Rob Lederer: So we take that previous demonstration on the board of the hydrogen and the iodine together. Well, here they are now colliding with each other to form HI but we see the HI can reform these two again. So, what we do is, we draw a top arrow pointing that way, a bottom arrow pointing that way and that signifies equilibrium. Something that's important to mention. These differing amounts of equilibrium of products and reactant concentrations that are held consistent. But I was never lost on any scientist at any time. they are able to take these numbers or concentrations at equilibrium and turn them into a constant, a constant, a constant number for instance, if we got 90 and 10% here, we can then divide 90 by 10 and get the number 9 and that can be a constant because, we are always going to get 90% of product formed. Well, okay. So, we take this information and we, while we, you kind of -- it out and get it a little bit more math than just taking 90 and 10 percent. We have the ability to be able to take the concentrations of products and reactants, set up a ratio and then get a constant number for that reaction at a given temperature. This is very important in the industrial world. Every industrial thing that we create, whether it be Aspirin or natural gas that comes out of the ground, all have equilibrium processes and that must be understood and values that need to be expressed. Okay, here it comes, watch this. we take the concentration of the HI and we write concentration as brackets around the chemical and then we divide it by the concentrations of these two chemicals which are H2 and the I2 and when we do that, but recognizing that you see that two in front, that actually makes a difference in terms of what we are going to get for our constant value and so we take the two, the coefficient in front of any chemical and we make it the power to which that concentration is taken. The concentration of the HI squared divided by the concentration of the H2 and I2 at a certain temperature will always give us a constant and this is how we write constant. Because, we like to write it, we add the constant K. K equals the concentration HI squared over H2 times I2, you just wrote for this reaction something called the equilibrium expression. While you didn't write it, I just did. You can be asked to write the equilibrium expression for a reaction, which is just writing what we just did in the previous one. We write down ratio and then make it equal to K. so, here is our reaction. We got N2 plus 3Cl2 gas makes or is in equilibrium with its product which is 2NCl3 gas and that's correctly balanced 1, 3, 2. How do we write the expression for that? You always write K equals that's the first thing and then it's always products over reactants, write their concentrations. It's the concentration of the NCl3. these brackets mean concentration. What we do with that number in front? The power to which the concentration is taken. That's divided by the concentration of the N2, don't have to put the one and the concentration of the Cl2, but its cubed. Now if we knew the concentrations of these chemicals at equilibrium and plug them in here cubing that one, squaring that one, dividing these two have to be multiplied together -- we will get a value of K. what's the significance of that? Like who cares. Why would do that? Because, what if somebody says to you, hey at equilibrium, I know the concentration of this and this. So, how much of this will actually form? This might be my product which could be very important and might be like Aspirin or Timolol you are making at the factory. I know my concentrations of ingredients. If I know the K value, then I can solve for this. I am going to figure out how much I am going to make. Wow that's what we --