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Topic: pKa and effect on temperture?  (Read 25952 times)

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Offline lespaul

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pKa and effect on temperture?
« on: August 14, 2011, 09:53:32 PM »
Does anyone know what effect temperature has on pKa for carboxylic acids?

Does increased temp make pKa go up (meaning the acid is less strong) or down (acid get stronger).

Offline gertrudetrumpet

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Re: pKa and effect on temperture?
« Reply #1 on: August 15, 2011, 12:38:52 AM »
if it is the same as water, as temp increases, pka increases. I think at boiling point, ph of water is like 6.1

Offline shirleyisgood

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Re: pKa and effect on temperture?
« Reply #2 on: August 16, 2011, 04:41:41 AM »
when the temp. increases, Ka will increase, so pKa will decrease,which means acid goes stronger.

Offline gertrudetrumpet

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Re: pKa and effect on temperture?
« Reply #3 on: August 16, 2011, 03:54:26 PM »
whoops, got the two mixed up, apologies.

Offline lespaul

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Re: pKa and effect on temperture?
« Reply #4 on: August 17, 2011, 11:51:21 PM »
Thanks gertrudetrumpet.

So if I understood you correct:

Temp goes up, Ka will decrease, pKa will increase - meaning acid will get WEAKER. Right?

Offline TomW

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Re: pKa and effect on temperture?
« Reply #5 on: August 26, 2011, 07:53:02 PM »
Does anyone know what effect temperature has on pKa for carboxylic acids?

You want to be careful about just memorizing the answer to this problem. Most general chem exams will have a question that relates temperature and acids, because it's a great way to also test your understanding of Le Chatelier and pKa. You want to be able to work through why raising the temperature will decrease pKa, because that way you will never get mixed up or confused on an exam.

The best way to think of this is to remember that breaking a hydrogen bond, even a weak one, requires energy. This means that the reaction described by a Ka value is endothermic. Now, what does Le Chatelier tell us about an endothermic reaction when heat is "added"?

HA + heat   :rarrow:  H+ + A-

Note that as you add heat, the reaction is "pushed" to the right, and so more products will be formed, ie, more H+. Now, Ka is defined as ([H+]*[A-]/[HA]). An increase in H+ will increase the value of Ka.

Thus, Ka increases with temperature, and the acid will become stronger. Since the -log(x) decreases as x gets large, an increasing Ka will have a correspondingly lower pKa.

Offline Borek

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Re: pKa and effect on temperture?
« Reply #6 on: August 27, 2011, 04:05:12 AM »
This means that the reaction described by a Ka value is endothermic.

This is overgeneralization. In most cases that's true, but - as usual in chemistry - there are counterexamples.
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Offline lespaul

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Re: pKa and effect on temperture?
« Reply #7 on: September 03, 2011, 11:13:53 AM »
Hey Tom,

Thanks for the input, but I think you may have it backwards.

The addition of MOST acids to water is almost always exothermic (remember this is why we add acid to water, not the other way around). Secondly, even if you didnt know this, take a practical approach to it. When we add HCL, H2SO4, etc to water or any other solvent - it dissociates automatically dissociates (yes, according to different extents based on pKa) but the important thing to remember is that I did NOT add heat to make this happen - heat was generated as a RESULT of the breakdown of HA to H+ and A-.

As such, the "heat" component should be on the products side, not on the reactants. Therefore, an increase in temp - according to LeChatliers should decrease Ka or INCREASE pKa, making acids WEAKER at higher temps. Exactly like you stated, but the other way around.

Im sure there are some select acids that are endothermic, dont have any that come to mind. But I think we can say that for the most part - when temp increases, pKa also increases, thus making acids weaker.


Here are some references:

http://en.wikipedia.org/wiki/Exothermic_reaction

http://en.wikipedia.org/wiki/Acid_dissociation_constant


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