Why autoionization relates to thermochem

Question

The autoionization of water, as represented by the equation below, is known to be endothermic. Which of the following correctly states what occurs as the temperature of water is raised?

H2O(l) H2O(l) H3O (aq) OH-(aq)

A. The pH of the water does not change, and the water remains neutral.

B. The pH of the water decreases, and the water becomes more acidic.

C. The pH of the water decreases, and the water remains neutral.

D. The pH of the water increases, and the water becomes more acidic.

E. The pH of the water increases, and the water remains neutral.

WHY IS C correct AND NOT B?

Answer 1

As the temperature increases, Kw for autoionization increases.  This means both [H+] and [OH-] increase.  Since [H+] increases, pH decreases.  But since [H+] still equal [OH-], the solution remains neutral.  So the answer is C.

The other way to look at this is that the number that defines neutral pH decreases as temperature increases.

See the source reference for values of Kw vs. temp.  Look at the subsection on acidity in the reference.