pH of water as temperature increases

[MedGrl@2022]

Hi everyone... I hope everyone is enjoying fun MCAT studying...

I came upon this problem in my examkracker 1001 chemistry text book.

718. The autoionization of water is an endothermic reaction. As the temperature of pure water increases the pH?

A. decreases because [H+] increases
B. increases because [OH-] increases
C. increases because [H+] increases
D. remains at 7 because [H+] equals [OH-] even after an equilibrium shift

The answer is A but I thought it would be D, because even though the autoionization would increase. I thought that the number of H3O+ molecules would equal the number of OH- molecules and therefore the resulting solution would be nuetral.

Could someone help me understand why the answer is A?

Thank you,

Verónica

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[slz1900]

First off, you have to know that pH+pOH = 14 only at 25 C, so it is possible for something with a pH of 7 to in effect be "neutral". pH is just the -log[H+]. If water autoionizes, then you get more protons, so the pH goes down. The confusion stems from the fact that you also get more OH, so the pOH goes down as well by the same amount. The solution is still a neutral solution, but pH + pOH is now less than 14.

Also, the second part of D is true. [H+] does equal [OH] after a shift, but the part about the pH being 7 makes it incorrect.

 

[brood910]

endothermic rxn means:

Heat + H2O =><= H+ + OH-

If heat increases, then there will be more "reactant" than the product, so the rxn will tend to shift to the right. In other words, there will be more H+ and OH-.

Do you remember equilibrium concepts?

 

[Postictal Raiden]

When you supply heat to an endothermic reaction, the equilibrium shifts to the right, favoring products.

at room temp. Ka = [H+][OH-] = 10^-7 = pH of 7

If heat increases, and the equilibrium shifts to the right, Ka will be slightly higher than 10^-7. -log of that will give you a slightly lower pH than 7. If pH decreases, it's because [H+] increases.

 

[sazerac]

Hi everyone... I hope everyone is enjoying fun MCAT studying...

I came upon this problem in my examkracker 1001 chemistry text book.

718. The autoionization of water is an endothermic reaction. As the temperature of pure water increases the pH?

A. decreases because [H+] increases
B. increases because [OH-] increases
C. increases because [H+] increases
D. remains at 7 because [H+] equals [OH-] even after an equilibrium shift

The answer is A but I thought it would be D, because even though the autoionization would increase. I thought that the number of H3O+ molecules would equal the number of OH- molecules and therefore the resulting solution would be nuetral.

Could someone help me understand why the answer is A?

Thank you,

Verónica

At higher temperatures, the solution will be neutral (an electrical concept) and the pH will be less than 7 (measures the concentration of hydrogen ion).

This is an awesome EK question and the kind you will see from time to time on the real MCAT, because it really separates those who sort of get it from those who really get it (and / or can think quickly on their feet). It separates the 11's from the 14's.

For extra fun, tell us what the pOH of the solution will be as the temperature rises!

 

[Fifty 3rds]

I'm guessing the pOH will equal pH, meaning both will go down, an odd concept since we are programmed to operate under the 25c framework.

Because there is more heat/energy applied to the system, free ions will not necessarily reform to make water but will instead bounce around with excess energy in their ionic form.

Is this correct?

 

[sazerac]

I'm guessing the pOH will equal pH, meaning both will go down, an odd concept since we are programmed to operate under the 25c framework.

Because there is more heat/energy applied to the system, free ions will not necessarily reform to make water but will instead bounce around with excess energy in their ionic form.

Is this correct?

 

[pacer]

I would answer this question based on the following:

As temperature increases, more water molecules break apart and there is an increase in [H+], thereby decreasing pH and increasing the acidity of the solution. Eventually, Le Chatelier's principle will take place and the solution will reach some sort of equilibrium but to answer this question, just think of the immediate effects of increasing temperature rather than taking a large leap forward to long term effects.

 

[SpecterGT260]

I would answer this question based on the following:

As temperature increases, more water molecules break apart and there is an increase in [H+], thereby decreasing pH and increasing the acidity of the solution. Eventually, Le Chatelier's principle will take place and the solution will reach some sort of equilibrium but to answer this question, just think of the immediate effects of increasing temperature rather than taking a large leap forward to long term effects.

We arbitrarily define the pH (-log[H]) as the standard for grading the caustic potential of water. pOH is otherwise just as valid and as someone pointed out both ions *may* increase but they would do so equally.

My advice would be to locate a non idealized equation and look for a temperature term. It's been too long since thermo and I don't remember if temp affects pH or not. My gut says not unless there is another substance in the water who's solubility is temperature dependent.

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[SpecterGT260]

http://www.reagecon.com/pdf/technicalpapers/Effects_of_Temperature_on_pH_v4-_TSP-01-2.pdf

This should help.

Regardless, it is always important to remember that these sources are not infallible. I've had bad and blatantly wrong questions in qbanks while studying for boards. It's more important to make sure you understand the concept and not just memorize a factoid derived from some dude's understanding of the concept.

 

[brood910]

I dont understand why ppl are arguing over this simple thing.
The question said it is "endothermic". Dont miss that part..