Recently, I learned some acids become more corrosive in the presence of water. Yet, this seems counter-intuitive since the water would dilute the acid and raise the pH (less acidic). Do you know the story here?
If this is the case, it must be the result of an additional reaction between the acid and the water (probably exothermic, I’d wager.)
Specifically, this had to do with a sulphur acid (but not sulphuric acid) reacting on steel.
Also, is this true for all acids? And, could it be just any metal?
You didn’t hear quite right. Acid needs some water to react or it can’t do anything but of course lots of water just makes weak acid. That’s why you always pour acid into water, never the other way around. Pouring acid into water makes a weak solution that gradually gets stronger. Pouring water into straight acid makes for an instantly high concentration that can sometmes react and heat up by itself. Bad juju.
First off, strong and concentrated mean different things.
Acetic acid (vinegar) is a weak acid no matter of its concentration. Sulfuric acid is strong regardless of how dilute it is.
As Padeye suggests, water allows the acid to dissociate: the acid molecule breaks into ions in solution. The hydrogen ions go off and bond with other substances, corroding them. The strength of an acid refers to how much of the acid stays as a molecule and how much dissociates into ions.
With vinegar, only a small proportion of the hydrogen becomes a H+ ion, so its capacity to corrode is weak. Where lots of the hydrogen becomes an ion in solution, the acid is strong.
“Concentration” refers to dilution.
So yes, for sulpuric acid, some water is required to get the corrosive reaction of the acid going.
All this is dimly remembered, so if a real chemist pops up later and says otherwise, they’re right.
While I had chemistry many years ago, this doesn’t sound right. I think “sulphur acid” is a vague, nebulous unscientific term. But I could be wrong.
Padeye said
Again, I will defer to a chemist, but if the solution gets stronger, I wonder how. I can’t imagine it does, except in such minute form that it isn’t worth mentioning.
It gets more concentrated (thanks for the correction picmr) as you continue to pour acid. You stop when you get to the desired concentration. Doing the dangerous opposite, pouring water into undilited straight acid, makes an extremely high concentrate from the instant water touches the acid. Some acids can heat up rapidly under this condition.
This is true of sulfuric acid(H[sub]2[/sub]SO[sub]4[/sub]). It’s also true of sulfurous acid (H[sub]2[/sub]SO[sub]3[/sub]). It’s most likely that the acid you’re thinking of is sulfuric, but it might be sulfurous or another acid with sulfur. H[sub]2[/sub]S is a gas in pure form and a very weak acid in aqueous solution. It’s also not called sulfur acid, I don’t think anything is.
The above are protic acids (they are acidic because they put protons (H[sup]+[/sup]) in solution.) In aqueous solution, the protons are stabilized by water; this is sometimes written as H[sub]3[/sub]O[sup]+[/sup], it’s actually many water molecules surrounding each proton. The waters are acting as a weak base to facilitate the dissociation of the protons.
In pure H[sub]2[/sub]SO[sub]4[/sub], there is no water to stabilize the protons. The only way to really free up a proton is a reaction like
2 H[sub]2[/sub]SO[sub]4[/sub] --> HSO[sub]4[/sub][sup]-[/sup] + H[sub]3[/sub]SO[sub]4[/sub][sup]+[/sup]
Where one H[sub]2[/sub]SO[sub]4[/sub] is acting as a base; this doesn’t happen much, as water is a far better base. So H[sub]2[/sub]SO[sub]4[/sub] is only a weak acid without any water. (In aqueous solution Water also acts to stabilize the anion, but that may be getting too complicated)
Once it’s an aqueous solution, adding more water will dilute the acid and weaken it. That’s where ‘Concentration’ generally comes up, not with pure H[sub]2[/sub]SO[sub]4[/sub]. High school and intro chemistry courses typically deal almost exclusively with aqueous solutions, particularly when discussing acid-base equilibria; this may be why this seems particularily confusing.
Sulfuric acid on steel acts to oxidize the metal, in this case literally adding oxygen to the metal. This is more complicated than simply how many protons are present…
Last I knew water was neutral on the Ph scale and if I remember right (and it’s been a couple of years) any time water was added it would bring the levels of acidity down and making it a little more base. Like I said it’s been awhile though.
Water is neutral, but just because we’ve defined neutral as “as acidic as water”. If you have 10^ -7 loose protons per liter of solution, you have a neutral solution. The reason that looks like a somewhat random number is that it kind of is.
Water is so universal and harmless that it makes a good basis for lot of things, including the gram, as I remember.
Actually, does anyone know which came first, the gram or the liter? They’re kind of defined in terms of each other as they relate to water. Which one was popular first?