pH of heated water

[mrmandrake]

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Hey guys,

I was doing a TPR passage and got this one wrong.

"If the rxn of an acid and base is exothermic, how would pH be affected by increasing the temp?"

I thought the equilibrium could be written as H2O <--> (H+) + (OH-) + heat, so increase heat, lower H+ concentration --> higher pH.

The solution TPR gives is that heat decreases pH because the EQ could be written as H2O + heat <--> (H+) + (OH-), so heat increases H+ and so lowers pH. Doesn't that represent an endothermic rxn though? Am I wrong? Thanks!

 

[mdc1027]

Because the reaction is exothermic, when it goes in the forward direction it produces heat (ie heat is a product). Therefore, the overall reaction looks like this:

H+ + Base <---> HBase+ + Heat

According to Le Chatlier's principle, when you stress a system it will move to reduce that stress. By increasing the temperature, you are adding more heat, which is a product. So to relieve the stress of increased products, the reaction shifts more toward reactants, which means more H+, and therefore lower pH.

 

[mrmandrake]

hmm ... good point, they wrote the rxn backwards so that threw me off seeing heat on the left side, but since its reversible that makes sense ... thanks!

 

[alexis1001]

But wouldn't you be increasing the base concentration as well? I would have though that this would have no effect on the pH

 

[Phlame217]

If your base is OH then possibly yes, but this could be a lewis acid base such as NH3 which in solution would act as a base and form NH4. You also have to account for many other types of bases becuase all a base is, is technically an electron donor.

 

[BerkReviewTeach]

Hey guys,

I was doing a TPR passage and got this one wrong.

"If the rxn of an acid and base is exothermic, how would pH be affected by increasing the temp?"

I thought the equilibrium could be written as H2O <--> (H+) + (OH-) + heat, so increase heat, lower H+ concentration --> higher pH.

The solution TPR gives is that heat decreases pH because the EQ could be written as H2O + heat <--> (H+) + (OH-), so heat increases H+ and so lowers pH. Doesn't that represent an endothermic rxn though? Am I wrong? Thanks!

The given information essentially amounts to the following:

HX + MOH <==> H2O + M+ + X- + heat

Given that M+ and X- are spectator ions, we can ignore their impact on the reaction. So, the reaction reduces to:

• H+ + OH- <==> H2O + heat

If you reverse the reaction, it means that the autoionization of water is endothermic, which makes sense given that a bond is being broken in the formation of hydronium and hydorxide. This leaves the following reaction:

• H2O + heat <==> H+ + OH-

The addition of heat pushes the reaction forward, resulting in more H+ and more OH-. This means that the pH drops (given that there is more H+). The odd thing to swallow here is that the pOH also drops (given that there is more OH-). This means that pH + pOH = 14 only at 25 C.

At 37 C (body temperature), pH + pOH = 13.6, which means that in physiological systems, neutral water actually has a pH of 6.8.

 

[mrmandrake]

The given information essentially amounts to the following:

HX + MOH <==> H2O + M+ + X- + heat

Thanks. This was very informative.