How does an acid dissociate in water

Question

I read somewhere that hydrogen cation combines with water to form hydronium ion. I also read it combines with various water molecules. Can someone please explain?

Answer 1

Usually a pure acidic molecule like HCl gives one proton $\ce{H+}$ to a base like NH3 or $\ce{CH3COO-}$, and the proton once fixed on the base gives a new species which is $\ce{NH4^+}$ or $\ce{CH3COOH}$. And that's all what's happening. With water, it is a bit different. The $\ce{HCl}$ molecule starts by giving one proton $\ce{H+}$ to one $\ce{H2O}$. This produces an ion $\ce{H3O+}$ and usually chemists consider that it is the final result. It is not, because it is possible to show that the proton may attract a second water molecule in order to make a heavier ion $\ce{H5O2^+}$ having a charged H atom in a central bridge $\ce{H2O-H^+-OH2}$. Thermodynamic measurements are even able to show that still more $\ce{H2O}$ molecules are attracted around this $\ce{H5O2^+}$ ion, to produce an ion $\ce{H_{2n+1}O_n^+}$. The trouble is that the exact value of the parameter $n$ is not known with precision. It should be between $6$ and $10$. That is why many chemists forget about this difficulty and have decided that the only species in water is $\ce{H3O^+}$. This simplifies also the determination of dissociation constants of acids in water.