Different acids and their action

[McCrunchy]

I often hear that sulfuric acid is the strongest /most corrosive/dangerous of acids, followed by nitric and chloric. But they all dissociate to the same extent ( for reasonably dilute solutions), so they all have the same concentrations of H+, which in my eye was the essential characteristic of an acid. Does the counterion (SO4 2-, NO3 - , Cl -) matter too ? If so, why ?

 

Thanks for your input,

 

McCrunchy

[Mr Skeptic]

The pH is not the only characteristic of an acid related to corrosion.

[hermanntrude]

it all depends on exactly what characteristic you use to define an acid.

 

You're right when you say they all dissociate to the same extent in water, and so the H+ concentrations are the same, but you also have to take some other things into account:

 

In other solvents, such as acetic acid, the acids dissociate to a different degree... out of the six "strong" acids usually quoted, HClO4 is the strongest when put in acetic acid.

 

HCl is often considered to be fairly weak. The main reason for this is that it's rarely found in higher concentrations than 37%. This is because HCl as a PURE substance is a gas, and so very concentrated solutions fume and give off HCl gas. Also, Cl- isn't very reactive, unlike the counterions for other acids.

 

H2SO4 is pretty nasty because you can make it up to about 98% concentration and it will still be a liquid. [ce]SO4^2-[/ce] is not usually a big part of the corrosiveness of sulfuric acid. One of the nastier things about sulfuric is that it like water a LOT. sulfuric acid is the main reason for the rule that you must add acid to water and not the other way round. Sulfuric acid releases so much heat when combined with water it can boil the water instantly.

 

HNO3 is viscious stuff, because while it can be very concentrated (usually up to about 70%), it's not only the H+ that causes the trouble, the NO3- is a very strong oxidiser, and it's the NO3- that makes nitric capable of dissolving silver (the other two acids can't do that).

 

if you mix HCl and HNO3, you get what's known as aqua regia, which can even dissolve gold.

 

There are even nastier acids (google "superacids"), but they're not commonly used.

[Mossydie]

I thought H2SO4 was more corrosive because it gives off two H+ ions? Diastic I think it's called.

[hermanntrude]

the word you're looking for is diprotic. Certainly one mole of sulfuric can make twice as many H+ ions as one mole of HCl, but nevertheless, if you put a lump of silver in sulfuric acid, very little will happen. Put a lump of silver in nitric acid, and there will be violent fizzing and lots of toxic brown gas (NO2)

[UC]

Look up the hammet acidity function. In determining acidity, regardless of solvent, what matters is how strongly the acidic hydrogen is bonded to the rest of the acid.

 

Believe it or not, water is quite capable of behaving as a base. You will never see free [ce] H^+ [/ce] ions in water to any appreciable degree because free protons would be the strongest possible acids by definition. Instead, you have a reaction with the water, forming the hydronium ion, [ce] H3O^+ [/ce], which is the strongest acid that can exist in aqueous solution. Any acids that are stronger than hydronium ion will effectively completely react with water to give hydronium ion and an anion.

 

[ce] HX + H2O -> H3O^+ + X^- [/ce]

 

A 0.01F solution of [ce] HNO3 [/ce] and a 0.01F solution of [ce] HBr [/ce] in water will have effectively the same pH. In other solvents, this will not be the

 

If you want to consider how dangerous things are, you need to consider a whole slew of other characteristics. The ability to concentrate acids is important. concentrated sulfuric, for example, is a very powerful dehydrating agent and has a very strong heat of hydration (add water and it gets very very hot), which makes spills extremely dangerous. Hydrofluoric acid, since it reacts will all sorts of things, and is extremely toxic to life, has it's own dangers even though it is a weaker acid in aqueous solution than many other acids. Nitric acid is a very powerful oxidizer, so like hermann said, it will put holes in many metals or react violently with organic compounds (and release clouds of toxic gas in both cases). These same metals might be completely immune to concentrated sulfuric or hydrochloric acid.

[amit]

dont u all think that corrosiveness cant be the scale of strength as it also depends on the surface on which acid is acting. dissociation constant of HCl >HF still HF can Corrode glass. also just for the fact wikipedia lists HeH+ as the strongest acid in terms of PKa values(-62).

[UC]

dont u all think that corrosiveness cant be the scale of strength as it also depends on the surface on which acid is acting. dissociation constant of HCl >HF still HF can Corrode glass. also just for the fact wikipedia lists HeH+ as the strongest acid in terms of PKa values(-62).

 

That particular power of HF is all about fluoride and it's affinity for silicon. Since all the acids do different things to different substrates, I don't think it's really fair to pick based on reactions of the anion. The only fair grounds to compare acids is the acidity and the acidity alone (what you are saying).

 

The pKa for HeH+ is entirely theoretical, by the way. There is no way that compound could ever be isolated and used in a lab. (perhaps a few molecules in some fancy physics equipment at most)

[amit]

its written there "HeH+ cannot be stored any where as it can protonate almost every known compound" hence its usage etc are out of questions( atleast for now) its just testing how far can we can go!

 

this compound is called helium hydride there then shouldn't it be HeH-