Ka and Pka concept

[greenseeking]

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Hello,

I learned that you cannot change KA without a change in temperature. and pKa is -log [KA] so pKa doesn't change unless you change KA.

So with that reasoning, I tried to answer this question:

The PKA for the side chain of histidin in aqueous medium is 6.1. What is true of its pka in a hydrophobic environment?

a. it is the same, because log functions cannot be altered.
b. it would increase...
c.it would decrease..
d. it would be decreased because the proton leaves more readily to make the species neutral.

I was debating between a and d and chose a instead because of my reasoning above. Otherwise I would have chosen D, which is the answer. Could anyone explain why KA can't change unless there is a change in Temp, but pkA can change?

 

[Jamil]

To a certain degree you are correct, the ka and the negative log of Ka (pKa) will not change unless a change in temperature occurs. However, this question may be one instance were limiting yourself to one particular view, leads to the incorrect answer. The explanation indicates that the pKa decreases in order to decrease the charge on the side chain, which in the case of histidine is a +1 charge. It does this because in a hydrophobic environment, a neutral charge is more favorable. Thus, since a proton must be removed in order to create a neutral molecule, the pKa would decrease. Remember, the stronger the acid, the smaller the pKa, and since the proton is removed more easily this would be indicative of a stronger acid and would lead to a smaller pKa.

Now in regards to your question, the pKa and the Ka would both change due to the change in the environment of the molecule. The pKa would decrease, again because the proton (H+) is more more easily removed, and the Ka would increase, again indicating a more easily removed proton. The larger the Ka constant, the stronger to acid.

 

[greenseeking]

Hi,

thanks for your reply! i totally understand what you are saying and it does make sense that pka/ka will BOTH change for those reasons. But I still can't get my head around the fact that KA changes when there is no change in temperature. I guess this is just an exception to the "rule" that KA only changes with temperature?

 

[Jamil]

Hi,

thanks for your reply! i totally understand what you are saying and it does make sense that pka/ka will BOTH change for those reasons. But I still can't get my head around the fact that KA changes when there is no change in temperature. I guess this is just an exception to the "rule" that KA only changes with temperature?

Remember, Ka (and pKa for this matter) are ratios. They tell us the ratio of deprotonated species to protonated species. Due to the change in environment, from hydrophillic to hydrophobic, we have increased the amount of deprotonated species and in effect increased the Ka and decreased the pKa.

Generic:
Ka = [H+][A-] / [HA]

Specific:
Ka = [Histidine neutral][H+]/[Histidine +1]

As you can see we have increased the numerator in the equation and thus would expect a higher Ka value.

 

[greenseeking]

Hm. So I guess that would kind of be like Q. Maybe the problem is not talking about the equilibrium constant Ka but Q?

 

[nindra]

Hm. So I guess that would kind of be like Q. Maybe the problem is not talking about the equilibrium constant Ka but Q?

That's what I would think as well. Because, like the above person, I can't get around into convincing myself that k or pka would change without changing T. I remember reading in my TPR book that unless the question states that its in equilibrium, never assume it to be in equilibrium. Maybe it IS actually Q rather than K...?

 

[greenseeking]

Yeah. maybe they mean that Ka is actually Q and that you can derive pka from Q? Does anyone know have any other insights?

Can you derive pka from Q?

 

[Jamil]

I highly doubt you would see a question like this, unless the passage indicated conditions that could alter equilibrium constants.

One aspect of my explanation that may have been vague, that I wish to elaborate on is that the reason the Ka and pKa change is because the equilibrium has changed. The ratio of deprotonated species to protonated species is different due to the change of the chemical environment.

Equilibrium conditions which establish specific ka and pkas are created for specific predetermined conditions, and unless I am wrong, the Ka and pKa values are created in regards to the acidity of basicity of a substance in water, and in this particular case water is not the solvent. We can then assume that the pKa and Ka values of a substance in a hydrophobic environment would not be the same as those that were created when dealing with water.

You guys are right to think that, usually, the Ka (or any equilibrium constant for that matter) would not change unless a change in temperature has occurred, however, this assumes the standard conditions were the same.

Sorry, if I have repeated myself, the concept that this question deals with is quite funky. Hopefully someone else can elaborate or explain better than I can.

 

[greenseeking]

Actually that makes sense jamil. I am not confused anymore. Thanks so much for your responses! 🙂