[peterschmidt3943]

Hello,

normally, every book describes oxonium ion as H3O+, but some say that it is normally written as H9O4+ ... why?

[Borek]

Hydronium is a solvated proton, solvation is an equilibrium, multistep process. Technically neither of these two formulas is correct, as solutions contains several separate cations of a general formula H(H₂O)_n⁺. Determining their individual concentrations is not trivial, different approaches give different values of n for which the form dominates the solution.

[peterschmidt3943]

Hello,

could you give me an example for better visualization?

[Borek]

Not sure example of what you want. Reactions taking place are basically like that:

H⁺ + H₂O H₃O⁺

H₃O⁺ + H₂O H₅O₂⁺

and so on, with a general form of

H(H₂O)_n⁺ + H₂O H(H₂O)_n+1⁺

H₉O₄⁺ is just one of these forms - H(H₂O)₄⁺.

As these are equilibrium reactions all forms are present in the solution together, so there is no single correct answer to the question "what form does the H⁺ take in water". Valid question is "which form dominates the solution?" - but the answer depends on the acid concentration and is not easy to determine. If memory serves me well n=3,4,5 are the most often cited values.

[Enthalpy]

Several H₂O per H⁺, but in a variable number.

As the H₂0-H⁺ bond is not very strong, it is regularly broken and it forms again, so the number of H₂0 is only a mean value. I expect it to depend on the H⁺ concentration too.

Water also organizes further than the first layer of molecules around H⁺. Water molecules stick to an other with proper orientation even without any ion in the vicinity, and the presence of the H⁺ orients the molecules several layers away. This influence only gets weaker with the distance and is broken by heat more often.

So H₃O⁺ should be understood as a mere reminder that H⁺ isn't alone. But it isn't more accurate than writing H⁺ (aq).