Rattata2
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What is stronger nitric acid or hydrochloric acid?
Alright I shouldn't have to ask this, should know but I'm having a blonde-moment I spose.
I always thought nitric acid was a stronger acid than hydrochloric acid - as in if you tried to acidify NaCl with nitric acid you would get HCl and
NaNO3, rather than the other way around HCl + NaNO3 -> NaCl + HNO3.
I thought to figure it out by the pKa values then I realized that hydrochloric acid has a lower pKa (-8) than does sulfuric acid (which is -3) yet
sulfuric acid is the stronger acid (which is obvious because sulfuric acid acidifies NaCl). Nitric acid's pKa is -1.4 (and for comparison acetic's is
4.76, which is obviously the weakest)
So I'm really confused why don't these pKa values match up and which is the stronger acid? HCl or HNO3? I feel like I really should know this :/
[Edited on 1-8-2009 by Rattata2]
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12AX7
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WTF, I tried posting twice and on pressing "add reply", I was redirected to sciencemadness.org/ ???
Anyway fuck writing again.. matter equilibria trumps acidity. The end.
[Edited on 8-1-2009 by 12AX7]
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Rattata2
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Well it worked that time :p
Think could you go into a little more detail? (sorry, it just really seems like the forces are out to prevent me from figuring this out and it's been
bothering me for hours) Or even just a place I can go to figure it out - a book and page number, whatever.
Edit: Guess not 
That's a damn shame, there isn't a single person on the internet who can answer this question or point to an answer...including me
[Edited on 1-8-2009 by Rattata2]
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Sobrero
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http://classes.uleth.ca/200603/chem2500a/sorrtd.pka.jpg
pKa of HCl = -7
pKa of HNO3 = -1.4
This means HCl in an aqueous solution is more deprotonated than HNO3, and so HCl is a stronger acid than HNO3. That's it.
Don't get confused with those NaCl+H2SO4 situations. HCl is a gas and can be eliminated from the reaction mixture, which allows the reaction to occur,
even if HCl is the stronger acid.
[Edited on 1-8-2009 by Sobrero]
"There exists a world. In terms of probability, this borders on the impossible." (Jostein Gaarder)
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MagicJigPipe
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Did you get the information you needed Rattata2? Do you understand now or do you need more explanation?
Also, I usually tend to shy away from the "strong" vs. "weak" terminology when it is inappropriate/unhelpful (unless it helps to explain to a
novice the difference between say, acetic and sulfuric acids) because people have strange connotations about what a "strong acid" is. When a beginner
(average person) hears "strong acid" they tend to think of a liquid that will "eat through everything, including--DUH DUH DUH--metal!". You know, the
stereotypical "movie acid" like on Alien or Richie Rich. There is so much more to acids and bases than pH and pKa and strong vs. weak.
Acid/base "theory" (I forget if it is actually called that) is very interesting. I have an old sophomore level chemistry textbook that is very good
at explaining the concepts and, subsequently, helping one learn the equations that come along with them. I am in the process of making it into an
ebook. I will send you a copy when I'm done if you wish. (it's very good for reference)
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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chloric1
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Quote: Originally posted by Sobrero  | http://classes.uleth.ca/200603/chem2500a/sorrtd.pka.jpg
pKa of HCl = -7
pKa of HNO3 = -1.4
This means HCl in an aqueous solution is more deprotonated than HNO3, and so HCl is a stronger acid than HNO3. That's it.
Don't get confused with those NaCl+H2SO4 situations. HCl is a gas and can be eliminated from the reaction mixture, which allows the reaction to occur,
even if HCl is the stronger acid.
[Edited on 1-8-2009 by Sobrero] |
Exactly right. In my experiences as a hobbyist the pKa values or strength are of only academic importance. When you make acids and remove acids from
there salts, you are doing hands on chemistry. In hands on chemistry, you consider the individual chemical properties of the acids you are working
with such as boiling points and decomposition temperatures, side reactions etc. An examply silicic acid is extremely weak as an acid but will
completely displace sulfuric acid from sulfates forming silicates if heated to high enough temperatures(1000 C or higher). This happens alot in
firing ceramics and glass making but usually carbonates are used so no corrosive fumes damage kilns.
Fellow molecular manipulator
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12AX7
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Anyway, acidity refers to the equilibrium ionization, of how far
[HA] <---> [H+] + [A-]
proceeds under certain conditions (temperature, solvent, concentration). Mass equilibrium gladly trumps this, for example a rather weak acid (in
aqueous solution) like boric acid will easily displace chloride from NaCl if you simply melt them together at >500C. HCl leaves the reaction as
gas, a reaction to which it cannot return, hence the equilibrium is forced ahead, regardless of anything else about the reaction.
Tim
Edit: found the problem. Apparently this forum shits on diagonal brackets. I have used the HTML entities above (&lt; ).
[Edited on 8-1-2009 by 12AX7]
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Ozone
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Gah...
Apples and oranges, HCl and HNO3.
While the dissociation constants differ, as do the pKa's, they are both *negative*, say, practically, that both are completely dissociated in water
and are, hence, "strong acids".
They are both 1N/M.
The big differences are 1) that HCl(conc) = ~37% and HNO3(conc) =~68 % and 2) HNO3 is a strong oxidizer and HCl is *not*.
So, it depends on what you call a "Strong acid" in terms of aqueous solutions. Both are on the negative side of water and are dissociated (although
HCl might do so faster). The concentation indicates that, all else equal, an equal mass of HNO3aq will give 1.83 times more H+ than HClaq. But--HNO3
is also oxidizing, which then would entail again re-defining "strong".
Capiche?
O3
[Edited on 1-8-2009 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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Formatik
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| Quote: Originally posted by chloric1 | | An examply silicic acid is extremely weak as an acid but will completely displace sulfuric acid from sulfates forming silicates if heated to high
enough temperatures(1000 C or higher). This happens alot in firing ceramics and glass making but usually carbonates are used so no corrosive fumes
damage kilns. |
Right, the weak acids can displace strong acids if the theoretical equilibrium is shifted. Another example of a weak acid displacing salts of a strong
acid is oxalic acid and some inorganic chlorates. Ka of oxalic acid: 5.4x10^-2. Ka of chloric acid: 1. The reaction to form the chloric acid works
since a precipitate forms showing the equilibrium drives into that direction.
By the way, not only can you prepare HCl from NaCl and H2SO4, but H2SO4 from Na2SO4 and HCl. Where as mentioned before with the former the HCl is a
gas subjected to the dehydrating power of H2SO4. In the latter NaCl remains insoluble in the HCl, too much water and HCl are present for the H2SO4 to
be able to drive off the HCl.
[Edited on 1-8-2009 by Formatik]
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Globey
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HCL is the stronger acid, but I'd rather get a little constand boiling HCL (aq) on my skin than conc. HNO3
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Formatik
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Corrosiveness has nothing to do with acid strength. That has to do with reactivity (e.g. oxidizing capability, electronegativity, etc) more than
anything else.
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dann2
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Hallo,
Page
http://www.sensorex.com/support/education/pH_calculator.html
for calculating pH values if thats any use/help. It's useful for lazy bastards like myself anyways.
The forum cannot stomach angle brackets. If you use square brackets instead of angles you will at least not get sent to the home page but they only
seems to work with some html tags.
CROSSED OUT
[TABLE] [TR][TD] SOME DATA[/TR][/TABLE] won't work
Dann2
[Edited on 1-8-2009 by dann2]
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Rattata2
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Ah thanks you guys this helps a lot  I eventually 'sorta' found the answer last
night but this is much more detailed and useful! And yes I'd be interested in that ebook when you are finished writing it...just send me a message or
the like and I'd be glad to check it out.
I always thought that acid/base chemistry was a lot simpler than that and that the stronger acid by pKa would always acidify salts of weaker acids, so
I'm really glad to learn about this important tidbit
Cheers people!
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