Question

8.3. The pH value of pure water is not a universal constant, but dependent on temperature. At 0C it is 7.5, and at 40C it is 6.8. Explain this phenomenon and comment on why it is numerically reasonable.

Answer 1

2H2O H₃O⁺ + OH^-

Kw = [H₃O⁺][OH^-]

on increase in temperature H2O dissociation increases. [H₃O⁺] concentration increase and [OH^-] concentration increases.

Kw value increase

PH = -log[H₃O⁺]

[H₃O⁺] conc increase, PH value decrease (at high temp) (at 40⁰C PH = 6.8)

[H₃O⁺] conc decrease, PH value increase (at low temp)    (at 0⁰C PH = 7.5)

[OH-] conc increase,POH value decrease,PH value increase (at high temp)

[OH-] conc decrease,POH value increase,PH value decrease (at low temp)