The pH of pure water at 25°C is 7.0. The enthalpy change of the autoionization of water is + 55.89 kJ/mol. What is the pH of pure water at 100°C?

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Physics

Taniyah Dyer

14 January, 15:24

The pH of pure water at 25°C is 7.0. The enthalpy change of the autoionization of water is + 55.89 kJ/mol. What is the pH of pure water at 100°C?

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Darius · Answer 1

6.14

Explanation:

If the pH falls as temperature increases, this does not mean that water becomes more acidic at higher temperatures. A solution is acidic if there is an excess of hydrogen ions over hydroxide ions (i. e., pH < pOH). In the case of pure water, there are always the same concentration of hydrogen ions and hydroxide ions and hence, the water is still neutral (pH = pOH) - even if its pH changes.

The problem is that we are all familiar with 7 being the pH of pure water, that anything else feels really strange. Remember that to calculate the neutral value of pH from Kw. If that changes, then the neutral value for pH changes as well. At 100°C, the pH of pure water is 6.14, which is "neutral" on the pH scale at this higher temperature. A solution with a pH of 7 at this temperature is slightly alkaline because its pH is a bit higher than the neutral value of 6.14.