# What do we mean by writing H_3O^+?

Apr 21, 2017

When we write ${H}_{3} {O}^{+}$, this is a $\text{representation}$ of the hydronium ion, the characteristic cation of water.........we could also use ${H}^{+}$ as a $\text{representation}$.

#### Explanation:

We know that water undergoes autoprotolysis, which we represent as:

2H_2O(l) rightleftharpoons H_3O^+ + ""^(-)OH

So what is the hydronium ion? As far as anyone knows it is a cluster of 3 or 4 water molecules WITH AN EXTRA ${H}^{+}$ ion, i.e. formally, ${H}_{7} {O}_{3}^{+}$ or ${H}_{9} {O}_{4}^{+}$. Of course, the hydrogen ion can be facilely exchanged between clusters. If you have ever played rugby, I have always thought of it as when the forwards form a maul and can pass the pill from hand to hand while bound in the maul. (Of course, in such an acidic cluster, the hydrogen has the possibility to tunnel across the cluster).

Of course, the acid-base equilibrium is characterized by the equilibrium constant, ${K}_{w} = {10}^{- 14}$, under standard conditions. How do you think this equilibrium would evolve if we elevated the temperature to $100$ ""^@C.