Equilibrium Constants

This video explains the equilibrium constants Kc, Kp, Ka, Kb, and Ksp.

TRANSCRIPT:
So there’s quite a few equilibrium constants, but don’t worry. They’re all really easy to remember and straightforward. So we have Kc for concentration, Kp for pressure, Ka for acid, Kb for base, and Ksp for solubility product. Not bad. So let’s start with Kc. We’re given this example reaction, and what we need to remember is that the equilibrium expression is written in terms of products over reactants. So we’re going to have these products over these reactants. Now I see for the products, we have CO2 and H2O. Since we’re doing it for Kc which is concentration, we’re going to write out the concentrations of the products like so. So we have CO2 and concentration of H2O. Now, what we need to do is also remember that the coefficients are what we’re going to raise the concentration to the power of. We have 3 moles of CO2 which we’re going to turn into concentration of CO2 cubed. Same thing here, we’re going to raise it to the 4th power. Now that we have our products, we can move on to the reactants. We have C3H8, and we’re going to write out the concentration sign for it, and it has no coefficients so we’re just going to leave it as it is. The next thing we have is O2, and we’re going to raise it to the 5th power. Notice that I included all of these into the equilibrium expression; that’s not going to happen all the time. Because these are all gases, you can find the concentration of a gas so it makes sense to write them in the Kc equilibrium expression. But if you have solids or pure liquids, you can’t really find the concentration of those, so you would just leave them out of the equilibrium expression. For Kp, we have all gases so it makes sense to find the pressure for them. It’s going to be basically the same thing as this, except we’re going to write out the partial pressure of the products over partial pressure of the reactants. So we have partial pressure of CO2 and partial pressure of H2O. Now we need to raise them to these powers, 3 and 4, so we’ll have PPCO2 cubed and PPH2O to the 4th power. Now we have reactants on the bottom, so we’re going to have pressure of C3H8 times pressure of O2, and we’re going to raise it to the 5th power because that’s the coefficient. So that’s going to be your equilibrium expression for Kp. Now moving onto Ka, we have a liquid here, so remember, you can’t really find the concentration of a pure liquid such as H2O so we’re going to leave that out of the expression. It’s just going to be these two over this. For Ka, Kb, and Ksp, we’re just going to write it out as concentrations. These are basically just really specific ways of denoting the concentration equilibrium expression. So we have CH3COO- and H3O+. There are no coefficients so we’re just going to leave it that way over acetic acid. So remember, Ka is when you have an acid and Kb is when you have a basic equation.
Let’s move on. We’re not going to put the H2O in the expression, we’re just gonna leave it there. So we have NH4+ times concentration of OH- over concentration of NH3. Now the last one – solubility product. That’s when you have a solid and some of it is basically dissolving in a solution. So Fe(OH)3 solid is going to dissolve into Fe3+ ions and OH- ions. We’re not going to have the solid in the expression, so we’re just going to set Ksp = Fe3+ and OH-. And the OH- is going to be to the 3rd power.