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Why HCl is a much stronger acid than H2O?

  1. Jun 23, 2013 #1
    I want to know why HCl is a much stronger acid than H2O.They have similar bond energies (H-Cl = 427 kJ/mol, O-H = 467 kJ/mol), so there should be easy to ionize both. Besides, O is more electronegative than Cl, so it should "steal" the electron easier. Why is HCl Ka so superior than water Kw?
     
  2. jcsd
  3. Jun 23, 2013 #2
    Consider the size of chlorine and oxygen
     
  4. Jun 23, 2013 #3

    symbolipoint

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    Consider what kind of bond each is. HCl is, at least when dissolved in water, ionic. H-OH is not mostly ionic, so ionizes only weakly.
     
  5. Jun 23, 2013 #4
    Ok, O-H is more covalent than Cl-H, but what contributes for that? Why is HCl ionic and H2O covalent? Is it the size of O and Cl? F is also very small (smaller than O) and HF is still much stronger than H2O (the bond energy of HF is superior too).
     
  6. Jun 24, 2013 #5
    HCl and HF are stronger acids than H2O because Cl- and F- are much weaker bases than OH-. Those bond energies you wrote refers only to homolitic dissociation: H-Cl --> H. + Cl. and not to ionic dissociation: H-Cl --> H+ + Cl-. Ionic dissociation energies are much different; looking for them you should find that OH- is much more reactive than Cl- or F- with H3O+, for this reason the dissociation costant of water is much lower.
     
  7. Jun 25, 2013 #6

    Borek

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    Also, to properly analyze the situation, you have to remember all processes involved take place in water, and ion solvation (hydration) plays crucial role.
     
  8. Jun 25, 2013 #7
    Exactly, thanks for clafication (you have anticipated me :smile:).
    Infact ion dissociation is favoured in a polar solvent as water.
     
  9. Jun 26, 2013 #8

    epenguin

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    Well I think the student should be a bit alert and critical.

    It seems to me that #3 and 5 have just restated in different terms the fact that HCl is a much stronger acid than H2O, not explained it.

    Though it is true that homolytic dissociation energies, if that is what the quoted figures are, are not relevant.
     
  10. Jun 27, 2013 #9
    I remember my book stated that we should look at the conjugate base to determine the equilibirum. Even though O is more electronegative than Cl, Cl has a larger volume so the charge density is greater allowing the negative charge to be spread out. So this makes Cl- more stable than OH- as a result at equilibrium there is more Cl- and H3O+ than for water with OH- and H3O+. Furthermore, the HCl bond is weaker than the HO bond further attributing to the equilibrium favouring the forward reaction for HCl than for H2O.

    I recall the H in OH- having an effect on the stability of the charge but I can't seem to recall it. Hope this helps.
     
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