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Next SectionThe Equilibrium Constant

Most chemical reactions never go to completion where all the reactants are converted into products. Instead these reactions reach a chemical equilibrium.

Chemistry & Equilibrium

Concept #1: Chemical Equilibrium

Transcript

Hey guys! In this brand new video, we're going to take a look at the Equilibrium State and figure out when exactly do 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. Our 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 going to be some amount of reactant left. Now that amount could be a very small amount or could be a very large amount. It all depends on the chemical reaction. Now since our reactant amount is always going to be some amount, we're going to say reaction concentrations do not go down to 0. Now we're going to say 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 said that these reactions have double arrows, one going in the forward one going in the reverse. And we're going to say because they can go in the forward and 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. And just realize we're going in the forward direction here with this forward facing arrow and we use K1 to signify this forward direction. Now on the same way the reaction could go in the opposite direction where some of our product actually breaks down itself in order to rebuilt 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. Now if we're going to look at this plot here, you can see on this plot that our blue line represents our reactant A, our red line represents our product B. And what you should take from this chart is this, we're going to be losing reactants, so we lose reactants to make products. So that's the most common thing that we have to take from this image. And 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. So there amounts were 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. So 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. Now reactions are still going on on a molecular level but for every amount of reactant we lose, it's okay because our products are going in their reverse direction to make up for what we just lost. So in that way their levels maintain a same number overtime. So we're going to say 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 figured out the answer, come back on 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!

A reaction reaches equilibrium once the rate of the forward reaction equals the rate of the reverse reaction. Once at equilibrium there is no net change in the concentration of reactants or products.

Practice: Which one of the following statements does not describe the equilibrium state?

a. While at equilibrium, a dynamic process is still occurring.  

b. The concentration of the reactants is equal to the concentration of the products.

c. The concentration of the reactants and products reach a constant level.

d. At equilibrium, the net concentration of all species is not changing.

e. All are true.

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Next SectionThe Equilibrium Constant