9. Yes, it will want to retain the electron more, and it will hog it towards itself. Oxygen will be more negatively charged than sulfur when compounded with hydrogen, so it will want to receive a proton more than sulfur (protons will be attracted more towards oxygen).
     
  10. chemisttree

    chemisttree 3,723
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    I'm surprised that nobody has mentioned the role hydrogen bonding plays in the comparison of these two compounds and the pH effects that difference leads to.
     
  11. But the bond between H+ and OH- is not ionic, is covalent; it means that the charge on the ions doesn't count very much, what counts is the capability of forming a covalent bond between one of the lone pairs on the oxygen and the proton, that is the possibility for the oxygen to *give* a pair of electrons to the H+.
     
  12. I thought the hydrogen bond were more responsible of properties like boiling point and vaporization enthalpy, but I'd like to know your explanation.
     
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