Kaplan FL5 P/C Question

[betterfuture]

Carbonic acid is a physiological buffer. The pKa/°C for carbonic acid has been calculated as -0.009. Based on this information, which of the following is always true about the dissociation of carbonic acid?

(High-lite for answer, just so people don't accidentally click on this post and see the right answer choice)
Answer: ∆H>0

I can not understand Kaplans explanation. Anyone can clue me in on how they got the answer. Like how do you decipher what the question means and is asking. I was seriously lost.

---

Members don't see this ad.

 

[aldol16]

What are the answer choices? We can't tell which one is true if there are no options given.

 

[betterfuture]

Haha I wrote that you need to highlight for the answer. Take your mouse and click down after where it says Answer: and you will see it.

 

[aldol16]

delta G0 = -2.303RTlogKa = 2.303RT*pKa = delta H - T*delta S. Delta S is positive since you have one species going to two. Therefore, -2.303RT(0.009) = delta H - some positive number. From this, it looks like delta H can be positive or negative.

Edit: Wait - is this a change in pKa of -0.009 per degree Celsius? That means that as you add heat, the dissociation becomes more favored, such that it's endothermic.

 

[betterfuture]

delta G0 = -2.303RTlogKa = 2.303RT*pKa = delta H - T*delta S. Delta S is positive since you have one species going to two. Therefore, -2.303RT(0.009) = delta H - some positive number. From this, it looks like delta H can be positive or negative.

Edit: Wait - is this a change in pKa of -0.009 per degree Celsius? That means that as you add heat, the dissociation becomes more favored, such that it's endothermic.

What does that even mean? pKa/°C? I have never encountered this in my Gen Chem Review Books or the AAMC outline.

 

[aldol16]

What does that even mean? pKa/°C? I have never encountered this in my Gen Chem Review Books or the AAMC outline.

... You wrote it. I'm assuming it means that the pKa changes by some amount per degree Celsius. Equilibrium constants are dependent on temperature so that's what I think it means.

 

[betterfuture]

So if it was a (+) number instead of the (-) for pKa/°C, that would mean that dissociation is less favored as temperature goes up thus it would release heat?

 

[aldol16]

So if it was a (+) number instead of the (-) for pKa/°C, that would mean that dissociation is less favored as temperature goes up thus it would release heat?

Yes, if I'm interpreting pKa/degree Celsius correctly. A positive number would mean that pKa would be going up, meaning that Ka is going down, meaning that dissociation is less likely.

 

[betterfuture]

Thank you so much! You literally know everything!

 

[geykey]

I was about to ask the same question

 

[NextStepTutor_2]

What does that even mean? pKa/°C? I have never encountered this in my Gen Chem Review Books or the AAMC outline.

@betterfuture

This is simply a ratio. The AAMC can make up any ratio it wants, so long as it conforms to the laws of science. In this instance they have given you a ratio of change. How does the pKa of the reaction change with temperature?

Normally we expect heat to raise Keq, and drive reactions forward (though not always). Here we are talking Ka, not Keq, which is fine as they are the same thing, but for specific reactions (Ka, Kb are reserved for acid/base reactions).

For the MCAT you should also remember that as X increases, pX will decrease. A pKa/C ratio < 0 indicates that as T rises, the pKa goes down.

Given our knowledge of logs, this means that as T rises, pKa falls (Ka rises), indicating the equilibrium constant is larger for the reaction. AKA raising T drives the reaction to favor the products.

The AAMC will often use 2 different relationships at once like this. they are hoping you get tripped up connecting the dots from T to Ka to pKa to dissociation to dH. That is why memorizing will nto help much on the MCAT (but it will in med school). Understanding allows you to arrive at the answers with often little to no calculation needed.

dH > 0 indicates an endothermic reaction, or that heat can be treated like a reactant. This also explains why increasing T seems to favor the fwd reaction and raise Ka (lower pKa).

Hope this helps, good luck!

 

[deleted852061]

If you could identify that during dissociation of weak acid molecules, they become ions and gained more kinetic energy, they need to absorb it and thus it's endothermic reaction, delta H is positive. Gaining more freedom as ions, the entropy goes up, delta S is positive.

Without knowing any equation, you could rule out A (which is dS<0) and select B (dH>0). But there are option (C) and (D); C talks about dG and let's look at the following two equations. We know both dH and dS are positive, so that dG could be positive or negative, it would depend on the temperature. Eliminate (C).
(1) dG= – RTln Ka » 2.303RTpKa
(2) dG=dH-TdS

Option (D) ask about pKa. The question stated that "Carbonic acid is a physiological buffer".... and we know that buffer is composed of the weak acid and the conjugate base. Ka for a weak acid is always small, and when Ka is very small, -log (a very small number) = positive (e.g. -log 10^-1000=3). So we can eliminate D.

The other things you may be interested is that the value of the pKa changes with temperature and can be understood qualitatively based on Le Châtelier's principle: when the reaction is endothermic, Ka increases and pKa decreases with increasing temperature; the opposite is true for exothermic reactions. -- but this information isn't needed for this question.


Thus the best choice here is B. You could get to this answer without knowing any equation. : D