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Are there Lewis acids that *don't* have pH's below 7?

  1. Aug 8, 2011 #1

    Simfish

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    Gold Member

    What about Bronsted acids?

    And what about Lewis bases that don't have pH's over 7? (same for Bronsted bases)
     
  2. jcsd
  3. Aug 8, 2011 #2
  4. Aug 8, 2011 #3

    Ygggdrasil

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    First, pH is a property of a solution, namely the concentration of hydrogen ions in that solution. It doesn't really make sense to talk about the pH of a molecule (e.g. what is the pH of acetic acid?) because it depends on a number of factors such as concentration.

    When discussion the relative acidity/basicity of molecules, it is useful instead to refer to the pKa of the molecule. This value describes the equilibrium between the acid and its conjugate base and can be used to calculate the pH of a solution containing that molecule.

    So, are there acids or bases with pKa values that are above 7? Yes, there are many. For example, some common substances include hydrogen sulfide (pKa = 7.04), hydrogen cyanide (9.21), phenol (9.99), water (15.74), ethanol (15.9), and acetylene (25). You can even start measuring the pKas of protons that almost never are donated to solution like the protons on ethylene (pKa = 44) or ethane (pKa = 51).

    For measuring basicity, one just looks at the pKa of the corresponding conjugate acid. Here, the lower the pKa of the conjugate acid, the poorer the base. Some particularly poor bases include the halides (e.g. Cl-, pKa of HCl is -7), acetate (pKa of acetic acid is 4.76), and water (pKa of H3O+ is -1.74).
     
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