# What are H+ ions?

Dec 5, 2015

${H}^{+}$ ions represent the characteristic cation of water. I will try to explain what I mean.

#### Explanation:

There are 2 common representations of autoprotolysis in water:

${H}_{2} O r i g h t \le f t h a r p \infty n s {H}^{+} + O {H}^{-}$ And,

$2 {H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + O {H}^{-}$

I write representations because these are precisely what they are: a conception of water at the molecular level, which are chemically reasonable and certainly useful but might or might not represent reality.

We know that at $298$ $K$, ${K}_{w}$ $=$ ${10}^{- 14}$ $=$ $\left[O {H}^{-}\right] \left[{H}^{+}\right]$, and for most acid/base problems this will suffice. But what is this species, which is also represented as ${H}_{3} {O}^{+}$, the acidium ion? As far as anyone knows this is a cluster of water molecules with an extra proton associated; so ${H}_{7} {O}_{3}^{+}$ or ${H}_{9} {O}_{4}^{+}$, 3-4 water molecules with an extra ${H}^{+}$ tacked on.

If you have ever played rugby, think of a maul where the forwards bind together and pass the pill from hand to hand. This may not be realistic, in that within a cluster of water molecules (unlike within a pack of forwards), proton transfer can tunnel across water clusters; it is the movement of charge rather than protons. This explains the ionic mobility of ${H}^{+}$ and $O {H}^{-}$ in comparison with other ions.

At undergraduate level, it is probably useful to think of the acidium ion as an actual entity. You must be able to use ${K}_{w}$ in some form in your calculations, and use $\left[{H}^{+}\right]$ and $\left[{H}_{3} {O}^{+}\right]$ to determine $p H$ and $p O H$.

Apologies if I have written info I have written elsewhere. I could not find the link.