How are strong acids different from weak acids?

Answer 1

Acidity is simply a matter of hydrogen ions. Strong acids release more of them. That's a good second question. The first question you've got to ask is, "what's an acid?" You know we have "acids and alkalis" or "acids and bases" depending on what term your chemistry teacher likes. These days they mostly seem to like calling anything with a pH over 7 a base, so we'll use that. You're a worker at a chemical factory, and you are told to make some hydrochloric acid. Lots of money in hydrochloric acid, right? And all it is, is hydrogen chloride gas bubbled through water. Very easy to make. When you do it, the hydrogen starts looking around. It sees the water molecules over there being water molecules. They're havin' a good time. They're in hot tubs, water parks, fishing lakes. It's fun to be a water molecule. It's not much fun being a hydrogen chloride molecule. You gotta drag around this chlorine atom. No one likes a hydrogen chloride molecule--it pollutes the atmosphere, it eats the paint off people's cars, it's really smelly. Why would anyone want to be a hydrogen chloride molecule? So the hydrogen atom tells the chlorine atom to go away, joins up with a water molecule and becomes an entirely new compound--Hydroxonium! (Chemical formula: H3O.) I gotta tell ya, hydroxonium is kinda like the evil superhero of the chemical world. It can eat through things, leap tall buildings in a single bound, stop trains (well, if it eats through the tracks it can), all that stuff. Hydrogen atoms LIKE being part of hydroxonium. And water molecules like to become hydroxonium molecules. They get to stay up all night, hang out on the street corner, smoke cigarettes and sing "Gee Officer Krupke" when the cops come around to hassle them. They're like the Jets in West Side Story. (Your chem teacher ain't gonna accept this as the description of hydroxonium, so tell her it's got a pKa of -1.7 and is the most acidic thing that can possibly exist.) And as for the chlorine? Well...chlorine can tear up some stuff on its own, so it's just as happy to be rid of the hydrogen atom as it was to have it. This combination of two fairly nasty chemicals, hydroxonium and chlorine, makes hydrochloric acid so good at dissolving things. Okay, that's not QUITE how it goes, but you get the idea. Now as for this strong acid versus weak acid: A strong acid is one where all, or a majority of, the molecules of whatever it is you put in the water dissociate. If you put a mole of hydrogen chloride molecules in five moles of water, every hydrogen chloride molecule has the opportunity to cast off its bonds and become hydroxonium and free chlorine. If you put five moles of hydrogen chloride in one mole of water, of course, not all of the HCl will react...but that's not the HCl's fault, is it? You just haven't given the chemical the conditions it needs to fulfill its potential. A weak acid is one where the majority of the molecules do NOT dissociate. Acetic acid is a weak acid--CH3COOH. Note there's carbon in it; this is an "organic acid." Put this in water and you'll still get some hydroxonium...but you only need one of the four hydrogen atoms in the acetic acid to make hydroxonium, right? When that hydrogen atom at the end takes off, you're left with acetate--CH3COO. This is why they call acetic acid a weak acid--once the dissociation occurs, you've still got this huge molecule left to contend with. Normally, strong acids are really corrosive and weak acids aren't very corrosive--you eat a lot of weak acids, like acetic acid (vinegar), amino acids (proteins) and ascorbic acid (vitamin C). There's one very special case: hydrofluoric acid, which they want you to call HF because "hydrochloric" and "hydrofluoric" sound a lot alike. HF is a weak acid--fluorine is extremely electronegative, so it's almost impossible for the hydrogen atom to leave it and become hydroxonium. But at the same time, HF is very corrosive--so corrosive it will eat glass, any metal except iridium, a lot of plastics...oh, and because fluorine would rather bond to calcium than to hydrogen, if you get enough of this on your skin it will pull the calcium out of your blood and kill you.