Acid Strength and Reactivity Questions

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Offline atbear

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Acid Strength and Reactivity Questions
« on: 23/08/2003 09:45:29 »
I have a bet with my cousin....  we were talking about HF, and he said that HF is the strongest acid (assuming that from it's electronegativity), and I told him no, that it's sooo electronegative that it holds on to the Hydrogen too well and that it was actually a weak acid....  Well we got all wound up about it, because he didn't believe me.  He also insists that HF is the most reactive acid (I questioned this) but I didn't tell him definately no.

So my questions are in regard to HCl and HF (I used HCl my strong acid example):  
1.  What is the stronger acid, HF or HCl?
2.  What is the more reactive acid, HF or HCl?
3.  Why for both (if you don't mind telling)?
 

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Offline cuso4

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Re: Acid Strength and Reactivity Questions
« Reply #1 on: 23/08/2003 11:25:06 »
An acid is also called a proton donor. So a strong acid means it donates H+ readily. I agree with you that HCl is stronger than HF as fluorine attracts the hydrogen so strongly. I'm not sure which one is more reactive.

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Offline Ylide

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Re: Acid Strength and Reactivity Questions
« Reply #2 on: 21/10/2003 10:39:37 »
Acids which are considered "strong acids" are classified only as a function of thier ability to completely dissociate in water.  It has nothing to do with their ability to donate a proton.  Any acid which does not completely ionize in water is considered a weak acid, no matter how corrosive it may be.

HF, while it is extraordinarily reactive, is not a strong acid because it does not obtain 100% ionization into H+ and F- in aqueous solution.  

The only strong acids are: HCl, HI, HBr, HNO3, HClO3, HClO4, and H2SO4.  (note that only one proton of H2SO4 dissociates completely...the resulting ion HSO4- is only weakly acid)  

In more mathematical terms...if you place any of the strong acids into aqueous solution, for every mole of the acid you'll receive a mole of H+.  For weak acids, you'll receive a smaller amount based on the ionization constant for that acid.  (pKa)

So, you are correct in your statement to your cousin...HF is a weak acid, but not exactly for the reason you thought.

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Offline chris

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Re: Acid Strength and Reactivity Questions
« Reply #3 on: 21/10/2003 12:20:43 »
Nicely explained. Thanks.

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jolly

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Acid Strength and Reactivity Questions
« Reply #4 on: 24/02/2007 16:15:09 »
deleted as inapproprate
« Last Edit: 06/03/2007 01:00:55 by jolly »