Understanding Acidity: A look at Hydronium Ions

[cepheid]

Hello,

I'm new here. I haven't done anything more than first year university chemistry, and am not too familiar with the Lewis model for acids and bases. Therefore, please don't be too harsh on me for the elementary nature of this question:

In grade school, we were taught that when a hydrogen atom shares its electron with a chlorine atom, a covalent bond is formed. It holds together the new molecule, hydrogen chloride. Since this compound exists as a gas at standard conditions, it is designated by the chemical formula $$HCl_{(g)}$$. We also learned that when hydrogen chloride gas is dissolved in water, it becomes aqueous hydrogen chloride $$HCl_{(aq)}$$. The proper name for this is hydrochloric acid.

Later on, we were informed that the molecule does not remain whole once in an aqueous solution. It ionizes to form $$H^+_{(aq)}$$ ions and $$Cl^-_{(aq)}$$ ions. We were left with this notion, relatively unaltered, until the Bronsted-Lowry unit, where we learned that the $$H^+_{(aq)}$$ ion actually reacts with a water molecule to form the hydronium ion: $$H_3O^+_{(aq)}$$. Another way of looking at it is that the water molecule accepts a proton “donation” from $$\HCl_{(aq)}$$. A crucial point in this unit was that a strong acid is defined as one that nearly completely dissociates (or is it ionizes?) in this manner.

Whoa! That’s the kicker. This means that the final products in the process of dissolving hydrogen chloride are almost entirely hydronium ions and chloride ions. This is the final “acidic” solution. It is the one that displays all of the empirical properties of an acid (i.e. conducts electricity, turns blue litmus red, has a pH < 7.0, etc.). These empirical properties can be attributed entirely to the hydronium ion. Put another way, in the final acidic solution, the so-called strong acid, $$HCl_{(aq)}$$, is present in negligible quantities. So what is an acid, and how can we call $$HCl_{(aq)}$$ an acid? Again, I emphasize that for all “strong acids”, the aqueous acidic solution is composed almost entirely of hydronium ions (and the corresponding anions). If this is true, then does the term “hydrochloric acid” really have any meaning at all?

Lest you are in danger of thinking that this little conundrum applies only to the strong acids, consider the following. Why are weak acids less acidic (i.e. have a higher pH)? They are less acidic because the concentration of hydronium ions in the aqueous acidic solution is less. If you ruminate on that answer for a while, I think that you will conclude that the entire notion of “acidity” can be linked solely to the concentration of hydronium ions in an aqueous solution. The remainder of the undissociated compound in the solution doesn’t actually do anything that’s characteristic of an acid. So an acid is an acid because when one places it in water it dissociates into something else that has acidic properties?

If what I have just said isn't true, where have I gone wrong in my thinking?

 

[Bystander]

Originally posted by cepheid
(snip)

You're doing okay to this point --- might be refinements to be made in your picture, but it's substantially correct.

If you ruminate on that answer for a while, I think that you will conclude that the entire notion of “acidity” can be linked solely to the concentration of hydronium ions in an aqueous solution.

"Acidities" of protic (H containing) acids are measures of their proton donating, or protonating, ability/tendency/capacity/strength in a specified solvent --- it's not restricted to aqueous solutions.


(snip)

Help any?

 

[Meninger]

In an pure aqueous solution (water) a reaction btw H20 + H20 reaches an equilibrium which yields a equal concentrations of the hydronium ion and the OH- (hydroxide) ion.

A strong base is any substance which has the ability to increase the hydroxide concentration; essentially all of the base reacts with the acid.

...and vice versa for the strong acid.

The near opposite of a strong base would be a feeble base in which none of the base reacts with the acid and thus there is no increase in the hydronium concentration.

However, a weak base (acid) is something in btw the latter two categories. Depending on the strength of the acid that the weak base is reacting with we can choose to calculate the equilibrium constant.
A weak base with a weak conjugate acid consitutes a buffer. You should study this area of acids and bases instead of focusing so much on the different concepts of acids and bases. It will help you to understand the relationships between these terms.