HYDRONIUM ION

THE HYDRONIUM ION

Introduction

The hydronium ion is an important factor when dealing with chemical reactions that occur in aqueous solutions. Its concentration relative to hydroxide is a direct measure of the pH of a solution. It can be formed when an acid is present in water or simply in pure water. It's chemical formula is H₃O⁺. It can also be formed by the combination of a H⁺ ion with an H₂O molecule. The hydronium ion has a trigonal pyramidal geometry and is composed of three hydrogen atoms and one oxygen atom.  There is a lone pair of electrons on the oxygen giving it this shape. The bond angle between the atoms is 113 degrees.

H₂O_(l) ↔ OH^-_(aq)+ H⁺_(aq)

As H⁺ ions are formed, they bond with H₂O molecules in the solution to form H₃O⁺ (the hydronium ion). This is because hydrogen ions do not exist in aqueous solutions, but take the form the hydronium ion, H₃O⁺. A reversible reaction is one in which the reaction goes both ways. In other words, the water molecules dissociate while the OH^- ions combine with the H⁺ ions to form water. Water has the ability to attract H⁺ ions because it is a polar molecule. This means that it has a partial charge, in this case the charge is negative.  The partial charge is caused by the fact that oxygen is more electronegative than hydrogen. This means that in the bond between hydrogen and oxygen, oxygen "pulls" harder on the shared electrons thus causing a partial negative charge on the molecule and causing it to be attracted to the positive charge of H⁺ to form hydronium. Another way to describe why the water molecule is considered polar is through the concept of dipole moment. The electron geometry of water is tetrahedral and the molecular geometry is bent. This bent geometry is asymmetrical, which causes the molecule to be polar and have a dipole moment, resulting in a partial charge.

An overall reaction for the dissociation of water to form hydronium can be seen here:

2H₂O_(l) ↔ OH^-_(aq)+ H₃O⁺_(aq)