Acid/Base Question on acidity of salts

  • #1
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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.
 

Answers and Replies

  • #2
Bystander
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Look in more detail at the dis-association constants for the bases.
 
  • #3
Borek
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If my ZnBr2 undergoes hydrolisis, the reaction is ZnBr2 + 2H2O <------> Zn(OH)2 + 2HBr.
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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