[pconst163]

Many people mention things like "Pure sulfuric acid" or "pure hydrochloric acid" but I learned that acids form when a compound is dissolved or disassociated in water, so how can sulfuric acid H2SO4 be an acid if it's pure, i.e, not dissociated in water? Also could HCL be an acid if not disolved in water? Isn't it a gas?

They say pure sulfuric acid is a slimy liquid, but is it really a dangerous acid if it's pure?

Thank you!

Mod Edit: All caps title revised.

[curiouscat]

They say pure sulfuric acid is a slimy liquid, but is it really a dangerous acid if it's pure?

Dangerous yes.

Say it spills on you. You do have water in you.

[Borek]

Sulfuric acid is so strong it dissociates even without water, protonating other molecules of the acid:

2H₂SO₄    H₃SO₄⁺ + HSO₄^-

[pconst163]

please dont be funny, i really need to understand these acids, could you have for example, pure hydrochloric acid? wouldn't it just be a gas at room temperature? And if you increased pressure, it would become a liquid, would it be an acid then?

[Dan]

I learned that acids form when a compound is dissolved or disassociated in water

As far as I can see, this definition is incorrect.

HCl(g) satisfies the requirements for being an acid - whichever you choose, it satisfies Arrhenius, Brønsted-Lowry and Lewis acid definitions.

http://en.wikipedia.org/wiki/Acid#Definitions_and_concepts

[Corribus]

Molecules like hydrogen chloride don't dissociate to an appreciable degree unless they're dissolved in water.  Breaking a bond like the H-Cl bond takes energy.  In the gas phase, there's no energetic payoff to break the bond - it is a highly endothermic process, and therefore the reactant is highly favored.  Excluding at very high temperatures and strange chases like photodissociation, hydrogen chloride is not generally a (Bronsted-Lowry) acid in the gas phase, unless there is another gas-phase reactant around that reacts favorably with it.  When dissolved in whater, however, while it still takes energy to break the H-Cl bond, there is an energetic (enthalpic AND entropic) payoff in the form of new intermolecular forces that form when water molecules solvate the ions.  This payoff is so large that HCl readily and (practically) completely dissociates into separate protons and chloride ions (well, hydronium ions).  This makes it a convenient source of "free" protons, which is usually what we think of - practically speaking - when we think of a strong acid.  Calling hydrogen chloride solutions "pure" doesn't really make a whole lot of sense, because they aren't homogenous mixtures.  There is a maximum solubility, of course, and this is what we think of when we refer to "concentrated" acids. 

As has been noted, however - the driving force for hydrolyzation of hydrogen chloride is so strong that it doesn't take much water to create a caustic solution.  Water on the surface of your skin or eyes or in your throat and lungs is easily enough to create a highly acidic hydrochloric acid solution from gas-phase hydrogen chloride. 

[Dan]

Corribus, excellent post as usual, but I take issue with this statement:

hydrogen chloride is not generally a (Bronsted-Lowry) acid in the gas phase, unless there is another gas-phase reactant around that reacts favorably with it.

A Brønsted-Lowry acid is a species that donates a proton to a Brønsted-Lowry base. HCl(g) reacts with NH₃, ergo HCl(g) is a Brønsted-Lowry acid by definition.

[Babcock_Hall]

@OP, I wonder if some of the confusion here relates to the fact that when we buy commercial HCl, HBr, HNO₃, etc. we obtain them as concentrated aqueous solutions.  For example, concentrated HBr may be obtained as a 48% solution (mass to mass) in water.  IIRC nitric acid is obtained as a 70% solution, and sulfuric acid is a 98% solution.  They are only impure if one considers water to be a contaminant.

[magician4]

maybe we should remember, that no molecule is an acid all by itself: it always depends on at least a second molecule that will make it ACT as an acid (by accepting a proton, for example)

this mostly becomes reduced to "an acid is something that will make a low pH in water"

... and in this simplification sometimes lies the source of a lot of confusion


regards


Ingo

[Corribus]

A Brønsted-Lowry acid is a species that donates a proton to a Brønsted-Lowry base. HCl(g) reacts with NH₃, ergo HCl(g) is a Brønsted-Lowry acid by definition.

I apologize if my post was unclear - this is why I wrote: "unless there is another gas-phase reactant around that reacts favorably with it."

Of course, anything (with a hydrogen) can be Brønsted-Lowry acid.  Like everything else, it's a matter of degree.  How readily the proton is removed is the factor that really matters.  This depends on the molecule itself as well as the nature and number density of other nearby molecules.

As a footnote, the Bronsted-Lowry definition of acid is

acid + base   conjugate base + conjugate acid

If one wants to be pedantic about it, the reaction of HCl(g) + NH₃(g) might be interpreted to not qualify, because the product is an undissociated solid salt (NH₄Cl), which is neither a conjugate acid nor a conjugate base.  Or is it both?

Historically, I think it's pretty clear that acid-base theories were formulated for aqueous systems, which means that application of these concepts to non-aqueous states of matter creates some minor definitional problems.  Thermodynamic considerations, though, still remain true and consistent across the board, so in nonaqueous systems it may be better to speak in terms of those rather than in terms of acids and bases.

[Dan]

this is why I wrote: "unless there is another gas-phase reactant around that reacts favorably with it."

Saying that HCl(g) is not an acid unless it is in the presence of a base is a strange statement to me. I agree that it will not donate a proton unless a base is present (true for all acids), but that doesn't mean it would not generally be considered an acid - it donates a proton in the presence of very weak bases. This is true for any reactant - would you say that MeLi is not generally considered a nucleophile because it only reacts nucleophilically if there is an electrophile present?

If one wants to be pedantic about it, the reaction of HCl(g) + NH₃(g) might be interpreted to not qualify, because the product is an undissociated solid salt (NH₄Cl), which is neither a conjugate acid nor a conjugate base.  Or is it both?

Fair point - I'd say both, but I don't think it's worth fighting about!

My point was just that I think HCl(g) is "generally considered" Brønsted-Lowry acid in the gas phase. I know it's a matter of degree, but surely if acetic acid and trifluoroacetic acid are generally considered Brønsted-Lowry acids, then HCl, being several orders of magnitude more acidic, should be too?