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

AI Thread Summary
HCl is classified as an Arrhenius acid because it contains hydrogen and reacts with water to produce hydrogen ions, despite the fact that these ions are not present in a simple form in solution. Hydrogen carbonate, while it can donate a proton to water and form hydronium and carbonate ions, is not considered an Arrhenius acid due to its more complex behavior in solution. The confusion arises from the historical context of the Arrhenius definition, which simplifies the understanding of acid-base reactions. Although the concept of H+ ions is a simplification, it serves as a useful model for beginners, while more advanced discussions require understanding the complexities of H3O+ and the coordination sphere. Ultimately, the Arrhenius theory provides a foundational framework for learning about acid-base chemistry, despite its limitations.
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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.
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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.
 
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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.
 
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.
 
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.
 
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+.
 
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