Why is H⁺ sometimes written as H₃O⁺ in equations?

Why are neutralization reactions written with H₃O⁺ instead of H⁺?

If a free hydrogen ion encounters a water molecule, it attacks the unshared electron pairs on the oxygen in the water molecule and forms a hydronium ion, H₃O⁺. The chemical bond that forms between the water and hydrogen ion is covalent and very strong. In an aqueous solution, essentially all of the H⁺ exists as H₃O⁺.

When people write "H⁺(aq)" in a chemical equation, it's understood that they really mean "H₃O⁺(aq)". Writing hydrogen ions rather than hydronium ions makes neutralization equations a little easier to read and balance.

Author: Fred Senese senese@antoine.frostburg.edu