Guys, in this brand new video, we're going to take a look at the equilibrium state and figure out when exactly some reactions reach equilibrium. Now, first we're going to say that most chemical reactions never go to completion. When we say the word "completion," all that really means is that our reactants do not completely convert into products. A reaction that goes to completion, every amount of reactant that's there gets broken down in order to build up to make our products. But we're going to say that this doesn't happen with all chemical reactions. So there's always going to be some amount of reactant left. Now, that amount could be a very small amount or it could be a very large amount. It all depends on the chemical reaction. Since our reactant amount is always going to be some amount, we're going to say reaction concentrations do not go down to 0. Instead, these chemical reactions reach a state of chemical equilibrium in which the reaction moves in the forward and the reverse direction. Remember, we talked about this. We say that these reactions have double arrows. 1 going in the forward, 1 going in the reverse. And because they can go in the forward or the reverse direction, they're called reversible reactions.
Here we have a typical reaction. We have reactant A going in the forward direction to give us product B. We're going in the forward direction here with this forward facing arrow and we use k1 to signify this forward direction. In the same way, the reaction could go in the opposite direction where some of our product actually breaks down itself in order to rebuild some of the reactant that was lost. When we're going in the reverse direction, we use k-1. The minus signifies that we're going in the reverse direction.
If we look at this plot here, you can see that our blue line represents our reactant A and our red line represents our product B. What you should take from this chart is this: We're losing reactants to make products. So that's the most common theme that we have to take from this image. What you should realize here is eventually product amount will stop increasing and reactant amount will stop decreasing, and they'll reach a plateau for both of them. Their amounts are going to be held constant after a while. It's at this 5-minute mark where they become constant in their concentrations. So we're going to say equilibrium happens at that exact moment. That's how we're able to tell a reaction has reached chemical equilibrium. The reactant and the product concentrations or amounts have been held constant. Reactions are still going on a molecular level but for every amount of reactant we lose, it's okay because our products are going in the reverse direction to make up for what we just lost. In that way, their levels maintain the same number over time. Once our amounts plateau, that's when equilibrium is reached.
Now knowing that, I want you guys to take a look at this practice question on the bottom and try your best to answer it on your own. Once you figure out the answer, come back to our next video and click on the explanation button after answering and see if your answer matches up with my own. Good luck on this question, guys.
