The autoionization of water, 2 H2O(l) <=> H3O+(aq) + OH-(aq), is an endothermic process (ΔH > 0). How does heating pure water from 25 °C to 50 °C affect Kw and the pH of neutral water?
Answer and explanation
Correct answer: A. Kw increases and the pH decreases below 7.0, but the water remains neutral
According to Le Chatelier's principle, increasing the temperature of an endothermic reaction shifts equilibrium to the right, increasing the autoionization constant Kw (at 50 °C, Kw ≈ 5.5 x 10^-14). Because both [H3O+] and [OH-] increase equally, [H3O+] > 1.0 x 10^-7 M, so pH = -log[H3O+] decreases to ~6.63. However, because [H3O+] = [OH-], the water remains chemically neutral.
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