Why is HCl an Arrhenius acid but hydrogen carbonate isn't?

[zenterix]

Homework Statement

From what I recall reading, $\mathrm{H^+}$ ions do not actually exist in water.

What we have are hydronium ions $\mathrm{H_3O^+}$ and hydroxide ions $\mathrm{OH^-}$ ions.

Relevant Equations

So what exactly is an Arrhenius acid then?

An Arrhenius acid is, apparently, a compound that contains hydrogen and reacts with water to form hydrogen ions.

HCl is considered an Arrhenius base.

But, after all, does it actually form hydrogen ions? I thought such ions were not actually in the solution.

Hydrogen carbonate, $\mathrm{HCO_3^-}$ also donates a proton to water forming hydronium and carbonate ion.

But it is not considered an Arrhenius acid.

I don't understand why HCl is an Arrhenius acid but hydrogen carbonate isn't.

 

[Borek]

Even H₃O⁺ is a simplification, what is present is more like a series of
H(H₂O)_n⁺ cations (similarly, OH^- combines with water molecules, producing series of anions).

You are trying to draw far reaching conclusions from simplified models, as if they were exact. They are not.

 

[zenterix]

I'm just reading books and finding that in my mind the way things are explained is contradictory.

"$\mathrm{H^+}$ doesn't exist in an aqueous solution."

"An Arrhenius acid produces $\mathrm{H^+}$ ions in aqueous solution"

I don't know how one can read those two things and then not be confused.

 

[Borek]

Arrhenius acid is a historical concept, much older than our knowledge about behavior of H⁺. It is still a reasonable initial approximation, for many practical applications exact form of H⁺ in solution doesn't matter, fact that it in some way present does.

In a way I understand how you find these statements confusing, but I feel like Arrhenius theory as an intermediate step makes the initial learning curve much easier to climb. Otherwise you would need to jump into full acid/base equilibrium and chemistry directly.

 

[DrJohn]

What we refer to as an H+ ion in solution is more complicated that just an H+ sitting there and not interacting with anything. Slightly more advanced ways of explaining what it is says call it H3O+, but even that is not a full explanation. And if we look at the entire coordination sphere, it looks complicated and confusing. So for ease of use we talk about H+ at simply levels, and H3O+ at slightly more advanced levels, just for convenience.
EG acid base titrations - think H+
but equilibria, think H3O+.