Equilibrium Constant

Now that we understand what an acid is, we need a method of quantifying which acids are stronger and which are weaker. pH doesn’t work for this, let me explain why:

pH vs. pKa

Concept: Why we use pKa instead of pH. 

Video Transcript

Alright guys, now I want to talk about that relationship between the equilibrium constant and the pKa. Remember that in General Chemistry we were always talking about something called pH, and pH was used to measure what? Do you guys remember? It was actually used to measure the concentration of hydronium ions in a solution. Oops, not is, in solution. Okay. So I know what’s really specific but it was used to basically measure how acidic a solution was. How acidic or how basic. Okay? But it turns out that in organic chemistry, we don’t care about the solution at all. We don’t care if it’s acidic or if it’s basic at all. What we care about is the actual molecule itself. And what I care about is how likely is that molecule to donate a proton or how likely is that molecule to accept a proton? pH doesn’t tell me that. pH just tells me how many H+ are circulating in this test tube. I don’t care about that. I care about the molecules inside. I care how much are they likely to give away a proton.
So that means that we’re going to use a different measure. We’re going to use the negative log of the equilibrium constant, of the dissociation constant. Remember that dissociation constant has to do with how likely a bond is to break. So we’re going to use the negative log of how likely that is to happen to figure out for the tendency of a molecule to donate protons. And that’s what we actually care about. So that’s why in Gen Chem, you used pH, but now now in Orgo we’re going to use pKa instead.

The Ka (dissociation constant) describes the tendency of a molecule to break apart. In the case of acids, that specifically means donating protons, which is exactly what we are interested in knowing!

Concept: The relationship between equilibrium constant and pKa.   

Video Transcript

So let’s just talk about stuff that you guys should remember: that strong acids are going to have a high dissociation constant. That means that they’re very likely to dissociate fully, and that means they fully dissociate in aqueous solution. Remember that weak acids are going to have a smaller dissociation constant, and what that means is that they’re only going to partially dissociate in an aqueous solution. And that makes a huge difference, because that means that they’re going to have different pKa’s, they’re going to have different tendencies to donate protons.
Now let’s remember, what is pKa? P, remember, stands for the negative log base 10. And then remember that Ka stands just basically for products over reactants. Now, I’m not going to make you guys calculate every single things, but check it out, products in this case are just dissociating into H+, and reactants are what happens before if fully dissociates. Does that make sense? So the K is basically the ratio of how much my acid is going to actually become a proton, and that’s what I care about.
So therefore, if we’re taking the negative log of this Ka, what that means is that the higher the Ka, basically the higher the changes of the molecule breaking apart and making ions, the lower the pKa is going to be. So just like we had in pH, remember that your strongest acid was actually the one that was the lowest pH. It was close to 0. Remember that basically, pH is on a scale of 0 to 14, and remember that down here was the very acidic solution, and then over at 14 was the very basic solution. So remember that if it was very acidic, it would have a very low pH, and in the same way, pKa is going to do the same thing. So your strongest acids are always going to have the lowest numbers for pKa. Does that make sense? And it’s because we’re using the negative log, not the positive. So it’s always going to be the opposite. Cool so far? Awesome.

Concept: The pH scale vs. the pKa scale.

Video Transcript

Now what I want to do is I want to go over the similarity of the pH scale and the pKa scale. For the pH scale, like I said, 0 is very acidic, 14 is very basic, and 7 was neutral. Remember that 7 was just water, and that was neutral and right in the middle. Well, for pKa we have something very similar, but for pKa it’s just going to be a different scale. And it’s going to mean something different. So for pKa, about the lowest pKa that you can get without being a crazy molecule is around negative 10, and these are going to be the most amazing acids. Remember that pKa has to do with how much it wants to be an acid. so negative 10 is going to be the most amazing acids ever. And then it turns out that the scale is about from negative 10 to about 50. So 50 would be about the same thing as me saying a really high number for pH. And what I’m wondering is, what do you think 50 means? Do you think that means that it’s a very good base? The answer is no, it has nothing to do with basicity, because remember, all this is, is we’re just trying to see how likely is it to dissociate into an acid.
So that means what is 50? 50 is just going to be—pardon my French—but it’s just going to be your very, very shitty acids. It’s the ones that are terrible at dissociating. So basically, 50 doesn’t mean that you’re basic, it just means that you’re really bad at being an acid. It means that you suck as an acid. You’re never going to dissociate. Then what is 16? 16 is actually water. And water is going to be kind of your neutral pKa, where basically anything above water, we’re going to say that those are the good acids, and then anything below water, we’re going to say that those are the bad acids. Does that make sense? With 50 being the worst of all, so much so that I said “shitty”. Does that make sense?
So basically, when we’re talking about pKa’s, we’re always looking at things that are lower than 16 as our good acids.

The pH and pKa scales really are completely different. Pardon my French! pKas are obviously something I’m really passionate about. 

Calculating pKa

This is the easiest kind of question you could get. Calculating pKa’s just takes some very simple math. 

Example: Calculate the pKa of ammonium and determine if it is a stronger acid than acetic acid.