The natural equilibrium of water Why can the equilibrium reaction which exists in water be simply represented as

$$\ce{H2O &lt;=&gt; OH- + H+}$$

rather than

$$\ce{2 H2O &lt;=&gt; OH- + H3O+}$$

Doesn't this change both the equilibrium constant AND the enthalpy change?

Why would this still give the correct answer when you use the ionic product of water in solution?
The former equation is just a simplified version of the latter.

The important factors are the concentrations (activities) of the positive and negative species.

However, since these values are small ($10^{-7}\,\mathrm{mol\cdot l^{-1}}$) compared to the concentration of water ($55,56\,\mathrm{mol\cdot l^{-1}}$) this distinction would not affect the value of the dissociation constant either.