Equilibrium Constant

in this video, we're going to 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 X organic reactions and organic 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 uh 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 reactant or products to react ints. 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 going to proceed in a forward direction or 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 reactant 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.

so recall it. A reaction at equilibrium has all of the following characteristics, and the first is that there's no net change in the concentrations of reacting to products. And that's because the rate of the Ford 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 zero at equilibrium and also at equilibrium, a reaction systems 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 reacted, and this ratio is constant for a set condition. And again because that equilibrium, the reaction systems 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 reacting a into product be. 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 noticed 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 three units of A for every two units of B. And so the concentration of a does not equal the concentration of be at equilibrium, and this is totally fine. So the concentration of a does not have to equal the concentration of Beit equilibrium. But the Ford reaction rate has to equal the reverse reaction rate at equilibrium, and so this is totally acceptable. And so the next thing that I want to tell you guys is that when the concentration of a is greater than the concentration of a at equilibrium. So, like four A is greater than three a. Then the reaction's 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 being equilibrium, like four B is greater than to be here. Then the cut. The reaction's gonna proceed in a reverse, uh, 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.

so recall that the ratio of product concentrations over reacting 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 gonna be right around Kelvin, or about 25 degrees Celsius. Now, in our example below were showing the equilibrium constant equation. And we already know that it's the concentration of products specifically at equilibrium, over the concentration of reacting specifically at equilibrium and again recalled, 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 or predominate ing or the reactant air predominate ing and by predominate in what I mean is they have a higher concentration when they're predominate ing. And so if we take a look at this chart over here, we can see that when the equilibrium constant is exactly equal toe one. What that means is that the concentration of products at equilibrium is exactly equal to the concentrations of reacting to that equilibrium. And so in this scenario, actually, neither of the two are predominate ing because their concentrations are exactly equal at equilibrium. Now, in this scenario, if the equilibrium constant is really small, smaller than one, what that means is that the concentration of products that equilibrium is going to be smaller than the concentration of reacting to equilibrium and the reactant are gonna be predominate ing over the products because their concentration is higher at equilibrium. Now, in the last scenario, if the equilibrium constant is large and greater than one, what that means is that the concentration of products that equilibrium is going to be greater than the concentration of reactant that equilibrium. And so, in this scenario, the products are gonna be predominant because their concentrations are higher. And so it turns out that most reactions actually have multiple products and reactant 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, uh, coefficients and coefficients are simply numbers that air in front of the molecules and their 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 reactant and coefficients, we can see how this works. And so, in our reaction, we have to react. It's reacting a and reacted be in capital letters, and we have to products, products, see and products D in capital letters and then the lower case letters that air colored are the coefficients and so again recall that are equilibrium constant. Over here is the concentration of products at equilibrium over the concentration of reacting to that equilibrium. And so one of our products is already inputted into our equilibrium. Constant. That's product D, and so notice that the capital under D is in the brackets and then the coefficient is included as an exponents, so we could do the same for our other product. So we'll put C capital letter in the brackets and then the lower case letter, the coefficient will go as an exponent, and the same applies for the reactant which are down below. Reacting 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 exponents. And so we'll be able to get mawr practice with this in our practice video, so I'll see you guys there.