Pure water is neutral when the rate of its autoionization is exactly balanced by the…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Pure water is neutral when the rate of its autoionization is exactly balanced by the reverse recombination of ions. Laboratory measurements show that the autoionization constant of water increases with temperature; for example, at 50 °C the value is reported as Kw=5.48×10−14.
(a) Explain qualitatively, without performing a numerical calculation, how the larger value of Kw at 50 °C affects the numerical value of the hydronium-ion concentration in neutral water when compared with its value at 25 °C.
(b) Describe how the change in the hydronium-ion concentration you discussed in part (a) influences the pH of neutral water at 50 °C. In your explanation, reference the definitions of pH and pOH together with the condition required for neutrality.
(c) A student claims that a measured pH of 7.00 always indicates a neutral solution, regardless of temperature. Evaluate the validity of this claim using the reasoning developed in parts (a) and (b).