# Where do "OH"^(-) and "H"^(+) ions come from?

Nov 30, 2015

From the dissociation or autoprotolysis of water.

#### Explanation:

Water is conceived to undergo an acid-base dissociation:

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

Alternatively,

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

Both reactions are somewhat conceptual, in that there is no species that corresponds to ${H}^{+}$ or even ${H}_{3} {O}^{+}$ (aka, the hydronium ion). As far as anyone knows, this acidium ion is 4-5 water molecules with an extra ${H}^{+}$, i.e. ${H}_{9} {O}_{4}^{+}$; a cluster of water molecules with an extra ${H}^{+}$. Whatever it is, we conceive that the species in solution is ${H}_{3} {O}^{+}$, and we write our reactions and model our equilibria accordingly.

We can certainly write the equilibrium reaction for the above equation, because it has been extensively measured:

${K}_{w} = \left[O {H}^{-}\right] \left[{H}_{3} {O}^{+}\right]$ $=$ ${10}^{- 14}$, at $298 K$. Would you expect ${K}_{w}$ to increase or decrease at temperatures $>$ $298 K$?