 |
 |
Acid Strength Revised
Name: Ivan
Status: student
Age: 16
Location: N/A
Country: N/A
Date: Around 1999
Question:
at http://www.newton.dep.anl.gov/askasci/chem99/chem99182.htm
you
say that Hydrofluoric acid (HF) is the strongest acid. However, from
what
I understand its not even a super acid meaning it doesn't ionize 100%.
When I looked up the Ka value for it it was said to be 3.53X10^-4
(according to the CRC hand book of chemistry and physics) giving it a pH
of 1.73 and a %ionization of 1.8%. How can this be the strongest acid?
Replies:
Different scales of acidity can give different rankings. It's all a matter
of what is being measured. It is true that HF does not fully dissociate in
water unless you add a base to it. (Incidentally, a "superacid" can be
defined in several ways; I have never before heard 100% dissociation as one
of them. The two definitions I have heard are 1. more acidic than
anhydrous AlCl3, and 2. more acidic than concentrated H2SO4.) Anhydrous HF,
however, is considered one of the most acidic substances known. This means
that it readily protonates other reference bases, even though it does not
extensively dissociate in water.
Why is this? I don't know. I imagine that it might have something to do
with hydration preferentially stabilizing HF over H+ + F-. Indirect
measurements indicate that HF is much better hydrated than any of the other
hydrogen halides. This fact alone will make it less ionized in aqueous
solution. The fact that anhydrous HF exists as hydrogen-bonded polymers
probably also has something to do with it.
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
Dear Ivan,
Upon rereading that answer I wrote long long ago,
I see that I made at least one error. I said that
"all strong acids are fully ionized in dilute solution."
This is much too strong a statement. There is in reality
a continuum of behaviors as one goes from weak acids
to strong acids. And many strong acids, HF being
one of them, are not 100% dissociated in aqueous solution
(although one tends to treat them that way as a first
approximation).
As I tried to explain in the message you referred to,
it is my understanding (although I could be mistaken) that
one cannot correlate the reactivity of a strong acid
with its Ka value. Weak acids can indeed be so
correlated, but not strong ones. This is, as I mentioned,
due to the fact that no acid in aqueous solution can have
a greater Arrhenius acidity than H3O+. Therefore,
one ranks the strength of strong acids according
not to the Arrhenius definition (tendency to
dissociate in aqueous solution) but according to the
Bronsted-Lowry definition (tendency to transfer a proton
to a given base in an acid-base reaction).
A superacid is not one which is 100% dissociated
but one which has a tendency to undergo BL acid-base
reactions more readily than anhydrous sulfuric acid
(this is, I believe, the IUPAC definition, but again
I could be mistaken).
I hope this helps. I suggest you consult the same primary
reference I consulted, i.e., the McGraw-Hill Encyclopedia
of Science and Technology.
Best Regards,
Prof. Topper
Dept of Chemistry
The Cooper Union
Click here to return to the Chemistry Archives
| |
Update: April 2012
|
|
