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pH of water

Discussion in 'MCAT Study Question Q&A' started by unsung, Jun 19, 2008.

  1. unsung

    10+ Year Member

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    EK1001 Q718:

    The autoionization of water (shown below) is an endothermic reaction.

    2H2O --> H3O+ + OH-

    as the temperature of pure water increases the pH:

    A. decreases because [H+] increases
    B. increases because [OH-] increases
    C. increases because [H+] increases
    D. remains at 7 because [H+] equals [OH-] even after an equilibrium shift.

    I thought it would be D, but the answer is A. Why?

    It's endothermic, so adding heat should drive the equilibrium towards the products... but I don't see why pH would change with heat.
     
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  3. psy

    psy Lazy Bum Extraordinaire
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    pH has to do with H+ concentration or more specifically -log10 (H+). It doesn't mean the whole solution is more acidic with more heat because there is an equal amount of OH-. It just means there are more H+ around. Tricky eh?
     
  4. unsung

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    ohhhh I see. So the idea is that if the equilbrium shifts to the right, there would be more H+, even if [H+] = [OH-]? Gah. That boggles my mind. :laugh:
     
  5. HopHurdlerXXS

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    Hey that question is a tricky SOB. Don't feel too bad, I had to think about it before seeing why D was wrong.
     
  6. Raryn

    Raryn Infernal Internist / Enigmatic Endocrinologist
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    pH+pOH only equals 14 at a specific temperature. a pH of 8 doesnt necessarily mean a pOH of 6 if the temperature is 80 degrees. they could both be 8...
     
  7. nfg05

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    Yeah this whole question is more about Le Chatlier's principle and considering heat as a reactant than using what you know about [H+] and pH to answer it. That stuff is just used so that they don't have to say "which way does the equilibrium shift," making it very obvious that you just need to use Le Chatlier.
     
  8. supafield

    supafield Dream Big
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    yikes, that's tricky, I knew the pH scale changed with temperature... but immediately would have thought D too..... haha well hopefully now that comes up and I'll get it lol...
     
  9. niranjan162

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    How come it can't be B, if there is a shift to the right won't [OH-] also rise?
     
  10. supafield

    supafield Dream Big
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    Kw = 1X10^-14 which explains via the -log why the pH is 0-14 (minus the rare expection)
    Water dissociates into 1X10^-7 M [H+] and 1X10^-7 [OH-]
    This all goes down at a temperature at 25 C

    So imagine you heat up the water (and using arbitrary #'s)

    Kw may become 1X10^-12 making the pH scale 0-12
    and it dissociates equally again due to the auto-ionization of water
    So you'll get 1X10^-6 [H+] and 1X10^-6 [OH-] giving a pH of 6

    So you'll see even though both concentrations of ions increase due to the pH scale itself decreasing it becomes theoretically a lower pH.

    haha at least this is how I reason it to myself... and that would reason out B
    The question is worded pretty poorly... the pH increases due to the increased ionization of water shrinking the pH scale
    Although there is an increase in H+ ions the language screws me up because the way A is worded it makes it sound like H+ is increasing compared to OH- but it's not it's just increasing in general
     
  11. eikenhein

    eikenhein Supreme Commander Anesthesiologist
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    With increase temperature, [OH-] increase and pOH decrease. At the same time, [H+] increase and pH decrease. pH + pOH = 14 is only true at standard temperature.
     

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