Acid Base Equilibrium

So now we've got into its pretty much the Holy Grail of acid base chemistry and that is predicting acid-base equilibrium. So let's go ahead and get started on this. For these types of questions what you are going to be asked is, “What's the direction of the reaction? Is it going to go to the right, is if going to go the left?” I even know it sounds like, “Hey, I’ve got 50-50 shot. It can't be that bad.” They are surprisingly tricky. So we’re going to learn a set of rules and we're going to use pKa information in order to figure out these questions.
So basically here is, I’m just going to give you an example. Here's a common question that you can see in an exam. Would the following reaction go to the left or would it go to the right? Label all the species. Draw the correct arrows. There's a lot in there and there's a lot you have to understand. So let's go ahead and break it down one step at a time. The very first thing you always want to start off with is identifying the Lewis acid and the Lewis base. Remember that I said the Lewis is the most general definition of acids and bases. So that just means that you’re just going to find on the left hand side which one is the acid which one is the base. Now how do you do this? A lot of times that could be the trickiest part. Well, many times there’s going to be known charges or a known acid will present. So if there’s charges or a known acid that makes it easy.
So for example if you see a positive charge, that’s always going to be acid. And if you see a negative charge that's always going to be the base. So that part is easy. Also let's say that you have a carboxylic acid. If you have a carboxylic acid then that would make it easy as well or HCl or whatever.
But sometimes you're not going to have a negative charge and you’re not going to have a positive charge So if you don't, then you go to the second step. The second step is to make sure that all spectator ions
are dissociated. Do you guys remember spectator ions from Gen Chem? Those were just your first column cations. So that would be Lithium (positive), Potassium (positive) and Sodium (positive). Now some professors even go as far as to add Cesium. I have seen that before. Cesium is also in that first column. Alright, so these actually, I mean I know that I said that it was three but actually it could be four depending on how tricky your professor wants to get. These are cations that always associate in solutions to these basically make ionic bonds.
So for example if I had NaOH and I was trying to determine if that was my acid my base I would be confused possibly because I would say, “Well, I don't see a negative charge. I don't see a positive charge. What is it?” Well you have to disassociate it first. So that means you have to take this and make it Na+ and that means my OH becomes OH-. And now member that any positive is a spectator ion that I don't care about. So that means that now I have a base. Does that make sense? So if you don't see an obvious positive or negative you have to go ahead and start dissociating.
But now let’s say that you do both those steps and your compounds are both still neutral. So let’s say there's nothing to dissociate, there's no charges, have no clue what to do. So for example if you had something like, let me think a good example, so let's say that you had… so let's say that you had an H2O and you were comparing it to carboxylic acid and you didn't remember that carboxylic acid was an acid. Let’s just say you had a brain fart and forgot that carboxylic acid was an acid. Well then you can just go ahead and look at pKa’s then you could say—that’s basically the next rule. If they’re still neutral, assign the lowest pKa as the acid.
So then I would say the pKa of my water is 16. The pKa of my carboxylic acid is five. So that means that my carboxylic acid is going to be the acid and that means that my water is going to be the base. Does that make sense? So basically, if you are still neutral at the end just go with the lowest pKa. So that's going to really take care of you.
The next step is now that you know your acid and your base are, go ahead and label the conjugates. That means draw them out if you have to draw them. Or just label them based on which one is the acid, that one is the conjugate base, the base becomes the conjugate acid.
So then the last step is compare the acidity or pKa—basically acidity and pKa are same thing—of the Lewis acid to the conjugate acid. And remember that in order to go in that direction you're going to have to go from stronger to weaker. Now one way I like to say that is that the strongest acid is going to be the one at the lowest pKa. So I always say that the pKa should go from lower on the left to higher on the right pKa. Because remember pKa is the opposite of strength. So you want a low pKa to start with and you want a high pKa at the end.