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Which compound is more acidic? (without using pkA value) HCl or H2SO4

  1. Mar 8, 2012 #1
    1. The problem statement, all variables and given/known data
    Which compound is more acidic? (without using pkA value)

    HCl or H2SO4

    2. Relevant equations
    Inductive effect.

    3. The attempt at a solution
    Since sulfur is more electronegative compared to cholrine, H2SO4 is more acidic than HCl...
    So then I think of how I could use inductive effect instead of electronegativity to verify this claim.

    Since sulfur is more electronegative compared to oxygen it shall pull away electrons slightly, more than chlorine on hydrogen?
    I don't think it'll work since there are a 3 oxygens around sulfur and so the combined electronegativity outweighs that of sulfur?

    What do you think?
  2. jcsd
  3. Mar 9, 2012 #2


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    Staff: Mentor

    I think I have no idea what it means to be "more acidic". And I am deadly serious, I am not aware of any definition of acidity. The way I see it it can mean one of two things - acid strength (in which case HCl is "more acidic"), or ability to neutralize base (similarly to alkalinity) - in which case H2SO4 as diprotic is more acidic.

    Poorly worded question.
  4. Mar 9, 2012 #3
    Original question:
    For the following pairs of compounds, predict which one will be the strongest acid
    and justify your choice.

    Between HCl and H2SO4, H2SO4 has a lower pKa (~-9) than HCl (~7) and so it is a stronger acid compared to HCl.

    I wanted to answer this question without using a pKa table so I tried comparing the electronegativity of the central atoms, and as wel as their inductive effects. Not sure if inductive effect would work though.
  5. Mar 9, 2012 #4


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    Science Advisor
    2017 Award

    Also consider that resonance stabilization of the conjugate base occurs in H2SO4.
  6. Mar 9, 2012 #5


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    Staff: Mentor

    Apparently it depends on the source used. Kotrly & Sucha (Handbook of Chemical Equilibria in Analytical Chemistry) list sulfuric acid pKa1 as -3, Hulanicki (Reactions of acids and bases in analytical chemistry) lists pKa of HCl as -4.
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