Pure liquid water undergoes auto-ionization according to the equilibrium expression…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Pure liquid water undergoes auto-ionization according to the equilibrium expression shown in the reference sheet. A student is asked to analyze how the neutral pH of water changes with temperature.
Assume no dissolved gases or solutes are present.
(a) Describe qualitatively how the value of the equilibrium constant for the auto-ionization of water, Kw, is expected to change as temperature increases from 25 ∘C to 50 ∘C. Base your answer on thermodynamic reasoning about the forward and reverse processes.
(b) Without performing any numerical calculation, predict whether the pH of
neutral water at 50 ∘C is greater than, equal to, or less than 7.0. Justify your prediction in terms of relative hydronium and hydroxide ion concentrations.
(c) Some introductory texts define neutrality as “pH = 7.” Explain why this definition is incomplete and clarify the more precise criterion chemists use to decide whether a solution is neutral at any temperature.