Equilibrium Constant - Video Tutorials & Practice Problems
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concept
Biochemical Reaction Features
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In this video, we're gonna begin our discussion of the equilibrium constant. So the equilibrium constant is a specific number and a feature that characterizes biochemical reactions, and there are actually four features that characterize all biochemical reactions. And so let's take a look at our example below to clarify that. And so spontaneity is the first feature of a biochemical reaction, and we covered spontaneity in our previous videos when we talked about the second law of thermodynamics. And so you can see that we've got exergonic reactions and endergonic reactions, and all reactions will fall under one of these two categories. Now the second feature of a biochemical reaction is the equilibrium constant. And again, recall that the equilibrium constant is a specific number, so the equilibrium constant is a specific number that characterizes a biochemical reaction, and so notice that it has to do with a balance. And so the balance has to do with the ratio of reactants or products to reactants, and we'll talk more about that as, we move forward in our course. Now the next feature is the reaction direction, and that's determining whether the reaction is gonna proceed in a forward direction or a backwards direction. And we'll talk about this when we get to our next lesson. Now, velocity has to do with how fast a reaction is going to occur, so the rate at which the reactants, convert into products. And so we'll talk about velocity when we talk about enzymes later on in our course. So in our next video, what we're gonna do is refresh on what equilibrium actually means. So I'll see you guys in that video.
2
concept
Equilibrium
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2m
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So recall that a reaction at equilibrium has all of the following characteristics, and the first is that there's no net change in the concentrations of reactants or products, and that's because the rate of the forward reaction is equal to the rate of the reverse reaction. And also a reaction at equilibrium, there's no change in free energy, and so that means that delta G or the change in Gibbs free energy is equal to 0 at equilibrium. And also at equilibrium, a reaction system's energy is at its minimum or at its lowest. And because the energy is at its lowest, that means that the system is most stable at equilibrium. And then finally, at equilibrium, there's a very specific ratio of the concentrations of products over the concentration of reactants, and this ratio is constant for a set condition. And again, because at equilibrium, the reaction system's energy is at its lowest and most stable, all reactions proceed towards restoring equilibrium. And so, let's take a look at our example below where we have this graph that your professor is likely not gonna ask you guys to analyze, but this graph is used all the time to explain equilibrium. So, let's take a look at it. And this graph is analyzing a simple reaction where we're converting reactant a into product b. And so on the y axis, we have the free energy of the system, and on the x axis, we have the component concentrations. So on the far left of the x axis, we have pure a concentrations, and on the far right, we have pure b concentrations. And so notice that equilibrium is indicated by this blue box here and notice that the energy is at its very lowest at equilibrium. And also notice that at equilibrium shown by these equilibrium arrows here, that there are 3 units of a for every 2 units of b and so the concentration of a does not equal the concentration of b at equilibrium. And this is totally fine. So the concentration of a does not have to equal the concentration of b at equilibrium, but the forward reaction rate has to equal the reverse reaction rate at equilibrium. And so this is totally acceptable. And so, the next thing that I wanna tell you guys is that when the concentration of A is greater than the concentration of A at equilibrium, so like 4 A is greater than 3 a, then the reaction is gonna proceed in a forward direction in order to restore the equilibrium concentrations. And, also, when the concentration of b is greater than the concentration of b at equilibrium, like 4 b is greater than 2 b here, then the cut the reaction is gonna proceed in a reverse, direction in order to restore equilibrium. And so we'll talk more about equilibrium in our next video when we talk about the equilibrium constant. So I'll see you guys in that video.
3
concept
Equilibrium Constant
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4m
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So recall that the ratio of product concentrations over reactant concentrations, specifically at equilibrium, is called the equilibrium constant. And the equilibrium constant can actually be abbreviated by this symbol shown here. Now, the equilibrium constant actually changes with different conditions and it changes with temperature. But the good thing is is that in biological systems, if the temperature is not already given to us, then we always assume the temperature is going to be right around 298 Kelvin, or about 25 degrees Celsius. Now, in our example below, we're showing the equilibrium constant equation. And we already know that it's the concentration of products, specifically at equilibrium, over the concentration of reactants, specifically at equilibrium. And again, recall that the brackets here means the concentration of. And so it turns out that the equilibrium constant is just a number, and by looking at the value of this number, we can actually determine whether the products are predominating, or the reactants are predominating. And by predominating, what I mean is they have a higher concentration when they're predominating. And so if we take a look at this chart over here, we can see that when the equilibrium constant is exactly equal to 1, what that means is that the concentration 2 are pred 2 are predominating because their concentrations are exactly equal at equilibrium. Now, in this scenario, if the equilibrium constant is really small, smaller than 1, what that means is that the concentration of products at equilibrium is going to be smaller than the concentration of reactants at equilibrium and the reactants are gonna be predominating over the products because their concentration is higher at equilibrium. Now, in the last scenario, if the equilibrium constant is large and greater than 1, what that means is that the concentration of products equilibrium is going to be greater than the concentration of reactants at equilibrium. And so in this scenario, the products are gonna be predominating because their concentrations are higher. And so it turns out that most reactions actually have multiple products and reactants. And so in order to adjust our equilibrium constant, what we need to do is we need to multiply their concentrations together to get the equilibrium constant. And so it also turns out that a lot of molecules have multiple coefficient or they have, coefficients. And coefficients are simply numbers that are in front of the molecules. And they're included into the equilibrium constant as well, but they're included as exponents. And so if we take a look at our example below, which has multiple products and reactants and coefficients, we can see how this works. And so in our reaction, we have 2 reactants, reactant A and reactant B, in capital letters. And we have 2 products, product C and product D, in capital letters. And then the lower case letters that are colored are the coefficients. And so, again, recall that our equilibrium constant over here is the concentration of products at equilibrium over the concentration of reactants at equilibrium. And so one of our products is already inputted into our equilibrium constant. That's product D. And so notice that the capital letter D is in the brackets, coefficient is included as an exponent. So we can do the same for our other product. So we'll put c, capital letter, in the brackets and then the lowercase letter, the coefficient, will go as an exponent. And the same applies for the reactants, which are down below. Reactant a is already input, so we can put in reactant B, which is going to have a capital B in the brackets, and then the coefficient will be included as an exponent. And so, we'll be able to get more practice with this in our practice video, so I'll see you guys there.
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Problem
Problem
What is the equilibrium constant expression for the following reaction?