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Homework Help: Quick chem. question!acids/bases

  1. Apr 24, 2005 #1
    The ionization of water is an endothermic process.
    a.)What happens to the value of Kw as water is heated? explain..
    b.) What happens to the pH of pure water as the temperature increases?
    c.)as the temperature of pure water rises, will the water become mor acidic, more basic or remain neutral...

    all i know is that if its endothermic
    A + B + Heat = C + D
    and if we increase the heat, the system shifts to the left..so what happens to the Kw?
  2. jcsd
  3. Apr 25, 2005 #2


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    [tex] \color{blue} 1: \ \ \ \ \mathsf{ 2\,H_{2}O \ + \ Heat \ \ \rightleftharpoons \ \ H_{3}O^{+} \ + \ OH^{-} } [/tex]

    [tex] \color{blue} 2: \ \ \ \ K_{w} \ = \ [H_{3}O^{+}]\cdot[OH^{-}] [/tex]

    Since water ionization is ENDOthermic, increasing temperature will drive the reaction to greater ionization ("to the right") and hence will increase "Kw". Since the latter also increases "[H3O+]", the "pH" of pure water will decrease with increasing temperature. Finally, even with these temperature dependencies, pure water will remain neutral since { [H3O+] = [OH-] }.

    Last edited: Apr 25, 2005
  4. Apr 25, 2005 #3


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  5. Apr 25, 2005 #4


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    Actually, it's NOT a contradiction!!

    Although it may appear that PURE WATER is becoming "more acidic" when its "pH" decreases with increasing temperature, it remains a fact that PURE WATER is ALWAYS NEUTRAL!! What's happening is that the "pH" considered "Neutral" changes with temperature. We are so accustomed to {pH = 7.000000000} being considered neutral that we forget the origins of the "pH" scale and that this scale's neutral point is defined by:

    [tex] \color{blue}\blacktriangleright\blacktriangleright \ \ \ \ \mbox{\textbf{Neutral pH}} \ \ = \ \ \mathbf{ -\log_{10}(\sqrt{K_{w}}) } [/tex]

    It turns out that {Kw 10(-14) mol2/Lit2} at std temp (25 degC) so that "Neutral pH" is conveniently "7" at std temp. When temperature rises, so does Kw and hence the corresponding "Neutral pH".

    Again, "Neutral pH" depends on temperature and is only "7" at std temp. Here's more info:

    The table below shows the effect of temperature on Kw. For each value of Kw, a new "Neutral pH" has been calculated.
    Code (Text):

    T (°C)  Kw (mol2/L2) "Neutral pH"
    0   0.114 x 10-14   7.47
    10  0.293 x 10-14   7.27
    20  0.681 x 10-14   7.08
    25  1.008 x 10-14   7.00
    30  1.471 x 10-14   6.92
    40  2.916 x 10-14   6.77
    50  5.476 x 10-14   6.63
    100    51.300 x 10-14   6.14
    You can see that the pH of pure water falls as the temperature increases.

    A word of warning!

    If the pH falls as temperature increases, does this mean that water becomes more acidic at higher temperatures? NO!

    A solution is acidic if there is an excess of hydrogen ions over hydroxide ions. In the case of pure water, there are always the same number of hydrogen ions and hydroxide ions. That means that the water remains neutral - even if its pH changes.

    The problem is that we are all so familiar with 7 being the pH of pure water, that anything else feels really strange. Remember that you calculate the neutral value of pH from Kw. If that changes, then the neutral value for pH changes as well.

    At 100°C, the pH of pure water is 6.14. That is the neutral point on the pH scale at this higher temperature. A solution with a pH of 7 at this temperature is slightly alkaline because its pH is a bit higher than the neutral value of 6.14.

    Similarly, a solution at 0 degC with a pH of 7 is slightly acidic, because its pH is a bit lower than the neutral value of 7.47 at this temperature (0 degC).

    Last edited: Apr 25, 2005
  6. Apr 25, 2005 #5


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    Yeah, you're right
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