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confusing EK question about PH

Discussion in 'MCAT Study Question Q&A' started by mikexima, Aug 13, 2011.

  1. mikexima

    7+ Year Member

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    This came across as a super easy question at first but It wasnt and i dont unerstand the explanation behind it.

    The autoionization of water shown below is an endothermic reaction, as the temperature of pure water increase, what happens to pH?

    2H20 --> H3O+ + OH-

    I thought the pH remained unchanged because you produced equal amounts of H+ and OH- but the EK explanation is that the pH decreases beause ph is defined as log of H+

    okay so pH decreases and pOH increases. but doesnt the overall solution remain neutral? isnt the pH of a netural solution 7? I'm so confused now
     
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  3. geeyouknit

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    I think pH of water is only 7 at 25 deg C. It varies as temp varies. So changing the temp (via Le Chatliers Principle) pushes the rxn forward, dec both pH and pOH.
     
  4. Majik

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    Yeah, remember that any of these equilibrium reactions are temperature dependent! The only thing that alters the equilibrium of a reaction IS changing the temperature. This includes the auto ionization of water (Kw). Provided that this is an endothermic reaction (ie. heat is a reactant), then adding more heat (ie. more reactants), pushes the reaction in the forward direction. As a result, the amount of H+ AND OH- that dissociates would increase. As such, the Kw would increase at this different temperature (assuming we increased it). Kw and pH are inversely related. If Kw goes UP, pH must go down (and vice versa). Usually for pH questions, we're told the solution is at 25 degrees C most of the time (in which case Kw=1x10^-14), however the MCAT could easily throw you off your game by involving temperature somehow. Hope this helps.
     
  5. mikexima

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    that makes sense but overall does the solution stay neutral ?
     
  6. liveoak

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    yes.

    neutral is when [OH] = [H]
     

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