Autoionization

[plzNOCarribbean]

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seems simple, but the TPR drill passage and wikipedia are contradictory. On the drill passage it says that:

given that the reaction of an acid and a base is always an EXOthermic one, how would the pH of pure water be affected by increasing the Temperature (ignore changes in volume due to expansion)

B)Increase, the dissociation of water will be disfavored at higher temperatures
C)Decrease, the dissociation of water will be favored at higher temperatures.

I chose B, but the answer says C. Their explanation is this:

H20 + Heat (equilibrium arrow) H30 + OH

"Thus, if the temperature increases, the equilibrium will shift to the right and the [H3O] will increase, and thus pH will decrease"

This makes no sense. I though it was exothermic, so shouldn't heat be on the product side and it should shift left, thus increasing pH? I understand that at equilibrium there is very few H3O and OH molecules, so I can see why increasing temp would increase collisions and cause for the dissociation of water. I also looked this up on wikipedia, and they said "kw increases as the temperature increases, so hot water has a higher concentration of hydronium than cold water, but this doesn't mean it is more acidic, as the hydroxide concentration is higher by the same amount" WTFFF!!!😱

WTF, could some explain to me WTH happens to water when T increases. It seems like TPR was completely off

 

[orgohacks]

seems simple, but the TPR drill passage and wikipedia are contradictory. On the drill passage it says that:

given that the reaction of an acid and a base is always an EXOthermic one, how would the pH of pure water be affected by increasing the Temperature (ignore changes in volume due to expansion)

B)Increase, the dissociation of water will be disfavored at higher temperatures
C)Decrease, the dissociation of water will be favored at higher temperatures.

I chose B, but the answer says C. Their explanation is this:

H20 + Heat (equilibrium arrow) H30 + OH

"Thus, if the temperature increases, the equilibrium will shift to the right and the [H3O] will increase, and thus pH will decrease"

This makes no sense. I though it was exothermic, so shouldn't heat be on the product side and it should shift left, thus increasing pH? I understand that at equilibrium there is very few H3O and OH molecules, so I can see why increasing temp would increase collisions and cause for the dissociation of water. I also looked this up on wikipedia, and they said "kw increases as the temperature increases, so hot water has a higher concentration of hydronium than cold water, but this doesn't mean it is more acidic, as the hydroxide concentration is higher by the same amount" WTFFF!!!😱

WTF, could some explain to me WTH happens to water when T increases. It seems like TPR was completely off

My (quite possibly wrong) take on it is that autoionization will be favored by high temperature, because there is more entropy in having two chemical species in solution versus one (more ways for them to be disordered).

If you look at delta G = delta H - T delta S, then as you increase T, you're also increasing the size of that "T delta S" term. This becomes more negative, which makes the delta G more negative, hence the reaction becomes more favored.

Not sure if it's right, but that's my stab at it.

 

[SoftwareKevin]

given that the reaction of an acid and a base is always an EXOthermic one, how would the pH of pure water be affected by increasing the Temperature (ignore changes in volume due to expansion)

B)Increase, the dissociation of water will be disfavored at higher temperatures
C)Decrease, the dissociation of water will be favored at higher temperatures.

I chose B, but the answer says C. Their explanation is this:

H20 + Heat (equilibrium arrow) H30 + OH

"Thus, if the temperature increases, the equilibrium will shift to the right and the [H3O] will increase, and thus pH will decrease"

This makes no sense. I though it was exothermic, so shouldn't heat be on the product side and it should shift left, thus increasing pH?

It said that an acid base reaction is exothermic... so the reaction of OH and H+ is exothermic, not the reaction which is the dissociation of water. That's why heat goes on the side with water. Try writing it the other way:

OH + H+ <---> H2O + heat

now heat is on the product side - does that help you see it?

 

[SoftwareKevin]

I also looked this up on wikipedia, and they said "kw increases as the temperature increases, so hot water has a higher concentration of hydronium than cold water, but this doesn't mean it is more acidic, as the hydroxide concentration is higher by the same amount" WTFFF!!!😱

WTF, could some explain to me WTH happens to water when T increases. It seems like TPR was completely off

I thought a little more about this. pH does go down because concentration of H3O+ goes up. However, concentration of OH- also goes up, so pOH also goes down. pH and pOH both went down. This is because Kw is only 10^-14 at 25C. If temperature changes, Kw changes and so our pH scale changes as well. pH + pOH is no longer 14.

None of this matters though since the question asked what happened to the pH. pH is always -log([H3O+]) so if H3O+ goes up, pH goes down, no matter what happens to the OH- concentration or Kw.

 

[plzNOCarribbean]

Hm, the exothermic explanation and the heat on the water side makes sense. I'm just trying to get an intuitive understanding bc I thought maybe the pH would remain the same bc the hydroxide an hydronium ion concentration would still be equal, even though each would be increased. So, based on yal's logic, autoionization itself is an endothermic rxn and the more heat is added, the more the water molecules dissociate and thus the solution becomes more acidic at higher temperatures? Would this be a good general guideline to go by on the MCAT? (bc a lot of passages have water related and acid-base related principles)