CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
For a series of experiments, a chemist measures the natural logarithm of the ion-product constant of water, lnKw, at several temperatures.
Temperature (K) lnKw
298 −31.4
308 −30.7
318 −30.0
328 −29.4
338 −28.8
Assume the enthalpy of the auto-ionization process is effectively temperature-independent over the range studied.
(a) Plot (or describe how you would plot) the data in a way that produces a straight-line relationship suitable for extracting the enthalpy change, ΔH, for the auto-ionization of water.
(b) Determine the numerical value of ΔH (in kJ mol−1) from your straight-line representation. Show all calculations, including any conversions.
(c) Use the value of ΔH obtained in part (b) to predict Kw at 333 K. Clearly state any assumptions made.
(d) From your predicted Kw, calculate the pH of neutral water at 333 K and explain whether the solution would be classified as acidic, basic, or neutral according to the precise criterion you identified in Question 1 (c).