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Equilibrium and Equilibrium Constants

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INSTRUCTOR: All chemical reactions will eventually reach a state of equilibrium. Equilibrium is the point at which the concentration of products and reactants no longer change because the rate of the forward reaction equals the rate of the reverse reaction. It may look like nothing is changing, but in reality reactants are going to products and products are still going to reactants. Here I have two vessels with the same volume of solution in them. They will both remain at the same volume, but one I will not add or remove anything. The other I will add and remove, but the volume will remain unchanged. As you can see, the volume remains the same as the vessel that I haven't added or subtracted anything from. This vessel is considered static, unchanging. This is dynamic and is constantly changing. But as you see, the volume in both have remained the same. Which vessel, the vessel that is static or the vessel that is dynamic, would represent chemical equilibrium? Since chemical equilibrium is defined as the state of a chemical reaction in which the ratio of the concentration of products to reactants do not change, you would expect that both of these vessels would represent chemical equilibrium. But the reason the ratio of products to reactants do not change is because the rate of the forward reaction equals the rate of the reverse reaction. So essentially, there is always a reaction occurring both forward and reverse. Thus, the vessel representing dynamic equilibrium is the one that represents chemical equilibrium. Because in equilibrium the ratio of the products to reactants does not change, we can calculate what's called an equilibrium constant, which is the ratio of concentration of products and concentration of reactants. Here I have a bottle with a solution that is at equilibrium. For this reaction the equilibrium constant is greater than 1, which means that the concentration of products compared to reactants is greater than 1. With this reaction, the products are colorless whereas the reactants are blue. Part of this reaction includes oxygen as a reactant. If I introduce more reactant, the solution will turn blue. Now that I've added more reactant, what do you predict will occur? Will the solution remain blue or will it go to colorless? As you look at the solution in the bottle, you'll notice that it has now become colorless. You will see at the very top of the solution there is a slight blue color still there which is due to the continual reaction with oxygen above the solution. But overall, the reaction has become colorless. Now, we can predict that it would become colorless at equilibrium because we know that the equilibrium constant is greater than 1 so that there will be more products than reactants once it reaches equilibrium. The products in this scenario are colorless, whereas the reactants are blue. This highlights a use for our equilibrium constant in that we can use it to determine how many products we have at equilibrium compared to reactants, or vice versa how many reactants we have compared to products.