Acids and Bases Question 9

\ce{2H2O <--> H3O+(aq) + OH-(aq)}

The autoionization of water is represented by the equation above. Values of p $K_w$ at various temperatures are listed in the table below.

\begin{array}{|c|c|} \hline \text{Temperature} (^\circ \text{C}) & \text{p}K_w \\ \hline \hline 0 & 14.9 \\ \hline 10 & 14.5 \\ \hline 20 & 14.2 \\ \hline 30 & 13.8 \\ \hline 40 & 13.5 \\ \hline \end{array}

Based on the information above, which of the following statements is true?

The dissociation of water is an exothermic process.

The pH of pure water is 7.00 at any temperature.

As the temperature increases, the pH of pure water increases.

As the temperature increases, the pH of pure water decreases.

Approach

We can use the equation:

\text{pH} + \text{pOH} = \text{p}K_w

As the temperature increases, p $K_w$ decreases. If p $K_w$ decreases, pH and pOH also decrease.

Generally, as temperature increases, ions will dissociate easier. Since there are more hydroxide and hydronium ions in water, the pH and pOH will decrease.