Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: peterschmidt3943 on January 17, 2018, 12:45:11 PM
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Hello,
normally, every book describes oxonium ion as H3O+, but some say that it is normally written as H9O4+ ... why?
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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.
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Hello,
could you give me an example for better visualization?
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Not sure example of what you want. Reactions taking place are basically like that:
H+ + H2O ::equil:: H3O+
H3O+ + H2O ::equil:: H5O2+
and so on, with a general form of
H(H2O)n+ + H2O ::equil:: 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.
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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).