Acid confusion

[Chuck Norris]

I am confused about how an acid can have a -pH. For example fluoro anti-monic acid with a pH of -10^31 or something like. Besides 100% dissassociation in whatever solvent how can something like the acid mentioned above be that ''corrosive''?

 

Also why does aluminium foil really, really, hurt if you bite down on it moderately hard (not too hard)?

I think I might of put the above question in the wrong section.

Edited September 5, 2009 by Chuck Norris

[MDJH]

I am confused about how an acid can have a -pH. For example fluoro anti-monic acid with a pH of -10^31 or something like. Besides 100% dissassociation in whatever solvent how can something like the acid mentioned above be that ''corrosive''?

Well, first off, pH is a measure of hydronium ion concentration, not corrosiveness... and to me "corrosiveness" sounds like a property that would depend on what is getting corroded. (ie. strong acids "corroding" metals when strong bases would tend not to do so, whereas both kinds of chemicals would burn human skin)

 

More specifically, pH is the negative base-10 logarithm of the hydronium concentration, and 1 mol/L of hydronium ions would have a pH of 0. So I guess anything with more than 1 mol/L of hydronium ions would have a negative pH.

 

EDIT: As for question 2, I didn't even know that about biting aluminum; I guess it has something to do with aluminum being a metal.

[hermanntrude]

remember the definition of pH is [math]-log[H^+][/math], where [H+] is the concentration of H+ ions in solution (bear in mind that most textbooks beyond a certain level will actually refer to the concentration of H3O+ instead of H+, since this is more technically correct).

 

If you think of that, it's no surprise that a negative pH can exist. Any concentration greater than 1mol/L will give a log which is positive and therefore a negative log (and pH) which is also negative.

[MDJH]

remember the definition of pH is [math]-log[H^+][/math], where [H+] is the concentration of H+ ions in solution (bear in mind that most textbooks beyond a certain level will actually refer to the concentration of H3O+ instead of H+, since this is more technically correct).

 

If you think of that, it's no surprise that a negative pH can exist. Any concentration greater than 1mol/L will give a log which is positive and therefore a negative log (and pH) which is also negative.

... why does it even have to be "beyond a certain level" though? Why can't they start off with the explanation that it's hydronium from the start? I remember doing grade 10 science class and finding the chemistry component interesting, but finding it confusing that stuff like Ammonia would give off hydroxide instead of hydrogen... of course, that summer I got my grade 11 chem textbook early and looked it up, but it doesn't sound too complicated...

[Chuck Norris]

Now I understand the mathematics of pH. Anyway what factors dictate corrosiveness (in general)? for example some acids will corrode almost anything whereas other acids will only corrode a certain substance. I any one out there could provide a answer to the bitey aluminium foil question that'll be ace.

[John Cuthber]

pH is nice and simple- the log of teh reciprocal of the hydrogen ion concentration (activity to be precise).

However, for really strong acid it doesn't help much. The acidity stops being directly related to the concentration.

Consider triflic acid; here's some data about it.

http://en.wikipedia.org/wiki/Trifluoromethanesulfonic_acid

It has a density of about 1.7 and a molar mass of 150 so a litre contains a little over 10 moles. It couldn't, even if it were fully dissociated, have a hydrogen ion concentration more than about 12M

But it's a much stronger acid than, for example, sulphuric acid which (by the same sort fo calculation) is about 18M

The strength of the acid isn't related to the "number of H= ions in a litre" for these sort of things- it is a measure of how weak a base can the acid palm off a proton on.

[MDJH]

Now I understand the mathematics of pH. Anyway what factors dictate corrosiveness (in general)? for example some acids will corrode almost anything whereas other acids will only corrode a certain substance. I any one out there could provide a answer to the bitey aluminium foil question that'll be ace.

I would think the "bitey" question would be a separate subject from acid strength. And yeah, an acid's "corrosiveness" isn't necessarily proportional to its strength, supposedly hydrofluoric acid burns are considered worse than even sulfuric acid burns, (supposedly because of the electronegative fluoride) yet sulfuric acid is considered a strong acid because 100% of its molecules lose at least one H+ in water, (if I recall correctly) whereas hydrofluoric acid is considered a weak acid because it ionizes only partially.

 

Ironic that in terms of acid molecules stronger means MORE likely to fall apart.

[Melvin]

I thought the "aluminum foil in mouth" was electrochemical and requires that you have fillings. The fillings and the foil are electrodes, and saliva is the electrolyte.

 

At least, that's what I remember...

[Chuck Norris]

Thanks Melvin. That explains the pain, I have fillings.