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% Ionization vs. Concentration of Strong Acid

  1. Dec 4, 2008 #1
    I solved an assignment problem and I came to the conclusion that the % ionization of a strong acid decreases as the initial concentration of the ionized acid is increased.

    I don't fully understand why this is..?

    Any clarification would be greatly appreciated!

    Thanks in advance!!
     
  2. jcsd
  3. Dec 4, 2008 #2

    Borek

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    Staff: Mentor

    Strong acid is - by definition - always dissociated in 100%. So your solution must be somehow wrong.

    Well, there are no THAT strong acids, so you are generally right. The higher the concentration, the lower the ionization percent.

    I am not sure it makes sense to try to look for some specific explanation. You may try to analyze it in terms of LeChatelier's principle (very weak approach), you may treat it just as a conclusion of the dissociation constant definition (the one that you arrived at), you may think about it this way - pure acid is not dissociated, infinitely diluted acid is dissociated 100%, everything inbetween must follow the trend. Not sure if any of these adds anything new to the problem.
     
  4. Dec 4, 2008 #3
    Thanks for the prompt reply!

    I did some further research and found that the % ionization of a weak acid/base increases as its initial concentration decreases (becomes more dilute). I guess this conclusion for strong acids follows the trend of the weak.

    But why can't pure acid not be dissociated?
     
  5. Dec 4, 2008 #4

    Borek

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    Staff: Mentor

    It is not that they can't, but dissociation means releasing the proton. This proton has to be attached to something, it doesn't flow in the liquid on itself. In water it finds its place in the lone electron pair of water molecules, in most pure acids there are no lone pairs that could accept it. Still, to some extent it happens. For example for pure sulfuric acid reaction

    2H2SO4 -> H3SO4+ + HSO4-

    has an equilibrium constant 10-2.9 - much higher than analogous reaction for water (10-14 - nothing else but water ion product). But sulfuric acid is an exclusion.
     
  6. Dec 4, 2008 #5
    I see... Thanks for the clarification!
     
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