Sciencemadness Discussion Board

What is stronger nitric acid or hydrochloric acid?

Rattata2 - 1-8-2009 at 02:09

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]

12AX7 - 1-8-2009 at 02:30

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]

Rattata2 - 1-8-2009 at 02:35

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]

Sobrero - 1-8-2009 at 04:54

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]

MagicJigPipe - 1-8-2009 at 07:04

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)

chloric1 - 1-8-2009 at 07:39

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.

12AX7 - 1-8-2009 at 08:31

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 (< ).

[Edited on 8-1-2009 by 12AX7]

Ozone - 1-8-2009 at 09:56

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]

Formatik - 1-8-2009 at 10:45

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]

Globey - 1-8-2009 at 13:31

HCL is the stronger acid, but I'd rather get a little constand boiling HCL (aq) on my skin than conc. HNO3

Formatik - 1-8-2009 at 14:36

Corrosiveness has nothing to do with acid strength. That has to do with reactivity (e.g. oxidizing capability, electronegativity, etc) more than anything else.

dann2 - 1-8-2009 at 14:46

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]

Rattata2 - 1-8-2009 at 18:44

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!