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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Hello everybody, welcome back. Today we're going to be looking at unit 7.3, which is introducing the concepts of the reaction quotient and the equilibrium constant. So let's get started. So the first thing we're going to look at here is one of the most basic fundamental things that you'll be asked to do in this unit, and it's very, very simple. Okay, and what it is is I've got an equation represented up there. A and B are reactants, C and D are products. The lowercase letters, j, k, l, m, those are representing coefficients in a balanced equation.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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So the first thing we're going to look at here is one of the most basic fundamental things that you'll be asked to do in this unit, and it's very, very simple. Okay, and what it is is I've got an equation represented up there. A and B are reactants, C and D are products. The lowercase letters, j, k, l, m, those are representing coefficients in a balanced equation. What you'll be asked to do very, very often, as like part A in many, many equilibrium free response questions, is to write what's called a mass action expression, or sometimes it's usually it's called the equilibrium constant expression. And what you're looking at in orange there is an example of a mass action expression, an equilibrium constant expression. So let's break this down.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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The lowercase letters, j, k, l, m, those are representing coefficients in a balanced equation. What you'll be asked to do very, very often, as like part A in many, many equilibrium free response questions, is to write what's called a mass action expression, or sometimes it's usually it's called the equilibrium constant expression. And what you're looking at in orange there is an example of a mass action expression, an equilibrium constant expression. So let's break this down. The capital K, that is called the equilibrium constant. Okay, and if you're wondering why it's the letter K, most of the scientists that were working on this equilibrium studies back in the day were German, and the word constant in German starts with a K, so that's why it's a K. So equilibrium constant equals, and then you see a bunch of brackets. Remember guys, brackets mean molarity.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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So let's break this down. The capital K, that is called the equilibrium constant. Okay, and if you're wondering why it's the letter K, most of the scientists that were working on this equilibrium studies back in the day were German, and the word constant in German starts with a K, so that's why it's a K. So equilibrium constant equals, and then you see a bunch of brackets. Remember guys, brackets mean molarity. Okay, concentration. And what you see are the concentrations of the products in the numerator and the concentrations of the reactants in the denominator. And the coefficients become exponents in that expression.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Remember guys, brackets mean molarity. Okay, concentration. And what you see are the concentrations of the products in the numerator and the concentrations of the reactants in the denominator. And the coefficients become exponents in that expression. So an equilibrium constant expression is always written as products, molarity of the products, divided by molarity of the reactants. Okay, the molarities of C and D, those are being multiplied, A and B are being multiplied times each other, and then of course the coefficient, or excuse me, the coefficients become the exponents. At the bottom of the slide, sometimes you'll see the equilibrium constant written as just K. Other times you'll see it written as KC.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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And the coefficients become exponents in that expression. So an equilibrium constant expression is always written as products, molarity of the products, divided by molarity of the reactants. Okay, the molarities of C and D, those are being multiplied, A and B are being multiplied times each other, and then of course the coefficient, or excuse me, the coefficients become the exponents. At the bottom of the slide, sometimes you'll see the equilibrium constant written as just K. Other times you'll see it written as KC. That little subscript C represents concentrations. And if you ever forget this format, guys, it is on your equation sheet right here. Okay, at the very top it's reminding you how to write an equilibrium expression.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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At the bottom of the slide, sometimes you'll see the equilibrium constant written as just K. Other times you'll see it written as KC. That little subscript C represents concentrations. And if you ever forget this format, guys, it is on your equation sheet right here. Okay, at the very top it's reminding you how to write an equilibrium expression. So that's there for you should you ever need it. And I want to just show you an example, okay, of what an equilibrium expression would look like. So here's a real balanced equation that you have in front of you, and if this were a free-response question, maybe part A would simply say, write the equilibrium expression for this reaction, and guys, this is exactly what it would look like.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Okay, at the very top it's reminding you how to write an equilibrium expression. So that's there for you should you ever need it. And I want to just show you an example, okay, of what an equilibrium expression would look like. So here's a real balanced equation that you have in front of you, and if this were a free-response question, maybe part A would simply say, write the equilibrium expression for this reaction, and guys, this is exactly what it would look like. Okay, now we're going to talk about this a little bit more, but just to make sure that you understand this, can you include all parts of a balanced equation? Well, no you can't. If you will notice, everything in this equation is in the gaseous state.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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So here's a real balanced equation that you have in front of you, and if this were a free-response question, maybe part A would simply say, write the equilibrium expression for this reaction, and guys, this is exactly what it would look like. Okay, now we're going to talk about this a little bit more, but just to make sure that you understand this, can you include all parts of a balanced equation? Well, no you can't. If you will notice, everything in this equation is in the gaseous state. The other state of matter that can always be included in an equilibrium expression are aqueous solutions, things that say AQ after them. We will not ever include solids or liquids. Let me say that again.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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If you will notice, everything in this equation is in the gaseous state. The other state of matter that can always be included in an equilibrium expression are aqueous solutions, things that say AQ after them. We will not ever include solids or liquids. Let me say that again. When you're writing out an equilibrium expression, you exclude, do not include, solids and liquids. And why? Well, guys, remember brackets mean molarity.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Let me say that again. When you're writing out an equilibrium expression, you exclude, do not include, solids and liquids. And why? Well, guys, remember brackets mean molarity. You can't, we don't talk about the concentrations of solids and liquids. That's just, that doesn't exist. It's only gaseous and aqueous situations, okay.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Well, guys, remember brackets mean molarity. You can't, we don't talk about the concentrations of solids and liquids. That's just, that doesn't exist. It's only gaseous and aqueous situations, okay. Those are the things that are to be included. So let's say, for example, this water at the end, the final product here, wasn't in the gaseous state. Let's say it was in the liquid state, okay.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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It's only gaseous and aqueous situations, okay. Those are the things that are to be included. So let's say, for example, this water at the end, the final product here, wasn't in the gaseous state. Let's say it was in the liquid state, okay. That means that this would not be there, okay. What it really is, is anywhere you have a solid or a liquid, you just put the number one in its place, or you can just leave it out entirely, okay. If, let's say, I mean this would be kind of silly, but let's say, I know this is sort of ridiculous, but let's say this oxygen here was in the solid state.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Let's say it was in the liquid state, okay. That means that this would not be there, okay. What it really is, is anywhere you have a solid or a liquid, you just put the number one in its place, or you can just leave it out entirely, okay. If, let's say, I mean this would be kind of silly, but let's say, I know this is sort of ridiculous, but let's say this oxygen here was in the solid state. We don't include solids and liquids, so oxygen would not be there in that equilibrium expression. We would just leave it out, okay. Now, you can also write equilibrium expressions, instead of using concentrations, molarities, you can also write expressions using partial pressures, okay.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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If, let's say, I mean this would be kind of silly, but let's say, I know this is sort of ridiculous, but let's say this oxygen here was in the solid state. We don't include solids and liquids, so oxygen would not be there in that equilibrium expression. We would just leave it out, okay. Now, you can also write equilibrium expressions, instead of using concentrations, molarities, you can also write expressions using partial pressures, okay. So you'll notice this time, this expression, it doesn't say Kc. Remember, C, that little subscript C, that stood for concentration. Now it says Kc.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Now, you can also write equilibrium expressions, instead of using concentrations, molarities, you can also write expressions using partial pressures, okay. So you'll notice this time, this expression, it doesn't say Kc. Remember, C, that little subscript C, that stood for concentration. Now it says Kc. Kp, p for pressure. Now, think a couple of units back. What state of matter is the only state of matter that we ever talk about?
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Now it says Kc. Kp, p for pressure. Now, think a couple of units back. What state of matter is the only state of matter that we ever talk about? Pressure. Gases, ladies and gentlemen. So if you are asked to write a Kp expression, and the AP exam will be very clear about which kind of K they want you to write.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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What state of matter is the only state of matter that we ever talk about? Pressure. Gases, ladies and gentlemen. So if you are asked to write a Kp expression, and the AP exam will be very clear about which kind of K they want you to write. If they said write Kp, you would include gases only, all right. So let's do an example. Let's say that SO3, my product here, I know this is sort of silly, but let's say it was a liquid.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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So if you are asked to write a Kp expression, and the AP exam will be very clear about which kind of K they want you to write. If they said write Kp, you would include gases only, all right. So let's do an example. Let's say that SO3, my product here, I know this is sort of silly, but let's say it was a liquid. Okay, well how would that change my Kp expression? This would not be there at all, okay, and what would be in its place would be the number one. So the Kp expression, if it, we have this situation where the SO3 is a liquid, Kp would be one over the partial pressure of SO2 squared times the partial pressure of oxygen.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Let's say that SO3, my product here, I know this is sort of silly, but let's say it was a liquid. Okay, well how would that change my Kp expression? This would not be there at all, okay, and what would be in its place would be the number one. So the Kp expression, if it, we have this situation where the SO3 is a liquid, Kp would be one over the partial pressure of SO2 squared times the partial pressure of oxygen. So the setup is exactly the same. The coefficients turn into exponents, okay, but let's make sure we've got this. So if you're asked to write a Kc expression, Kc for concentration, you can include gases and aqueous compounds.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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So the Kp expression, if it, we have this situation where the SO3 is a liquid, Kp would be one over the partial pressure of SO2 squared times the partial pressure of oxygen. So the setup is exactly the same. The coefficients turn into exponents, okay, but let's make sure we've got this. So if you're asked to write a Kc expression, Kc for concentration, you can include gases and aqueous compounds. If you're asked to write a Kp expression, you include gases only, okay, and again Kp is on your equation sheet as well, so in case you forget that, it's there for you, all right. Okay, now this term right here, if you ever see this term, heterogeneous equilibrium or equilibria, okay, that is referring to the states of matter. Remember, hetero means different, so if you look at this reaction in front of you, you'll notice that the reactant is solid, one of the products is also solid, but the other product is gaseous.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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So if you're asked to write a Kc expression, Kc for concentration, you can include gases and aqueous compounds. If you're asked to write a Kp expression, you include gases only, okay, and again Kp is on your equation sheet as well, so in case you forget that, it's there for you, all right. Okay, now this term right here, if you ever see this term, heterogeneous equilibrium or equilibria, okay, that is referring to the states of matter. Remember, hetero means different, so if you look at this reaction in front of you, you'll notice that the reactant is solid, one of the products is also solid, but the other product is gaseous. So are all parts of this reaction in the same state of matter? No, they're not. Some are solids, one is a gas, that is what's called a heterogeneous equilibrium.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Remember, hetero means different, so if you look at this reaction in front of you, you'll notice that the reactant is solid, one of the products is also solid, but the other product is gaseous. So are all parts of this reaction in the same state of matter? No, they're not. Some are solids, one is a gas, that is what's called a heterogeneous equilibrium. If everything involved in this reaction, if they were all gases or all solids or all aqueous, that would be a homogeneous equilibrium. Okay, so if you ever see that term, it's just referring to the states of matter, okay. Now look at this equilibrium expression, all right, this is the equilibrium expression for that reaction.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Some are solids, one is a gas, that is what's called a heterogeneous equilibrium. If everything involved in this reaction, if they were all gases or all solids or all aqueous, that would be a homogeneous equilibrium. Okay, so if you ever see that term, it's just referring to the states of matter, okay. Now look at this equilibrium expression, all right, this is the equilibrium expression for that reaction. Now wait a second, I thought equilibrium expressions were always products over reactants, like where's the where's the rest of it? Why does it look so simple? Remember guys, we exclude solids and liquids, so calcium carbonate is left out, calcium oxide is left out, the only thing to be included is that gaseous carbon dioxide.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Now look at this equilibrium expression, all right, this is the equilibrium expression for that reaction. Now wait a second, I thought equilibrium expressions were always products over reactants, like where's the where's the rest of it? Why does it look so simple? Remember guys, we exclude solids and liquids, so calcium carbonate is left out, calcium oxide is left out, the only thing to be included is that gaseous carbon dioxide. Okay, now there is another variable that you need to know, capital Q, okay, which is called the reaction quotient, and it is calculated in the exact same way as the equilibrium constant, okay, and I've shown you an example here back from that that first equation I showed you, A plus B yields C plus D. This setup looks exactly the same, the only difference is it now says Q on the left-hand side instead of K. So what makes these things different? Q, that reaction quotient, is calculated using molarities that are not yet at equilibrium, and very often that's initial conditions, not always, but the key here is, and we're going to talk more about this in our next subunit, the molarities that I would plug in here are going to be molarities where the system is not yet at equilibrium, whereas molarities that get plugged into a K expression, an equilibrium constant expression, are molarities once the reaction has reached equilibrium. Okay, and you might say, well why would we ever want to calculate that?
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Remember guys, we exclude solids and liquids, so calcium carbonate is left out, calcium oxide is left out, the only thing to be included is that gaseous carbon dioxide. Okay, now there is another variable that you need to know, capital Q, okay, which is called the reaction quotient, and it is calculated in the exact same way as the equilibrium constant, okay, and I've shown you an example here back from that that first equation I showed you, A plus B yields C plus D. This setup looks exactly the same, the only difference is it now says Q on the left-hand side instead of K. So what makes these things different? Q, that reaction quotient, is calculated using molarities that are not yet at equilibrium, and very often that's initial conditions, not always, but the key here is, and we're going to talk more about this in our next subunit, the molarities that I would plug in here are going to be molarities where the system is not yet at equilibrium, whereas molarities that get plugged into a K expression, an equilibrium constant expression, are molarities once the reaction has reached equilibrium. Okay, and you might say, well why would we ever want to calculate that? I mean this whole unit is on equilibrium, why would we want to calculate something that is not at equilibrium? Well, that's something we're going to talk about a little bit later on in this unit, but it's going to help us to determine the direction the reaction needs to move towards to get to equilibrium, but we'll talk more about that later. So let's look at a practice problem, and we're just going to talk through this one together.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Okay, and you might say, well why would we ever want to calculate that? I mean this whole unit is on equilibrium, why would we want to calculate something that is not at equilibrium? Well, that's something we're going to talk about a little bit later on in this unit, but it's going to help us to determine the direction the reaction needs to move towards to get to equilibrium, but we'll talk more about that later. So let's look at a practice problem, and we're just going to talk through this one together. If you would like to try it on your own, you can of course pause the video and try it, but it says the graph to the right shows a relationship between concentration and time for a reversible reaction involving reactants A and B and product C, and you'll notice they're all gases. Letter A says write a balanced chemical equation that could be represented by this graph. Okay, well we know that the reactants are A and B, and we know C is a product, but that word balanced might be troubling you a little bit, like how are we supposed to know what the coefficients are?
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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So let's look at a practice problem, and we're just going to talk through this one together. If you would like to try it on your own, you can of course pause the video and try it, but it says the graph to the right shows a relationship between concentration and time for a reversible reaction involving reactants A and B and product C, and you'll notice they're all gases. Letter A says write a balanced chemical equation that could be represented by this graph. Okay, well we know that the reactants are A and B, and we know C is a product, but that word balanced might be troubling you a little bit, like how are we supposed to know what the coefficients are? Guys, this is actually going to have you going back to the kinetics unit, okay, and if you will notice, I want you to notice how A and B, how their concentration changes in this time range from 0 to 20 seconds. Do you all agree that both A and B change their molarity between in those 20 seconds? Both A and B change by 0.2 molar.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Okay, well we know that the reactants are A and B, and we know C is a product, but that word balanced might be troubling you a little bit, like how are we supposed to know what the coefficients are? Guys, this is actually going to have you going back to the kinetics unit, okay, and if you will notice, I want you to notice how A and B, how their concentration changes in this time range from 0 to 20 seconds. Do you all agree that both A and B change their molarity between in those 20 seconds? Both A and B change by 0.2 molar. Okay, look at letter A. It goes from 0.4 to 0.2. B goes from 0.5 to 0.3.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Both A and B change by 0.2 molar. Okay, look at letter A. It goes from 0.4 to 0.2. B goes from 0.5 to 0.3. So guess what? That means A and B are gonna have the same coefficient. All right, but now look at how, look at how the product changes in that same time frame.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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B goes from 0.5 to 0.3. So guess what? That means A and B are gonna have the same coefficient. All right, but now look at how, look at how the product changes in that same time frame. Letter C increases from 0 to 0.4. That's double the change that A and B are experiencing. So this is going back to that kinetics subunit or kinetics unit that we've covered previously.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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All right, but now look at how, look at how the product changes in that same time frame. Letter C increases from 0 to 0.4. That's double the change that A and B are experiencing. So this is going back to that kinetics subunit or kinetics unit that we've covered previously. So C, that product, is increasing twice as fast as A and B are decreasing. So ladies and gentlemen, this is what we will end up with. Okay, I know that is going way back in your brain, okay, but you got to remember that because we're now at the point in this, you know, series of units in AP Chem here that we can start pulling multiple different concepts into a question.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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So this is going back to that kinetics subunit or kinetics unit that we've covered previously. So C, that product, is increasing twice as fast as A and B are decreasing. So ladies and gentlemen, this is what we will end up with. Okay, I know that is going way back in your brain, okay, but you got to remember that because we're now at the point in this, you know, series of units in AP Chem here that we can start pulling multiple different concepts into a question. All right, so we've got our balanced equation A plus B, and you'll notice I have a double-headed arrow there. That's a reversible reaction. Yields 2C.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Okay, I know that is going way back in your brain, okay, but you got to remember that because we're now at the point in this, you know, series of units in AP Chem here that we can start pulling multiple different concepts into a question. All right, so we've got our balanced equation A plus B, and you'll notice I have a double-headed arrow there. That's a reversible reaction. Yields 2C. Okay, from that balanced equation we just found, it says write the equilibrium constant expression for the reaction at equilibrium. Okay, fine. Remember, coefficients turn into exponents, and that's what it should look like.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Yields 2C. Okay, from that balanced equation we just found, it says write the equilibrium constant expression for the reaction at equilibrium. Okay, fine. Remember, coefficients turn into exponents, and that's what it should look like. Products over reactants. But then it goes further. It says during what time range, meaning on the graph, is this expression valid?
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Remember, coefficients turn into exponents, and that's what it should look like. Products over reactants. But then it goes further. It says during what time range, meaning on the graph, is this expression valid? Well, it's, they've been asked us to write the equilibrium constant expression. So guys, where is equilibrium on this graph? What time frame?
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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It says during what time range, meaning on the graph, is this expression valid? Well, it's, they've been asked us to write the equilibrium constant expression. So guys, where is equilibrium on this graph? What time frame? It's valid between 20 and 40 seconds because this, remember, that's where the graphs have leveled out. That's where equilibrium has been established. Okay, last part.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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What time frame? It's valid between 20 and 40 seconds because this, remember, that's where the graphs have leveled out. That's where equilibrium has been established. Okay, last part. From the balanced equation again, write the reaction quotient expression for Q. During what time range is this expression valid? Okay, well, guess what?
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Okay, last part. From the balanced equation again, write the reaction quotient expression for Q. During what time range is this expression valid? Okay, well, guess what? Move that down out of the way here. The expression looks exactly the same. The only difference is what variable I have on the left-hand side.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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Okay, well, guess what? Move that down out of the way here. The expression looks exactly the same. The only difference is what variable I have on the left-hand side. Remember guys, if we were actually calculating this, which we'll get into in our next lesson, we would plug in molarities that are not yet at equilibrium. Okay, but then it again says during what time range is this expression valid? Well, Q is for when it is not at equilibrium.
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Unit 7.3 - Reaction Quotient and Equilibrium Constant.mp3
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The only difference is what variable I have on the left-hand side. Remember guys, if we were actually calculating this, which we'll get into in our next lesson, we would plug in molarities that are not yet at equilibrium. Okay, but then it again says during what time range is this expression valid? Well, Q is for when it is not at equilibrium. So that would be in the time range from 0 to 20 seconds. We are not yet at equilibrium in that space. So that's when Q would be a valid number to calculate.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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Ionization energy refers to the energy that's required to remove an electron from a neutral atom. So if we look down here, this A represents a neutral atom, meaning equal numbers of protons and electrons. And since the positively charged nucleus is going to attract those negatively charged electrons, it's going to take energy to pull an electron away from that attractive force of the nucleus. And so that's your ionization energy. If you take away an electron, you no longer have equal numbers of protons and electrons. You'd have one more proton than you do electrons. So you get a plus one charge here, so you form an ion.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And so that's your ionization energy. If you take away an electron, you no longer have equal numbers of protons and electrons. You'd have one more proton than you do electrons. So you get a plus one charge here, so you form an ion. And so ionization energy is always going to be positive. So it always takes energy to pull an electron away. So positive value for ionization energy.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So you get a plus one charge here, so you form an ion. And so ionization energy is always going to be positive. So it always takes energy to pull an electron away. So positive value for ionization energy. And our units are kilojoules per mole. And in this video, we're only going to be talking about the first ionization energy, so IE1, like that. Let's look at some actual ionization energies for elements in group one.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So positive value for ionization energy. And our units are kilojoules per mole. And in this video, we're only going to be talking about the first ionization energy, so IE1, like that. Let's look at some actual ionization energies for elements in group one. And so we can see here some elements in group one. And so for hydrogen, it would take 1,312 kilojoules per mole of energy to pull an electron away from hydrogen. For lithium, it would take about 520 kilojoules per mole to take an electron away.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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Let's look at some actual ionization energies for elements in group one. And so we can see here some elements in group one. And so for hydrogen, it would take 1,312 kilojoules per mole of energy to pull an electron away from hydrogen. For lithium, it would take about 520 kilojoules per mole to take an electron away. And we can see as we go down here, the number decreases. So sodium would be 496, potassium would be 419. So there's a clear trend.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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For lithium, it would take about 520 kilojoules per mole to take an electron away. And we can see as we go down here, the number decreases. So sodium would be 496, potassium would be 419. So there's a clear trend. As we go down a group in the periodic table, there is a definite decrease in the ionization energy. So it must be easier to pull an electron away. So let's see if we can figure out the reason why.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So there's a clear trend. As we go down a group in the periodic table, there is a definite decrease in the ionization energy. So it must be easier to pull an electron away. So let's see if we can figure out the reason why. And we're going to study in detail here these two elements, so hydrogen and lithium. So let's go ahead and look at these diagrams here. We're going to fill them in for hydrogen and lithium.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So let's see if we can figure out the reason why. And we're going to study in detail here these two elements, so hydrogen and lithium. So let's go ahead and look at these diagrams here. We're going to fill them in for hydrogen and lithium. And so for our first diagram, we will put hydrogen. So hydrogen has atomic number of 1. So there's one proton in the nucleus, so plus 1 charge in the nucleus.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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We're going to fill them in for hydrogen and lithium. And so for our first diagram, we will put hydrogen. So hydrogen has atomic number of 1. So there's one proton in the nucleus, so plus 1 charge in the nucleus. And in a neutral atom, there's one electron. So I'll go ahead and draw on hydrogen's one electron right here like that. The electron configuration would be 1s1.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So there's one proton in the nucleus, so plus 1 charge in the nucleus. And in a neutral atom, there's one electron. So I'll go ahead and draw on hydrogen's one electron right here like that. The electron configuration would be 1s1. So that one electron is in an s orbital in the first energy level. So this negatively charged electron feels an attraction for this positively charged nucleus. And so to pull it away, you must add energy.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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The electron configuration would be 1s1. So that one electron is in an s orbital in the first energy level. So this negatively charged electron feels an attraction for this positively charged nucleus. And so to pull it away, you must add energy. So if you add 1,312 kilojoules per mole of energy, you can pull that electron away. And if you do that, you'd be left with just a positive 1 charge in the nucleus and no electrons around it. And so you no longer have a neutral atom.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And so to pull it away, you must add energy. So if you add 1,312 kilojoules per mole of energy, you can pull that electron away. And if you do that, you'd be left with just a positive 1 charge in the nucleus and no electrons around it. And so you no longer have a neutral atom. You have an ion. You have H plus because you have a positive charge of 1 in the nucleus and 0 electrons, so H plus. So that's the concept of ionization energy here.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And so you no longer have a neutral atom. You have an ion. You have H plus because you have a positive charge of 1 in the nucleus and 0 electrons, so H plus. So that's the concept of ionization energy here. Let's look at lithium. So down here, we'll draw lithium. Lithium has atomic number of 3, so 3 protons in the nucleus.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So that's the concept of ionization energy here. Let's look at lithium. So down here, we'll draw lithium. Lithium has atomic number of 3, so 3 protons in the nucleus. And in a neutral atom, 3 electrons. So the electron configuration is 1s2, 2s1. So there are two electrons in the first energy level that are in s orbital.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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Lithium has atomic number of 3, so 3 protons in the nucleus. And in a neutral atom, 3 electrons. So the electron configuration is 1s2, 2s1. So there are two electrons in the first energy level that are in s orbital. So I'm going to go ahead and draw those in here. So these two electrons I just drew represent the two electrons in the first energy level. In the second energy level, there's one more electron.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So there are two electrons in the first energy level that are in s orbital. So I'm going to go ahead and draw those in here. So these two electrons I just drew represent the two electrons in the first energy level. In the second energy level, there's one more electron. So I'm going to put that electron down here like that. So for lithium, if we were to take an electron away, the one that's most likely to leave would be this outermost electron here, the one in the 2s orbital. So if you apply 520 kilojoules per mole of energy, you can pull away that electron.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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In the second energy level, there's one more electron. So I'm going to put that electron down here like that. So for lithium, if we were to take an electron away, the one that's most likely to leave would be this outermost electron here, the one in the 2s orbital. So if you apply 520 kilojoules per mole of energy, you can pull away that electron. And so if you did that, you'd be left with a plus 3 charge in the nucleus. And you would still have your electrons in the 1s orbital. So I'm going to go ahead and draw those in there.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So if you apply 520 kilojoules per mole of energy, you can pull away that electron. And so if you did that, you'd be left with a plus 3 charge in the nucleus. And you would still have your electrons in the 1s orbital. So I'm going to go ahead and draw those in there. But you've taken away that outer electron. And so therefore, you'd have a lithium cation here. You'd have Li plus 1 because you have three positive charges in the nucleus and only two electrons now.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So I'm going to go ahead and draw those in there. But you've taken away that outer electron. And so therefore, you'd have a lithium cation here. You'd have Li plus 1 because you have three positive charges in the nucleus and only two electrons now. So 3 minus 2 gives you plus 1. The electron configuration for the lithium cation would therefore be 1s2 because we pulled away that outer electron in the 2s orbital. So this is the picture for the ionization of hydrogen and lithium.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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You'd have Li plus 1 because you have three positive charges in the nucleus and only two electrons now. So 3 minus 2 gives you plus 1. The electron configuration for the lithium cation would therefore be 1s2 because we pulled away that outer electron in the 2s orbital. So this is the picture for the ionization of hydrogen and lithium. And we're going to examine some of the factors that affect the ionization energy. And so first, we'll talk about nuclear charge. So let me go ahead and write nuclear charge here.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So this is the picture for the ionization of hydrogen and lithium. And we're going to examine some of the factors that affect the ionization energy. And so first, we'll talk about nuclear charge. So let me go ahead and write nuclear charge here. So the idea of nuclear charge is the more positive charges you have in your nucleus, the more of an attractive force the electron would feel. And so therefore, the harder it would be to pull that electron away. So in general, you can think about increased nuclear charge.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So let me go ahead and write nuclear charge here. So the idea of nuclear charge is the more positive charges you have in your nucleus, the more of an attractive force the electron would feel. And so therefore, the harder it would be to pull that electron away. So in general, you can think about increased nuclear charge. That would want to increase the ionization energy because, again, there's a greater attractive force for the electron. So let's look at these two situations. And let's think about hydrogen first.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So in general, you can think about increased nuclear charge. That would want to increase the ionization energy because, again, there's a greater attractive force for the electron. So let's look at these two situations. And let's think about hydrogen first. So hydrogen has a plus 1 charge in the nucleus. And this one electron here would be pulled to the nucleus by that positive charge. If we look at lithium, plus 3 in the nucleus.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And let's think about hydrogen first. So hydrogen has a plus 1 charge in the nucleus. And this one electron here would be pulled to the nucleus by that positive charge. If we look at lithium, plus 3 in the nucleus. So that's a greater nuclear charge. So just thinking about nuclear charge alone, you would think, oh, well, this electron might be pulled in even more than with hydrogen because plus 3 is greater than plus 1. And so just thinking about nuclear charge for these two things, that would seem to indicate that lithium's outer electron would have a greater attractive force for the nucleus.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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If we look at lithium, plus 3 in the nucleus. So that's a greater nuclear charge. So just thinking about nuclear charge alone, you would think, oh, well, this electron might be pulled in even more than with hydrogen because plus 3 is greater than plus 1. And so just thinking about nuclear charge for these two things, that would seem to indicate that lithium's outer electron would have a greater attractive force for the nucleus. And so therefore, you might think it might take more energy to pull that electron away. So just thinking about nuclear charge, we might think an increase in the ionization energy. Next, let's talk about electron shielding.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And so just thinking about nuclear charge for these two things, that would seem to indicate that lithium's outer electron would have a greater attractive force for the nucleus. And so therefore, you might think it might take more energy to pull that electron away. So just thinking about nuclear charge, we might think an increase in the ionization energy. Next, let's talk about electron shielding. So electron shielding, or you could also call it electron screening. So the idea of electron shielding is the inner shell electrons are going to shield the outer electrons from the positive charge of the nucleus. And let's look at lithium for an example of that.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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Next, let's talk about electron shielding. So electron shielding, or you could also call it electron screening. So the idea of electron shielding is the inner shell electrons are going to shield the outer electrons from the positive charge of the nucleus. And let's look at lithium for an example of that. So we have these two inner shell electrons are going to repel the outer shell electrons. So this electron in blue is going to repel this electron in green. And this electron in blue is going to repel this electron in green.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And let's look at lithium for an example of that. So we have these two inner shell electrons are going to repel the outer shell electrons. So this electron in blue is going to repel this electron in green. And this electron in blue is going to repel this electron in green. And so they're going to shield that outer electron in green from that positive 3 charge because electrons repel other electrons. Like charges repel other like charges. And so that's the idea of electron shielding or electron screening.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And this electron in blue is going to repel this electron in green. And so they're going to shield that outer electron in green from that positive 3 charge because electrons repel other electrons. Like charges repel other like charges. And so that's the idea of electron shielding or electron screening. And so thinking about just this factor, for lithium, these two inner shell electrons are going to shield that outer shell electron. There's going to be a force in the opposite direction, if you will. And so that means that it would be easier to take that outer electron away due to the repulsive force of those electrons.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And so that's the idea of electron shielding or electron screening. And so thinking about just this factor, for lithium, these two inner shell electrons are going to shield that outer shell electron. There's going to be a force in the opposite direction, if you will. And so that means that it would be easier to take that outer electron away due to the repulsive force of those electrons. And so if you just think about electron shielding or electron screening by itself, it would be easier to take away lithium's outer electron due to the shielding effect. And so therefore, you would need less energy. So a decrease in the ionization energy for just thinking about this factor.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And so that means that it would be easier to take that outer electron away due to the repulsive force of those electrons. And so if you just think about electron shielding or electron screening by itself, it would be easier to take away lithium's outer electron due to the shielding effect. And so therefore, you would need less energy. So a decrease in the ionization energy for just thinking about this factor. Now, nuclear charge and electron shielding go hand in hand. And one way to relate those would be to think about what's called the effective nuclear charge. So I'm going to go ahead and write the effective nuclear charge, so ZEF, is equal to the nuclear charge, which is Z, minus the effect of the shielding electrons.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So a decrease in the ionization energy for just thinking about this factor. Now, nuclear charge and electron shielding go hand in hand. And one way to relate those would be to think about what's called the effective nuclear charge. So I'm going to go ahead and write the effective nuclear charge, so ZEF, is equal to the nuclear charge, which is Z, minus the effect of the shielding electrons. And so this is one way to think about it. This is a very simplistic way of doing the math here. So let's look at hydrogen first and calculate the effective nuclear charge that this electron experiences.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So I'm going to go ahead and write the effective nuclear charge, so ZEF, is equal to the nuclear charge, which is Z, minus the effect of the shielding electrons. And so this is one way to think about it. This is a very simplistic way of doing the math here. So let's look at hydrogen first and calculate the effective nuclear charge that this electron experiences. Well, there's a plus 1 charge in the nucleus. So that's the nuclear charge, Z. And there are zero shielding electrons.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So let's look at hydrogen first and calculate the effective nuclear charge that this electron experiences. Well, there's a plus 1 charge in the nucleus. So that's the nuclear charge, Z. And there are zero shielding electrons. So 1 minus 0 is, of course, plus 1. So this outer electron experiences an effective nuclear charge of plus 1. For lithium, there are three protons in the nucleus.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And there are zero shielding electrons. So 1 minus 0 is, of course, plus 1. So this outer electron experiences an effective nuclear charge of plus 1. For lithium, there are three protons in the nucleus. So Z would be plus 3. And there are two shielding electrons, these two inner shell electrons here. So it would be plus 3 minus 2.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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For lithium, there are three protons in the nucleus. So Z would be plus 3. And there are two shielding electrons, these two inner shell electrons here. So it would be plus 3 minus 2. So the effective nuclear charge would be a plus 1. So if you think about it, the effective nuclear charge that hydrogen's electron feels is about the same as lithium's outer electron, because they both have an effective nuclear charge of plus 1. So the fact that lithium has this electron shielding or electron screening, that kind of cancels out this effect of the nuclear charge.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So it would be plus 3 minus 2. So the effective nuclear charge would be a plus 1. So if you think about it, the effective nuclear charge that hydrogen's electron feels is about the same as lithium's outer electron, because they both have an effective nuclear charge of plus 1. So the fact that lithium has this electron shielding or electron screening, that kind of cancels out this effect of the nuclear charge. And so these two things kind of cancel out. Now, of course, this is a very simplistic way of calculating the effective nuclear charge. In reality, for lithium, if you do it the more complicated way, you actually get a value of approximately 1.3.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So the fact that lithium has this electron shielding or electron screening, that kind of cancels out this effect of the nuclear charge. And so these two things kind of cancel out. Now, of course, this is a very simplistic way of calculating the effective nuclear charge. In reality, for lithium, if you do it the more complicated way, you actually get a value of approximately 1.3. So we can say that lithium's effective nuclear charge is close to positive 1, even though it's a little bit more accurate to say it's around 1.3. And so for our purposes, the electron shielding for lithium cancels out that increased nuclear charge. And so we have to look at the last factor to understand this trend.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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In reality, for lithium, if you do it the more complicated way, you actually get a value of approximately 1.3. So we can say that lithium's effective nuclear charge is close to positive 1, even though it's a little bit more accurate to say it's around 1.3. And so for our purposes, the electron shielding for lithium cancels out that increased nuclear charge. And so we have to look at the last factor to understand this trend. And the last factor is the distance of that outer electron from the nucleus. So let's think about that. So for hydrogen, this electron is pretty close to the nucleus.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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And so we have to look at the last factor to understand this trend. And the last factor is the distance of that outer electron from the nucleus. So let's think about that. So for hydrogen, this electron is pretty close to the nucleus. And the closer it is, the more of an attractive force it has for the nucleus. So once again, in physics, Coulomb's law, it's distance dependent. The closer you are, the more of an attractive force you will feel.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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So for hydrogen, this electron is pretty close to the nucleus. And the closer it is, the more of an attractive force it has for the nucleus. So once again, in physics, Coulomb's law, it's distance dependent. The closer you are, the more of an attractive force you will feel. So that electron feels a very strong attractive force. So it's hard to pull that electron away. For lithium, this outer shell electron is on average a further distance away from the nucleus.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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The closer you are, the more of an attractive force you will feel. So that electron feels a very strong attractive force. So it's hard to pull that electron away. For lithium, this outer shell electron is on average a further distance away from the nucleus. And so therefore, it doesn't have as much of an attractive pull towards the nucleus. There's not as great of an attractive force. So it's easier to pull that outer electron away.
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Ionization energy group trend Atomic structure and properties AP Chemistry Khan Academy (2).mp3
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For lithium, this outer shell electron is on average a further distance away from the nucleus. And so therefore, it doesn't have as much of an attractive pull towards the nucleus. There's not as great of an attractive force. So it's easier to pull that outer electron away. If it's easier to pull that outer electron away, that of course would mean a decrease in the ionization energy. So because of distance, we can say that it's easier to pull that outer electron away from lithium because it's further away from the nucleus. And so thinking about all three factors at once, the nuclear charge and the electron shielding effect sort of cancel each other out.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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We're now at section 2.4 on the structure of metals and alloys. Our learning objective for this one is to represent a metallic solid and or alloy using a model to show essential characteristics of the structure and interactions present in the substance. So in section 2.1 I talked about how metals have their own type of bond which we call metallic bonds. It's sort of similar to ionic substances in the sense that they form like a crystal lattice. They form like a grid that has a certain geometric arrangement and with metals it can be sort of like a cubic square grid or a hexagonal grid. There's a couple different varieties that it comes in. But the big difference between the crystals that metals form and the crystals that ionic solids form is that in metals the electrons, the valence electrons are delocalized.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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It's sort of similar to ionic substances in the sense that they form like a crystal lattice. They form like a grid that has a certain geometric arrangement and with metals it can be sort of like a cubic square grid or a hexagonal grid. There's a couple different varieties that it comes in. But the big difference between the crystals that metals form and the crystals that ionic solids form is that in metals the electrons, the valence electrons are delocalized. Meaning the electrons aren't necessarily bound to the atom that they came from or bound to any atom at all. As you can see in the picture here we have a grid of positive metals that have been stripped of their valence electrons. And then those valence electrons kind of hover around in between making what people usually refer to as a sea of electrons.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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But the big difference between the crystals that metals form and the crystals that ionic solids form is that in metals the electrons, the valence electrons are delocalized. Meaning the electrons aren't necessarily bound to the atom that they came from or bound to any atom at all. As you can see in the picture here we have a grid of positive metals that have been stripped of their valence electrons. And then those valence electrons kind of hover around in between making what people usually refer to as a sea of electrons. And these electrons are able to like freely hop from cation to cation and move throughout the material. And that's what makes metals so conductive. So metals are usually a solid element meaning one type of atom.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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And then those valence electrons kind of hover around in between making what people usually refer to as a sea of electrons. And these electrons are able to like freely hop from cation to cation and move throughout the material. And that's what makes metals so conductive. So metals are usually a solid element meaning one type of atom. But different metals can be combined to make what are called alloys. So an alloy is when you blend two or more types of metal together to make one single structure. And these alloys can come in two different forms.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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So metals are usually a solid element meaning one type of atom. But different metals can be combined to make what are called alloys. So an alloy is when you blend two or more types of metal together to make one single structure. And these alloys can come in two different forms. One is called a substitutional alloy and in a substitutional alloy you can see here as an example. We have a bunch of red atoms and then we've replaced some of them with blue. So we've substituted some of the atoms for a different element.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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And these alloys can come in two different forms. One is called a substitutional alloy and in a substitutional alloy you can see here as an example. We have a bunch of red atoms and then we've replaced some of them with blue. So we've substituted some of the atoms for a different element. And you'll notice that even though the blue is somewhat smaller it's at least big enough that it can fit into a place where the red would without changing the structure of the whole. So substitutional alloys are made when the atomic radii of the two metals are similar. There's also something called an interstitial alloy where the atomic radii are not similar.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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So we've substituted some of the atoms for a different element. And you'll notice that even though the blue is somewhat smaller it's at least big enough that it can fit into a place where the red would without changing the structure of the whole. So substitutional alloys are made when the atomic radii of the two metals are similar. There's also something called an interstitial alloy where the atomic radii are not similar. So we can see here in our example of an interstitial alloy we have all of the red ones but we've sort of shoved some little green ones in between. And you can see that the atomic size of the green ones are way too small to fit in place of one of the red ones without changing the geometric arrangement. So let's look at some examples of substitutional and interstitial alloys.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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There's also something called an interstitial alloy where the atomic radii are not similar. So we can see here in our example of an interstitial alloy we have all of the red ones but we've sort of shoved some little green ones in between. And you can see that the atomic size of the green ones are way too small to fit in place of one of the red ones without changing the geometric arrangement. So let's look at some examples of substitutional and interstitial alloys. So a really popular alloy that's substitutional is brass which is mostly copper but you've replaced about 30% of the atoms with zinc in the structure. And if you look at the atomic radii of copper and zinc 140 picometers versus 139 they're almost identical in size. So the zinc is able to fit in there very nicely without changing the structure at all.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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So let's look at some examples of substitutional and interstitial alloys. So a really popular alloy that's substitutional is brass which is mostly copper but you've replaced about 30% of the atoms with zinc in the structure. And if you look at the atomic radii of copper and zinc 140 picometers versus 139 they're almost identical in size. So the zinc is able to fit in there very nicely without changing the structure at all. But what it does do is change the physical properties. Copper on its own is usually pretty soft and it oxidizes pretty quickly. And zinc is a pretty sturdy metal that's kind of hard to make things out of.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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So the zinc is able to fit in there very nicely without changing the structure at all. But what it does do is change the physical properties. Copper on its own is usually pretty soft and it oxidizes pretty quickly. And zinc is a pretty sturdy metal that's kind of hard to make things out of. So by blending these two, this soft metal with this sort of sturdy metal, you end up with something that's more in between. So it's stronger than copper and it's more ductile than copper meaning you can pull it and stretch it into wires and have it still hold its structure. And it's also done for aesthetic purposes because as you can see in this picture here the brass is a little bit more golden.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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And zinc is a pretty sturdy metal that's kind of hard to make things out of. So by blending these two, this soft metal with this sort of sturdy metal, you end up with something that's more in between. So it's stronger than copper and it's more ductile than copper meaning you can pull it and stretch it into wires and have it still hold its structure. And it's also done for aesthetic purposes because as you can see in this picture here the brass is a little bit more golden. And I think a lot of people like that for decorative purposes. But it also works good for decorative purposes as well because the brass does not tarnish as fast or oxidize as fast as the copper. You can also see this looks like a simulation or something but you can imagine that they hit this copper on another surface and it dented.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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And it's also done for aesthetic purposes because as you can see in this picture here the brass is a little bit more golden. And I think a lot of people like that for decorative purposes. But it also works good for decorative purposes as well because the brass does not tarnish as fast or oxidize as fast as the copper. You can also see this looks like a simulation or something but you can imagine that they hit this copper on another surface and it dented. But with the brass it did not dent as much because it is a much stronger alloy. So a good example of an interstitial alloy is steel which is actually not a combination of two metals. It's a combination of iron with a bit of carbon added to it where the carbon ends up making about less than 1% of the total atoms in the structure.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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You can also see this looks like a simulation or something but you can imagine that they hit this copper on another surface and it dented. But with the brass it did not dent as much because it is a much stronger alloy. So a good example of an interstitial alloy is steel which is actually not a combination of two metals. It's a combination of iron with a bit of carbon added to it where the carbon ends up making about less than 1% of the total atoms in the structure. And you can see with the sizes of iron and carbon, carbon is a bit more than half the size of iron. So it's not going to fit nicely in the structure the way that copper was with zinc. But by adding this carbon in there it actually makes the metal less malleable and more heat resistant.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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It's a combination of iron with a bit of carbon added to it where the carbon ends up making about less than 1% of the total atoms in the structure. And you can see with the sizes of iron and carbon, carbon is a bit more than half the size of iron. So it's not going to fit nicely in the structure the way that copper was with zinc. But by adding this carbon in there it actually makes the metal less malleable and more heat resistant. And it also gives it more strength and makes it less brittle. Brittle meaning easy to crack. I'm sure if you've ever seen these cast iron pots versus a steel pot.
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AP Chemistry, Section 2.4 Structure of Metals & Alloys.mp3
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But by adding this carbon in there it actually makes the metal less malleable and more heat resistant. And it also gives it more strength and makes it less brittle. Brittle meaning easy to crack. I'm sure if you've ever seen these cast iron pots versus a steel pot. The cast iron pots are good for absorbing a lot of heat but if you were to drop it off the top of a roof it might crack. Whereas a stainless steel one would probably just dent and roll away. Anyways, that is all I have to say about metals and alloys so I'll see you in section 2.5.
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Solution Concentration & Molarity - AP Chemistry Complete Course - Lesson 7.1.mp3
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Now when we talk about solutions, that's actually a fairly important concept in AP Chemistry, along with stoichiometry of course, as we saw in our previous videos. Now when we discuss the concentration of a solution, we can discuss it using various units. For example, perhaps you've heard of formality. That's actually a unit of concentration. It's not very common in AP Chemistry, but it is one that occasionally is used in chemistry. We have molality, then we could talk about percent by mass, we could discuss molarity, we have percent by volume, and then there's also normality. All of these are legitimate units of concentration.
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