TPR question H+/OH-

[GRod18]

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given that the reaction between H+ and -OH is highly exothermic and assuming that the primary coolant water is completely pure, what would be its pH?

A. Less than 7.

TPR solution:

H+ + -OH --> H20 + heat Delta H rxn < 0 (exothermic)

I understand that increasing temp shifts reaction to the left increasing both [H+] and -[OH], -log [H+] = pH, but why does that necessarily mean that pH is less than 7? If I have -[OH] at a greater concentration than [H+] wouldn't it be more than 7?

 

[Rabolisk]

H2O --> H+ + OH- (endothermic)

The question states that the only substance is pure water. H2O dissociates at a greater rate at higher temperature, thus there is more [H+] and more [OH-]. As [H+] increases, pH decreases. Of course, pOH decreases too, and the result is that pH + pOH < 14. You wouldn't have more [OH-] than [H+] because they both come from water, and you have to have one to one ratio, per stoichiometry.