pKa > 14

[LoLCareerGoals]

So I know there is an equation that states pH + pOH = 14 and pKa + pKb = 14.
I am a bit confused by pKa values > 14. Does this mean this acid will not dissociate in water at all and need a different polar solvent to lose and gain H+s?

---

Members don't see this ad.

 

[chonc]

pKa > 14 just means it's a really weak acid or very strong base. In an acid dissociation reaction, you'll see a large pKa if the Ka is small (pKa = -log(Ka) ), which means that the acid doesn't dissociate well. It would probably need a very strong base to dissociate but generally, it just means a very weak acid.

 

[milski]

pH+pOH=14 is correct only when the solvent is water. And yes, you'll need a different solvent, since the substance will not dissociate (to any noticeable degree) in water. I would not even call it an acid at that point. The solvent does not need to be polar either - that's not directly related to pKa but more to the structure of the substance that you're trying to dissolve.

 

[LoLCareerGoals]

Ok then what does negative pKb mean? What does pKb = -3 mean? Drop one of these into water and it will turn 1000 water molecules into OH-? I know it means very strong base, I just don't get it in the context of water.
I thought pKa concept assumed water as solvent. Shouldn't we specify pKa_water or pKa_alcohol if the definition or value changes with solvent???

 

[BryanNextStep]

I am a bit confused by pKa values > 14.

Where are you seeing questions asking you about this? In theory, you can stick with the usual calculations you know for pH, pOH, pKa, pKb for values between 0 and 14. If the math gives you a negative number - or a number greater than 14 - you haven't done anything "wrong" per se. It's unlikely on the real test that you'd get a situation with an acid that weak (or base that strong), but I'd be willing to bet that to answer the question correctly, you wouldn't have to change any of the problem-solving techniques you've learned to use for when a pKa is, say, 5.

 

[BerkReviewTeach]

Where are you seeing questions asking you about this? In theory, you can stick with the usual calculations you know for pH, pOH, pKa, pKb for values between 0 and 14. If the math gives you a negative number - or a number greater than 14 - you haven't done anything "wrong" per se. It's unlikely on the real test that you'd get a situation with an acid that weak (or base that strong), but I'd be willing to bet that to answer the question correctly, you wouldn't have to change any of the problem-solving techniques you've learned to use for when a pKa is, say, 5.

Look beyond general chemistry and you'll find that this is actually a common question. For example, consider the typical alpha proton on a carbonyl compound. It has a pKa around 17. There's an entire chapter (at least) in every organic chemistry text book dedicated to the water based reactivity of the that proton, so understanding the impact of pKa on reactivity in a conceptual fashion is very useful. If you think about the aldol condensation reaction, you are adding OH- to remove the alpha proton and initiate the reaction. Because water has a pKa of 15.7, we know that OH- is a weaker base than the carbanion formed after deprotonating the alpha proton, but the values are close enough that OH- should be able to deprotonate about 1 to 5% of the alpha protons. This creates the perfect environment for the nucleophilic anion to attack a nearby neutral ketone and form a new carbon-carbon bond. If the base were too strong, then you'd deprotonate too many ketones and there wouldn't be any electrophiles to attack. If the base were too weak, then you wouldn't generate enough carbanions to see a reaction to any detectable extent. Knowing that the pKas are within two of one another is important to the reaction.

As far as calculation-based questions go, it's important to know the pKa so you know what you are dealing with. For instance, if they gave you a hypothetical acid with pKa = -7, then you should immediately think that it's a strong acid (like HCl and HNO3, it has a negative pKa). Knowing that, you in turn know that it fully dissociates and you can simply assume [HX] = [H+] and calculate pH using pH = -log[HX]. If the pKa lies between 2 and 12, then you know it's weak enough that the HH equation applies (if it's a buffer). So even in calculation questions it's helpful to know what pKa < 0 tells us, what pKa > 14 tells us, and what happens when Ka is too close to [H+].