Answer
The autoionization of water is endothermic.
Work Step by Step
The equation of the autoionization of water is:
$$2H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$$
Since $K_w = [H_3O^+][OH^-]$, when $K_w$ increases, the concentrations of the hydronium ion and the hydroxide ion also increase.
As we can observe in the chemical equation, $H_3O^+$ and $OH^-$ are the products.
A reaction that increases the products concentration after a raise in the temperature is classified as endothermic.
