Question

Is `H_3O^+` hard water? What occurs in an aqueous solution of acetic acid?

Answer 1

Here's what's going on.

Explanation:

First and foremost, `"H"_3"O"^(+)` is not hard water, it is the hydronium ion.

Hard water is simply water that has a high concentration of multivalent cations, which are cations that have charges greater than `("1+")`.

When acetic acid, `"CH"""_3"COOH"`, is added to water, it will ionize to form the acetate anion, `"CH"_3"COO"^(-)`.

SImply put, acetic acid is a stronger acid than water, so it will donate its acidic proton to a water molecule, which will act as a base and accept this proton.

The water molecule that accepts this proton will form the `"H"_3"O"^(+)` cation.

Keep in mind that the reaction does not go to completion because acetic acid is a weak acid in an of itself, so an equilibrium will be established between the unionized and ionized forms

`overbrace("CH"_3"COOH"_text((aq]))^(color(green)("acts as an acid")) + overbrace("H"_2"O"_text((l]))^(color(blue)("acts as a base")) rightleftharpoons "CH"_3"COO"_text((aq])^(-) + "H"_3"O"_text((aq])^(+)`

In fact, the equilibrium lies to the left, meaning that more acetic acid molecules will not ionize in aqueous solution.

Answer 2

Because acetic acid is a weak acid, that will to some extent retain the proton bound to the oxygen.

Explanation:

Acetic acid undergoes the following equilibrium in water:

`(H_3C)C(=O)OH + H_2O rightleftharpoons H_3O^+ + (H_3C)C(=O)O^-`

This is the normal behaviour for an acid, i.e. for the generalized acid, `HA`,

`HA + H_2O rightleftharpoons H_3O^+ + A^-`

The equilibrium constant, `K_a`, for acetic acid is `1.76 xx 10^-5`. This means that these equilibria will lie strongly to the left. Please note that what I write as `H_3O^+` or `H^+` are not "hard water". `H_3O^+`, the acidium ion, is our conception of a cluster of water molecules with an extra `H^+` attached.