The Conjugate Seesaw

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Emily Glaser 1F
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Joined: Thu Jul 27, 2017 3:01 am

HF vs HCl

Postby Emily Glaser 1F » Tue Dec 05, 2017 3:05 pm

Why is HCl a stronger acid than HF when F has a greater electronegativity. If the answer has to do with bond length, since HF has a shorter bond length than HCl, then how in the world do we distinguish which acid is stronger?

Bond Length or Electronegativity

Sabrina Dunbar 1I
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Re: HF vs HCl

Postby Sabrina Dunbar 1I » Tue Dec 05, 2017 3:13 pm

HCl is stronger than HF because of their size, so in turn that means that it is indeed because of their bond length. Fluorine atoms are smaller than chlorine atoms and that makes the bonds between the hydrogen atom and the fluorine atom shorter and stronger than that of a hydrogen and a chlorine atom. However, do not get the strength of the bond confused with the strength of the acid. A stronger acid dissociates more in solution than a weaker acid that does not dissociate fully in solution. In the case of HF and HCl, the longer bond length in HCl is more easily broken because the bond is weaker, but that makes the acid itself stronger. HF has a stronger bond between the two atoms, but because it is harder to break and dissociate, the acid is weaker. This trend goes all the way down the group of halogens as the periodic trend shows that the molecules get bigger and bigger as you go down the group. Hopefully this helps and my wording wasn't too confusing!!

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Postby Chem_Mod » Tue Dec 05, 2017 3:15 pm

Close to what I said in class. Well done Sabrina! :-)

Emily Glaser 1F
Posts: 156
Joined: Thu Jul 27, 2017 3:01 am

Re: HF vs HCl

Postby Emily Glaser 1F » Tue Dec 05, 2017 3:17 pm

In the book it states that HClO has a pKa of 7.53, and HBrO has a pKa of 8.69. In this case, why would HClO be stronger based off your explanation?

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