Why is HF a weak acid?

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David Chibukhchian 2G
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Why is HF a weak acid?

Postby David Chibukhchian 2G » Wed Dec 02, 2020 11:34 am

Hey guys, I was looking at a list of strong and weak acids and noticed that HF is considered to be weak. I was a bit confused because I know that HCl and HBr are both strong acids. I was wondering if anyone could explain why this particular acid is weak while other halogens that bind with hydrogen are considered to be strong. Is it because the bond between hydrogen and fluorine is stronger? I'm not exactly sure, any information would really help!

Marisa Gaitan 2D
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Re: Why is HF a weak acid?

Postby Marisa Gaitan 2D » Wed Dec 02, 2020 11:42 am

Because the HF bond is so short, it is very strong and therefore makes it difficult for the bond to be broken. In other words it won't dissociate completely. Stronger acids such as HCl and HBr have weaker bonds and are more likely to dissociate completely.

Claire_Latendresse_1E
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Re: Why is HF a weak acid?

Postby Claire_Latendresse_1E » Wed Dec 02, 2020 11:47 am

HF is a weak acid because it doesn't completely dissociate when mixed with water. Here's the equation when HF reacts with water:
HF + H2O --> H3O(+) + F-
That equation looks a lot like the strong acid dissociations, but H3O+ and F- are very attracted to each other and form a bond. None of the strong acids create hydronium-anion bonds like this. The hydronium can't act as an acid when it's bonded to the fluoride ion, so HF is not a strong acid.

Melis Kasaba 2B
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Re: Why is HF a weak acid?

Postby Melis Kasaba 2B » Wed Dec 02, 2020 2:55 pm

You're correct that HCl, HBr, and also HI are strong acids, while HF is weak. It's actually the only hydrohalic acid that isn't strong. It's considered a weak acid because it doesn't completely dissociate into its ions in water. However, very concentrated solutions of HF can behave like a strong acid due to chemical bonds forming in between a base and conjugate acid.

BKoh_2E
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Re: Why is HF a weak acid?

Postby BKoh_2E » Wed Dec 02, 2020 4:53 pm

HF is a weak acid because the bond between Hydrogen and Fluorine is so short. It is more difficult for the bond to be broken, so it will not completely dissociate in water.

David Chibukhchian 2G
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Re: Why is HF a weak acid?

Postby David Chibukhchian 2G » Thu Dec 03, 2020 10:08 am

Okay, that definitely makes sense. I was assuming that HF doesn't dissociate completely because the bond between H and F is so short and thus strong. That makes it easier to generally understand the difference between strong and weak acids. Also, I didn't know that very concentrated solutions of HF can actually behave like a strong acid. I'll definitely keep that in mind. Thank you all for your answers, they were very helpful!


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