Question

Why will 2M HCl burn a hole in your shirt whereas the same concentration of acetic acid (vinegar) will not?

Answer 1

This comes down to the very important difference between strong and weak acids. See the explanation below...

Explanation:

This is exactly what is meant when we speak of an acid as being strong or weak.

Both solutions have the same concentration. This is only a reference to how much solute you dissolved in each litre of solution you made.

But, when we consider HCl, every one of the molecules that dissolved into the solution also ionized according to

`HCl + H_2O rarr H_3O^+ + Cl^-`

so, the concentration of `H_3O^+` (or of `H^+` if this is more familiar to you) is 2M, and no molecules of HCl remain. This is the same as saying "HCl is a strong acid".

In the case of acetic acid, only a very small percentage (typically 1% or less) of the `CH_3COOH` molecules underwent this ionization.
In other words, the equilibrium

`CH_3COOH + H_2O rightleftharpoons H_3O^+ + CH_3COO^--`

favours reactants to a large extent. Nearly all the acid remains in molecule form and the concentration of `H_3O^+` will be roughly 0.006 M. We say this acid is weak, because it ionizes to such a small degree.

Because it is the action of the `H_3O^+` that causes the corrosive nature of the acid, the acetic acid shows a much weaker effect.