Acid Base Equilibrium: Study with Video Lessons, Practice Problems & Examples

Understanding acid-base equilibrium involves identifying the Lewis acid and base, often determined by charges or known compounds. If neutral, dissociate spectator ions and compare pKa values, with the lower pKa indicating the acid. Label conjugate acids and bases, then assess the reaction direction based on acidity: stronger acids (lower pKa) favor the formation of weaker acids (higher pKa). This systematic approach is crucial for predicting reaction outcomes in acid-base chemistry.

The 3 steps for determining the direction of acid and base equilibrium.

So now we've gotten to what's pretty much the holy grail of acid-base chemistry, and that is predicting acid-base equilibrium. Alright? So let's go ahead and get started on this. For these types of questions, what you're going to be asked is what's the direction of the reaction? Is it going to go to the right or is it going to go to the left? And even though it sounds like, hey, I've got a 50-50 shot, it can't be that bad, they are surprisingly tricky. Okay? 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. Alright? So basically here's I'm just going to give you an example. Here's a common question that you could see on 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. Okay. There's a lot in there and there's a lot that you have to understand. So let's go ahead and break it down one step at a time. The first thing that you always want to start off with is identifying the Lewis acid and the Lewis base. Remember that I said Lewis is the most general definition of acids and bases. Right? So that just means that you're just going to find on the left-hand side which one is the acid and which one is the base. Okay? Now, how do you do this? A lot of times that can be the trickiest part. Well many times there's going to be known charges or a known acid will be present. So if there's charges or a known, that makes it easy. So for example, if you see a positive charge, that's always going to be the acid. And if you see a negative charge, that's always going to be the base. Okay. So that part is easy. Or also let's say that you have like a carboxylic acid. If you have a carboxylic acid, then that would make it easy as well, or if you have HCl or whatever. Okay? 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 chem gen chem? Those were just your row 1 or your first column cations. So that would be lithium positive, potassium positive, and sodium positive. Okay? 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 3 but it's actually could be 4 depending on how tricky your professor wants to get. These are cations that always associate in solutions. So these basically make ionic bonds. So for example, if I had NaOH and I was trying to determine if that was my acid or my base, I would be confused 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 dissociate it first. So what that means is you have to take this and make it Na⁺ and that means my OH becomes OH^-. And now, remember that Na⁺ is a spectator ion so I don't care about it. So that means that now I have a base. Does that make sense? So you have to 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 of those steps and your compounds are both still neutral. So let's say you've there's nothing to dissociate. There's no charges. You have no clue what to do. So for example, if you had something like, let me think of a good example. So let's say that you had H₂O. Okay? And you were comparing it to carboxylic acid and you didn't remember that carboxylic acid was an acid. Well, then you could just go ahead and look at pKa's. So then you could say that's basically the next rule. If they're still neutral, assign the lowest pKa as the acid. Okay? So then I would say the pKa of my water is 16. The pKa of my carboxylic acid is 5. 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. Cool? So that's going to really take care of you. Okay. The next step is now that you know what 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 becomes the conjugate base. The base becomes the conjugate acid. Cool. So then the last step is to compare the acidity or pKa. Basically acidity and pKa are the same thing of the Lewis acid to the conjugate acid. Okay? And remember that in order to go in that direction, weaker. One way I like to say that is the strongest acid is going to be the one with 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. Okay?

Determining Acid/Base Equilibrium

So let's go ahead and do that with this example here. So for this example, first of all, we need to identify which one is the acid and which one is the base. Is there an obvious charge? Yes. So this is actually a really easy question because we already have that obvious charge. So that means that this is going to be my base and that means that this one's going to be my acid. Does that make sense? Because I had a negative charge and a negative charge is a dead giveaway. This is a base. Okay. So now I have my base and my acid. Now I have to figure out which one of our conjugates are. So which one is the conjugate acid? Good job. This would be the conjugate acid. Because of the fact that your base always turns into a conjugate acid. Okay. And that means that this one has to be a conjugate base. So now we know that this reaction is in equilibrium. We know that the green side is becoming the blue side and that the blue side can also become the green side. They're in equilibrium, but we don't know to what extent. Do we know that it's favored to be on the product side or it's favored to be on the reactant side? How do we know? And that's what we have to determine in these types of questions. So would it go to the right or would it go to the left? The way that I do that is I just compare p_Ka's. So what's the p_Ka of my acid? See this? It gets so easy. The p_Ka of my acid isn't it's an alcohol so this should be 16. Alright. So what's the p_Ka of my conjugate acid? Well, my conjugate acid is a carboxylic acid, so this should have a p_Ka of 5. Okay. So my question to you is, is this going from stronger to weaker? No. This is actually going the opposite way. This would be going from weaker to stronger. Okay? Another way you could think of it is the number getting is the number getting bigger. No. Because remember you always want to start with a low number and go to a big number. So in this case, I'm starting with a big number and going to a small number. That's really bad. So what that means is that this is actually going to go to the left. Does that make sense? What that means is that this reaction is actually favored to go more to the left towards the weaker side instead of towards the stronger side. Okay. So what that means is that actually these two reagents are going to be in the reagents and these are going to be the products over here. Okay? So if my products are on the left side, that means that my mechanism must be on the right side Because basically I'm going that way, so what that means is that I need to draw the mechanism up here and actually show where the electrons are going so that I make all that over there. Alright? So let's go ahead and get started. If I'm drawing my mechanism on the right side, remember that I said basically mechanism and products have to be on opposite sides. So products are on the left side here, so that means that mechanism has to be on the right. Always remember that. They always have to be on opposite sides because the mechanism is what makes the product. Okay? So where would I start this arrow from? I would start it from the negative charge because I always have to start from the base. Where am I going to grab a proton from? Well, I'm going to grab the most acidic proton that's going to be this H because remember that H is the one that has a p_Ka of 5. Before, I would make you guys draw dipoles. Now that you have the p_Ka's memorized, it's even easier. Just go with the one that has the lowest p_Ka. It's that easy. Am I done? Do I have to break any bonds? Actually, I do. If I make that bond, I have to break that bond. Okay? Because that hydrogen would not be happy with only one bond. I mean, with 2 bonds. So there we go. That's our mechanism. It said to draw arrows in the correct direction. So now I'm showing that this O is going to get the H right there and that this H is going to be lost right here. See that? So you guys just drew your first entire acid-base reaction with predicting equilibrium. Isn't that awesome?

Determining Acid/Base Equilibrium

Alright. So let's go ahead and get started. So what's the acid? Okay. Well where's my base? What's my acid? This one has to be my base because of the fact that it has a negative charge. That was a dead giveaway once again, which means that this is actually my acid. Okay? So I have my base and I have my acid. Now what are my conjugates? That means that my base must go into my conjugate acid and that means that my acid must go into my conjugate base. Alright. So now I've got my conjugates laid out. Now I just have to figure out the pKa values. So what's the pKa of a double bond? Do you guys remember? It was 44. Okay. So that's a really, really bad acid. That's actually one of the worst. Okay. Now what's the pKa of my conjugate acid? It's 16. Okay. So is this going to go to the right or is this going to go to the left? What do you guys think? This is once again going to go to the left. The reason is because it's favored that I'm going to go from the strongest acid, which is my alcohol, to the weakest acid, which is my double bond. Alright. So I'm actually going to go that way and that means that which side is my products? This is my product side. That means which side is my mechanism on? My mechanism has to be on the opposite side. Okay. Wherever your products are, your mechanism is on the other one. So that means I'm going to start my mechanism on the right side. So where does the mechanism start? With the negative. It always starts at the base. So I'd come over here and try to grab something. What am I going to grab? The acidic proton, which is the alcohol proton. Am I done? No. I'm not done because I have to break the bond between the oxygen and the hydrogen and give those electrons I'm going to lose a proton on the—. Now you might be saying where is that proton? All it is is that remember that that carbon before used to have 1 proton on it. Okay? I'm sorry. It didn't have any protons on it. Okay? Because it had 3 bonds and a lone pair. That's why it has a negative charge. So it had 0 hydrogens. Now it's going to have 1 hydrogen sticking off of it. And that 1 hydrogen is the hydrogen that came from here. So that hydrogen is the same as that one. And then notice that now we're missing a hydrogen there. Alright.

Here’s what students ask on this topic:

How do you determine the direction of an acid-base reaction?

To determine the direction of an acid-base reaction, follow these steps: First, identify the Lewis acid and base, often indicated by charges (positive for acids, negative for bases) or known compounds. If both species are neutral, dissociate any spectator ions (e.g., Na⁺, K⁺). Next, compare the pKa values of the acids involved. The species with the lower pKa is the stronger acid. The reaction will favor the formation of the weaker acid (higher pKa). Label the conjugate acids and bases, and ensure the pKa values go from lower on the left to higher on the right, indicating the reaction direction.

What is the role of pKa in predicting acid-base equilibrium?

pKa is crucial in predicting acid-base equilibrium because it quantifies the strength of an acid. A lower pKa value indicates a stronger acid. In an acid-base reaction, the equilibrium favors the formation of the weaker acid, which has a higher pKa. By comparing the pKa values of the acids involved, you can predict the direction of the reaction. The reaction will proceed from the stronger acid (lower pKa) to the weaker acid (higher pKa), ensuring the equilibrium lies towards the side with the weaker acid.

How do you identify the Lewis acid and base in a reaction?

To identify the Lewis acid and base in a reaction, start by looking for charges: a positive charge indicates a Lewis acid, and a negative charge indicates a Lewis base. If no charges are present, dissociate any spectator ions (e.g., Na⁺, K⁺) to reveal the charged species. If the compounds remain neutral, compare their pKa values. The species with the lower pKa is the Lewis acid, while the one with the higher pKa is the Lewis base. This systematic approach helps in correctly identifying the acid and base in the reaction.

What are spectator ions, and how do they affect acid-base reactions?

Spectator ions are ions that do not participate directly in the acid-base reaction. Common spectator ions include alkali metal cations like Na⁺, K⁺, and Li⁺. These ions dissociate in solution and do not affect the equilibrium of the reaction. When identifying the Lewis acid and base, it's essential to dissociate these spectator ions to reveal the actual reactive species. For example, in NaOH, Na⁺ is a spectator ion, and OH⁻ is the reactive base. Ignoring spectator ions ensures accurate identification of the acid and base in the reaction.

Why is it important to label conjugate acids and bases in acid-base reactions?

Labeling conjugate acids and bases in acid-base reactions is important because it helps track the transformation of species during the reaction. The acid donates a proton to become its conjugate base, while the base accepts a proton to become its conjugate acid. This labeling clarifies the relationship between reactants and products, aiding in understanding the reaction mechanism and predicting the direction of equilibrium. By comparing the pKa values of the conjugate acids and bases, you can determine which side of the reaction is favored, ensuring a comprehensive analysis of the reaction.