Acid Base Equilibrium - Video Tutorials & Practice Problems
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The holy grail of acid-base chemistry is to be able to draw the mechanism and predict the equilibrium for an acid-base reaction. Believe it or not, you’re ready to do this.
STEP 1. Identify the acid and base
Many times charges or a known acid will be present (+) = Acid (–) = Base
If you see no charges, dissociate ALL spectator ions, these are the cations Li+, Na+, K+ and Ca+
If both compounds are still neutral, assign the one with the lowest pKa as the acid.
STEP 2. Label the conjugate acid and base. Remember:
Acids LOSE their most acidic proton to become conjugate bases
Bases GAIN a proton to become conjugate acids
STEP 3. Compare the acidity/pKa of the acid to the conjugate acid. The equilibrium will favor the side with the weaker acid (arrow towards the highest pKa).
That’s all there is to it! Watch me solve the first example so you can get the hang of it.
1
concept
The 3 steps for determining the direction of acid and base equilibrium.
Video duration:
5m
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Video transcript
So now we've gotten to what's pretty much the holy grail of acid base chemistry, and that is predicting acid base equilibrium. All right, 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 gonna learn a set of rules, and we're gonna use PK information in order to figure out these questions. All right, so basically, here's I'm just gonna give you an example. Here is a common question that you could see on an exam with the following reaction. Go to the left or would go to the right label. All the species draw the correct arrows. Okay, 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 several time. The very 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 gonna find on the left hand side Which one is the acid? Which one is the base? Okay, Now, how do you do this? Ah, lot of times that could be the trickiest part. Well, many times there's gonna be known charges or unknown acid will be present. So if there's charges or unknown acid, that makes it easy. So, for example, if you see a positive charge, that's always gonna 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 carb oxalic acid. If you have a car oxalic acid, then that would make it easier as well. Or if you have HCL or whatever. Okay. But sometimes you're not gonna have a negative charge, and you're not gonna 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 or from Kim Jong Kim. Those were just your role one or your I'm sorry, your first column. Cat ions. So that would be lithium positive. Um, potassium positive and sodium positive. Okay, now some professors even go a Sfar as toe add cesium. Okay, I have seen that before. Caesium is also in that first column. All right, so these, actually I mean, I know that I said that it was three, but it's actually it could be four, depending on how tricky your professor wants to get these Air Catalans that always associate in solution. So these basically make Ionic bonds. So, for example, if I had n a o. H. And I was trying to determine if that was my acid with 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 dissociate it first, so that means you have to take this and make it in a positive. And that means my o h becomes a wage negative. And now remember that any positive is a spectator ions. I don't care about it. So that means that now I have a base. Does that make sense? So you have toe. If you don't see a obvious positive or negative, you have to go ahead and start dissociating. Okay, But now let's say that you do both of those steps and you're compounds are both still neutral. So let's say you there's nothing to disassociate. There's no charges. You have no clue what to do. So, for example, if you had something like on, let me think of a good example. So let's say that you had. So let's say that you had on H two Whoa, Okay. And you were comparing it to Carl oxalic acid. And you didn't remember that car Oxalic acid was an acid. So let's just say that you had a brain fart and you forgot that car oxalic acid was an acid. Well, then you could just go ahead and look at PK. So then you could say that zey sickly the next rule. If they're still neutral, assign the lowest PK as the acid. Okay, so then I would say the Peca of my water is 16. The PKK of my car looks like acid is five. So that means that my car books like acid is gonna 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 PK cool. So that's going to really take care of you. Okay. The next step is now that you know what you're asked in your base are go ahead and label the conscience. 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 compared the acidity or PK basically acidity and PK air. The same thing of the Lewis acid to the conjugate acid. Okay. And remember that in order, Thio, go in that direction, you're gonna have to go from stronger to weaker. Okay, Now, one way. I like to say that is that the strongest acid is gonna be the one with lowest PK. So I always say that the peak a should go from lower on the right. I mean, on the left to higher on the right PK because remember, Pekka is the opposite of strength. So you want a low P k to start with and you want a high peak at the end, okay?
Would the following reaction go to the right or left? Label All ALL species. Draw arrows in the correct direction.
2
example
Determining Acid/Base Equilibrium
Video duration:
4m
Play a video:
Video transcript
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. Which one is the base? Is there a obvious charge? Yes. So this is actually a really easy question because we already have the obvious charge. So that means that this is gonna be my base. And that means that this one's gonna be my acid. Does that make sense? Because I had a negative charge and negative charges. A dead giveaway. This is a base. Okay, so now I have my base in my acid. Now, if the draw figure out which ones are congregates are, so which one is the conjugate acid? Good job. This would be the conjugate acid. Okay, Because of the fact that your base always turns into a college, you get acid. Okay? And that means that this once we're gonna conjugate base. So now we know that this reaction is an equilibrium. We know that that that the green side is becoming the blue side and that the blue side can also become the green said 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 its favorite to be on the react inside? How do we know? And that's what we have to determine in these kinds 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 PK. So what's the Peca of my acid? See this? It gets so easy. The PK of my ass. It isn't. It's an alcohol. So this should be 16. All right, So what's the PKK of my conjugate acid? Well, my conjugate acid is a carb oxalic acid, so this should have a PK of five. Okay, so my question to you is is this going from stronger toe weaker? No. This is actually the out 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 re agents are gonna be in the re agents, and these were gonna 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 were going so that I make all that over there. All right, 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. Okay. 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. Okay. Where am I going to grab a proton from? Well, I'm gonna grab the most acidic proton. That's gonna be this H because remember that H is the one that has a PK five. Okay, Before, I would make you guys draw die polls. But now that you have the PKK's memorized, it's even easier. Just go with one that has the lowest PK. It's that easy. Okay, um, I done So 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 only with two bonds. So there we go. That's our mechanism. It's a draw. Arrows in correct direction. So now I'm showing that this oh is going to get the h right there. And that this age is going to be lost right here. See that? So you guys just drew your first entire acid based reaction with predicting equilibrium. Isn't that awesome?
Would the following reaction go to the right or left? Label All ALL species. Draw arrows in the correct direction.
3
example
Determining Acid/Base Equilibrium
Video duration:
2m
Play a video:
Video transcript
All right, so let's go ahead and get started. So what's the acid? Okay. Well, where's my base? Was my acid? This one has to be my base, because 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 congregates? That means that my base must go into my conjugate acid. And that means that my acid must go into my conjugal face. All right, so now I've got my conjure gets laid out. Now, I just have to figure out the PK a values. So what's the Peca 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 PKK of my conjugate acid? It's 16. Okay, So is this going to go to the right or is this gonna go to the left? What do you 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. All right, so if you're gonna 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 mechanisms on the other one. So that means I'm gonna start my mechanism on the right side. So where does the mechanism start? With the negative. It always starts with the base. So I'd come over here and try to grab something. What? I'm going to grab the acidic proton, which is the alcohol proton. Is that, um I done No, not done. Because I have to break the bond between the O and the H and give those electrons to the O. So now what that means is that I'm going to wind up gaining a proton on the double bond and I'm gonna lose a proton on the Oh, now you might be saying, Where is that proton? All it is? Is that Remember that that carbon before used to have one proton on it. Okay, I'm sorry it didn't have any protons on it, okay? Because I had three bonds and a lone pair. That's why. Has a negative charge. So I had zero hydrogen. Now it's gonna have one hydrogen sticking off of it. And that one 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. Ah, hydrogen there. All right.