Pure water is ionised according to the equation: 2H2O(l) −⇀↽−\ce{ } H3O+(aq) + OH−(aq) Kw = 10-14 and ΔH = + 57.2 kJ mol-1 As the temperature of pure water is increased, its pH would be expected to

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Pure water is ionised according to the equation: 2H2O(l) −⇀↽−\ce{ <=> }  H3O+(aq) +  OH−(aq) Kw = 10-14 and ΔH = + 57.2 kJ mol-1 As the temperature of pure water is increased, its pH would be expected to

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The question concerns how the pH of pure water changes with increasing temperature, given the autoionization: 2 H2O ⇌ H3O+ + OH− with Kw = [H3O+][OH−]. The key concept is that the autoionization of water is endothermic (ΔH > 0). For endothermic equilibria, increasing temperature shifts the equilibrium to the right, increasing Kw. In pure water, [H3O+] = [OH−], so pH = −log[H3O+]. If Kw increases, [H3O+] increases as the square root of Kw (since [H3O+] = [OH−] = sqrt(Kw)), which makes pH = −log(sqrt(Kw)) decrease as Kw grows.

To address potential options (even though the option list is not provided here):
- If an option stated that pH increases with temperature, that would be incorrect because increasing Kw raises [H3O+] and thus lowers pH in pure water.
- If an option claimed pH remains unchanged, that would be incorrect because Kw changes with temperature for water, altering [H3O+].
- If an option suggested pH becomes more acidic (i.e., decreases) as temperature rises, that aligns with the thermodynamic reasoning for an endothermic dissolution whose equilibrium constant Kw increases with temperature.

In summary, as temperature increases, Kw increases for pure water due to the endothermic nature of water autoionization, leading to a higher concentration of hydronium ions and a lower pH. The correct behavioral statement is that the pH decreases with increasing temperature, reflecting greater autoionization and higher [H3O+] in pure water.

MUF0041 Chemistry Unit 1 - Semester 1, 2025 Area of Study 3 Test 3 (Acids and Bases)

Pure water is ionised according to the equation: 2H2O(l) −⇀↽−\ce{ <=> } H3O+(aq) + OH−(aq) Kw = 10-14 and ΔH = + 57.2 kJ mol-1 As the temperature of pure water is increased, its pH would be expected to

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