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What is the hydronium ion?

The hydronium ion is a kind of positive charge that results from the protonation of a water molecule, and whose chemical formula is H 3 O + . It consists of the simplest of the oxoniums: ions where oxygen carries a positive formal charge and has three covalent bonds.

3 O + is one of the simplest but curious cations that exist. In water under normal conditions it has a concentration of 1 · 10 -7 M, a product of the autoionization equilibrium. However, its concentration increases exponentially when acids stronger than H 3 O + itself dissolve in water, making it acidic.

The concentration or activity of H 3 O + in water is used to measure the acidity of aqueous solutions: pH. The more H 3 O + ions there are, the less positive the pH will be, and the more acidic the solution in question will be. This acidity, on the other hand, resides in the hydrogen ion, H + , which is often confused with the H 3 O + ion .

Hydronium ion or cation formula

+ and its amazing ability to transfer between the hydrogen bonds of water molecules allows H 3 O + to associate in more complex cationic formations; such as the Eigen cation, H 9 O + , and the Zundel cation, H 5 O + , and many others.

Structure of H3O + with a model of spheres and bars

In the first image you could see the structural formula of the hydronium ion. Above now we see its representation with a model of spheres and bars. In both, the trigonal pyramid geometry stands out, whose bond angles (OH) is 113º; slightly deviated from 119º for the tetrahedron.

Although oxygen has a partial positive charge, this does not mean that the negative region is around the hydrogen atoms. Quite the opposite. The oxygen in H 3 O + is even more electronegative due to its electronic deficiency; so all the negative density displayed on an electrostatic potential map will be concentrated on it :

Electrostatic potential map for H3O +

Not to mention, oxygen has a pair of free electrons, which further reinforces that negative density.

A consequence of all the above is that the hydrogens of H 3 O + lose electronic density, since oxygen attracts it towards itself. Thus, H 3 O + can establish very strong hydrogen sources with a neighboring water molecule: H 2 O + -H — OH 2 .

This interaction is the key behind the astonishing ionic mobility of H + and why H 3 O + is able to associate with many water molecules at once.

Acidity

The hydronium ion is a very strong acid. In fact, it is the strongest acid that can exist in aqueous solution. Why? Because any other acid stronger than it will protonate a water molecule to originate H 3 O + :

3 O + is able to remain stable as long as there are no other bases in the medium that are stronger than water. Any HA acid that meets this will be classified as a strong acid. Meanwhile, if the acid HA is weaker than the H 3 O + , then part of the HA will not dissociate completely and we will speak of a weak acid:

HA + H 2 O ⇌ A  + H 3 O +

Therefore, since H 3 O + is the strongest acid that exists in water, its acidity will depend on the concentration of H 3 O + . This is the basis for defining, in simple terms, the acidity of an aqueous solution expressed as pH:

pH = -log [H 3 O + ]

+ vs. H 3 O +

Hydrogen ion and hydronium are not the same. H + is much more acidic than H 3 O + , since it consists of nothing more than a proton, which will seek by all means to bind itself to a molecule to gain electrons. When H + gets to a water molecule, H 3 O + is formed :

+   + H 2 O → H 3 O +

That is why H 3 O + can be represented as H + (aq), indicating that it is an H + in aqueous medium.

The strength of an acid is measured in its ability to donate, according to the Brönsted-Lowry definition, H + ions . The stronger you are, the more you will donate H + , not H 3 O + . The strongest acids ever synthesized (superacids) are those in which the H + is practically “naked”; that is to say, without any impediment to jump towards the molecule that will protonate.

The practice of representing H 3 O + as H + (ac) is so common that the two are often spoken of as the same, without this having a negative impact on the interpretation of the chemistry of the solutions.

Solvation

3 O + can form very strong hydrogen bonds with a neighboring water molecule. In doing so, we have the Zundel cation, H 5 O + :

Zundel cation

But the positive charge does not remain only on one side of the cation: it can be transferred to the other water molecule as an H + ion :

2 O— H- + OH 2 → H 2 O + -H— OH 2

Therefore, the positive charge is distributed between both oxygen atoms for the two water molecules.

In the case of Eigen’s cation, H 9 O + , an H 3 O + forms hydrogen bonds with three water molecules, the positive charge being distributed among all of them thanks to a “jumping” H + . These jumps are so fast that they explain the great ionic mobility of H + in water, using H 3 O + as a vehicle, and water molecules as a highway.

5 O + and H 9 O + are not the only cationic associations that H 3 O + can create in water. Some molecular dynamics calculations show the existence of a H 3 O + (H 2 O) 20 : 20 H 2 O molecules interacting with a H 3 O + cation and distributing the positive charge between them.

Therefore, H 3 O + and H + build a curious relationship with water molecules, beyond acidity.

You go out

Just as there are organic salts of oxonium, oxonium derived from the protonation of water is no exception. Its general formula is [H 3 O + ] [X  ], where X  is any anion that comes from the dissolution of a very strong acid.

These salts are sometimes called ‘monohydrate acids’, since the formula [H 3 O + ] [X  ] or H 3 O + · X  can also be written as HX · H 2 O. Thus, there may be acids dihydrates, HX · 2H 2 O, trihydrates, HX · 3H 2 O, etc.

For example, HCl can crystallize as HCl · H 2 O or H 3 O + · Cl  . Likewise, we have other hydronium salts such as H 3 O + · ClO  or HClO 4 · H 2 O, and HBr · 4H 2 O or H 3 O + · Br  · 3H 2 O.

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