Autoionization of water & pH

[MediCynical]

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The autoionization of water is an endothermic reaction
2H2O -> H3O+ + OH-

As the temperature of pure water increases the pH:

The given answer is decreases because [H+] increases, but I said it remains at 7 because [H+] equals [OH-] even after an equilibrium shift.

So the pH drops and so does the pOH? Why wouldn't the 2 cancel each other out?

 

[loveoforganic]

Neutrality is pH 7 only at certain conditions. When you change the temperature, you change those conditions, and thus change where neutrality is.

 

[MediCynical]

Neutrality is pH 7 only at certain conditions. When you change the temperature, you change those conditions, and thus change where neutrality is.

aha. thank you

 

[Hemichordate]

So how would you know if neutrality is >7 or <7 at other temperatures?

 

[RogueUnicorn]

you would have to know how Kw changes with temperature. you can find it on google i believe if you're just curious. if pKw is above 14, then you know neutral is higher than 7

 

[rls303]

you would have to know how Kw changes with temperature. you can find it on google i believe if you're just curious. if pKw is above 14, then you know neutral is higher than 7

http://www.chemguide.co.uk/physical/acidbaseeqia/kw.html

So according to the bottom of the link, the Kw can change, and it says that the H+ will increase as will the OH- according to this. Because we no longer have Kw=14 as our reference point now.Thus contributing to a lower pH and a lower pOH, correct?

 

[loveoforganic]

Yeah. Basically, it takes a certain amount of energy for a water to split into oh- and h+. When you increase te temp, you increase the available energy for that reaction.