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Acid/Base Question on acidity of salts

  1. Jan 19, 2016 #1
    Hi everyone!

    So I have ZnBr2 and CdCl2, and I am asked to choose the most acidic one.

    Firstly, I do not understand why both salts are acidic....

    If my ZnBr2 undergoes hydrolisis, the reaction is ZnBr2 + 2H2O <------> Zn(OH)2 + 2HBr.
    This would mean that this is acidic as the HBr dissociates completely.

    I was told that there is something about the size tend about the atoms.
    However, I do not understand how the size of the Zn or Cd matters in determining the acidity as the atoms will end up with the hydroxides as a base in the reaction...

    Please explain to me why one salt is more acidic than the other, and please explain why the size trend matters in this instance.

    I know that if I have an H-X acid, then the size of the X atom determines the acidity [the larger it gets, the weaker the bond, the more acidic]. However, my Zn and Cd are not attached in this instance.

    Thanks in advance.
  2. jcsd
  3. Jan 19, 2016 #2


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    Look in more detail at the dis-association constants for the bases.
  4. Jan 20, 2016 #3


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    Staff: Mentor

    It is better to write this reaction as a net ionic.

    Such predictions are quite tricky, as they work only if you assume "all everything was not unchanged". That's rarely the case.

    I wouldn't care about size here, rather about solubility of hydroxides. Which is less soluble (so more easily produced in the hydrolysis) - Zn(OH)2 or Cd(OH)2?

    (Actually I just checked their Ksp values and to my surprise they work the other way I thought they will. That only supports what I wrote about these predictions being tricky. They look obvious when you know the answer, but trying to rationalize and extend to predict properties in other cases is a waste of time, much better to check the numbers).
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