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Help me understand the concept of hydrolysis

  1. Oct 20, 2009 #1
    On our study guide, our teacher gave us some problems. This one baffles me for some reason.

    1. When solid NaCN is added to water, the pH ________________.

    a.remains at 7
    b.becomes greater than 7 because of the hydrolysis of Na+
    c.becomes less than 7 because of the hydrolysis of Na+
    d.becomes greater than 7 because of the hydrolysis of CN−
    e.becomes less than 7 because of the hydrolysis of CN−

    Answer: d. becomes greater than 7 because of the hydrolysis of CN−


    He gave us the answers as well, but I'm having a hard time conceptualizing this.

    Why does CN hydrolyze? I thought hydrolysis was the splitting of H2O, and obviously Na got a good chuck of that as well. Didn't Na hydrolyze too?


    Obviously, the equation goes:

    NaCN + H2O => NaOH + HCN


    but that doesn't help me.
     
  2. jcsd
  3. Oct 20, 2009 #2
    There is a more technical answer, but this should help:

    CN- hydrolyzes to form its conjugate acid, HCN, because HCN is a weak acid; weak acids want to form, unlike strong acids. Because the CN- picks up H+ ions, it decreases the amount of H+ in solution and so increases the pH.

    Na+ doesn't exactly pick up OH- because NaOH is a strong base. Like strong acids, strong bases don't want to form. So the Na+ doesn't hydrolyze. Your equation is correct, but it doesn't really happen like that in real life.
     
  4. Oct 21, 2009 #3
    Thanks so much for your reply! I think I keep forgetting that this is only theoretical.
     
  5. Oct 21, 2009 #4

    Borek

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    Staff: Mentor

    Not sure what you mean - hydrolysis is as real as anything can be.

    --
    methods
     
  6. Oct 22, 2009 #5
    I didn't mean Hydrolysis wasn't real! Rather, I meant that salts and water don't really react to make acid and a base. Right?
     
  7. Oct 22, 2009 #6

    Borek

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    Staff: Mentor

    Sure they do, in 0.001M solution of KCN over 13% of the cyanide is in the form of HCN - that is, it reacted with water producing acid and OH- (hence solution is basic, with pH slightly over 10).

    --
    methods
     
  8. Oct 22, 2009 #7
    I must have misunderstood my teacher. Is that only because HCN is a weak acid? I'm kind of weak when it comes to math.

    How would you go backwards with the Ka = [H][A]/[HA] equation? Or, how do you come up with 13%?
     
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