# Why are weak acids weak?

What are weak acids? My chemistry and biology classes skimmed over them briefly but nothing in depth. I understand that an acid is weak if it partially dissociates and breaks down into ionic compounds, but they never stated why.

So my question is why do weak acids break down in water? Do strong acids dissociate at all? What if they were in a different solution (or do they just stay themselves all the time?)

What are weak acids?

Examples of weak acids in water solution would be acetic acid (main component of vinegar) and ammonium ion (conjugate acid of ammonia).

I understand that an acid is weak if it partial dissociates and breaks down into ionic compounds but they never stated why.

An acid is weak if not all of the acid molecules ionize into hydrogen protons and its conjugate base in a particular solvent system. Alternately, if we were to use the broader, Brønsted definition, an acid is weak if it does not completely or nearly completely donate its proton to some base.

Acetic acid is a weak acid in water. Put acetic acid into pure liquid ammonia and now acetic acid will fully dissociate. The proton will be lost from acetic acid to ammonia and this is a favorable reaction. Makes sense, because the only thing that's changed is the solvent that's grabbing the proton - and ammonia is only a few billion times stronger as a base than water.

That and do the strong acids dissociate at all. Like if they were in a different solution or do they just stay themselves all the time.

Strong acids by definition ionize to an extent of 100% or nearly 100%. Strong acids will give up their protons completely to the strongest base in the system.

Note that in the above text I have used the word "ionize" to describe molecular acids because by definition when these acids react, they ionize. Or form ions. On the other hand salts of acids and bases such as sodium hydroxide are said to "dissociate," because ionic compounds are made of ions, and it wouldn't make much sense to say that ions ionize or become ions.

• That's right. The keyword is "levelling effect of the solvent". – Abel Friedman Oct 11 '14 at 19:43

There is nothing magic about weak acids. The dissociation reaction of an acid in water looks like this: $\ce{HA + H2O <=> A- + H3O+}$. There's an equilibrium constant K that's related to the free enthalpy $\Delta G$ in the usual manner: $$\Delta G=-RT \cdot \ln K$$

A weak acid is simply an acid where the equilibrium lies far to the left. A strong acid, on the contrary is an acid where the equilibrium lies far to the right, i.e. it is more acidic than the $\ce{H3O+}$ ion.

I think you meant that weak acids don't break down in water... you see, and acid is usually considered strong if a lot of it ionizes and donate it's protons (H+) to the water in which it's dissolved. While a base is considered strong depending on it's capacity to protonate (take the H+ from the water leaving OH- on the solution). This sort of behavior is defined by the affinity between the ions (that are in the acid/base) and the H+.

There are more complete definitions for acids and bases that don't necessarily require to consider it in a solution with water, but if your teacher skimmed over more basic definitions, then it's not likely that you will study it unless you get General or Inorganic Chemistry classes in college.

If you're interested in this sort of stuff you can always get a chemistry book on the library. I used "Chemistry The Central Science" by Brown during my General 'n Inorganic Chemistry classes on my first semester on college and I think it is a book that would be easily understood by someone who's still in high school given you put some effort in it.