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pKw vs temperature graph

I understand the initial decrease in pKw.

$\ce{Heat + H2O (l) <=> H+ (aq) + OH- (aq)}$

Temperature increases, reaction shifts to the right, so $\ce{H+} \text{ and } \ce{OH}$ concentration increases.

Higher $\ce{H+} \text{ and } \ce{OH}$ concentration results in lower $\ce{pK_w}$, as $\ce{pK_w = - log ([H+][OH-])}$

But why then does the $\ce{pK_w}$ start to increase beyond $250$ degrees C?

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    $\begingroup$ I'm afraid your logic behind the decrease isn't exactly OK, or maybe it's just lacking details. $\endgroup$ – Mithoron Nov 19 '17 at 1:25
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    $\begingroup$ I think there is a lot you're missing. 25 mPa is about 24.7 atmospheres. At that pressure water boils at 224 C. There is a semi-empirical formula. See maplesoft.com/applications/view.aspx?SID=154073&view=html $\endgroup$ – MaxW Nov 19 '17 at 6:46
  • $\begingroup$ The closer you come to the boiling point, the less energy the ions gain from hydratisation. $\endgroup$ – Karl Nov 19 '17 at 10:34

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