Well i was having a bad day and ******* (i won't say it) pisses me off. Your pKa values have absolutely NO impact on this discussion. pKa vales apply to dilute aqueaous solutions which we are NOT talking about. A different measure of acid strength is necessary. One that defines the strength based on the force of attraction for a counter ion, not pKa. Read this short description of what I mean. pKa measures an acids dissociation in water, that is not necessarily the same as its strength of attraction of a counter ion. Time to sign up for a few more classes, actually it sounds like those classes hurt you more than they helped.
Here is an online discussion that illustrates my point, I didn't do a good job of making my case earlier however I knew I was right.
Question: What is the strongest acid?
How do you distinguish between two acids that are both 100% dissociated?
jim day
Answer:
Interesting question there...the strongest acids are sometimes
referred to as "superacids." These have an extremely large
proton-donating ability. Usually an acid is considered to
be a superacid if it has a proton-donating ability greater
than / equal to that of 1nhydrous (100%) sulfuric acid.
Apparently the strongest acid is hydrogen flouride (HF).
Upon reading, it seems that it is actually tough to distinguish
between the superacids on the basis of their pH alone. All strong
acids are fully ionized in dilute solution, and therefore they
all appear to have the same strength, equal to that of the
hydronium ion (h3O+), which is the most highly acidic species
that can exist in water. So all of these have the same pH on a
per-proton basis.
So, to rank superacids, chemists measure how strongly they tend to
react with a given base, B;
HA + B --> BH+ + A
This way, even superacids can be ranked relative to one another
in terms of their "strength." HF is the strongest in the table I
am looking at, followed by HSO3F, H2S2O7, HSO3CF3, HSO3Cl,
and H2SO4 (sulfuric acid).
All my data is from the McGraw-Hill Encyclopedia of Sci/Tech. -topper
Question: 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?
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
new york, ny
Links
http://newton.dep.anl.gov/askasci/chem99/chem99182.htm
http://newton.dep.anl.gov/askasci/chem99/chem99669.htm
Fuckstick (take it as a term of endearment)
Now lets see a responce with something to back up what you have to say instead of pKa this and pKa that.
Do Your Part To Win The War