[atbear (OP)]

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)?

[cuso4]

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.

Angel

[Ylide]

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.

This message brought to you by The Council of People Who Are Sick of Seeing More People

[chris]

Nicely explained. Thanks.

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[jolly]

deleted as inapproprate