Chemical Equilibrium: Study with Video Lessons, Practice Problems & Examples

Chemical reactions typically do not go to completion, meaning some reactants remain unconverted into products. Instead, they reach a state of chemical equilibrium, characterized by reversible reactions where the forward and reverse processes occur simultaneously. At equilibrium, the concentrations of reactants and products remain constant, even though molecular activity continues. This balance is visually represented in concentration plots, where reactants decrease and products increase until they plateau. Understanding this concept is crucial for grasping reaction dynamics and the equilibrium constant (K) that quantifies this state.

Chemical Equilibrium Concept 1

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, one going in the forward, one going in the reverse. And we're going to say 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. And just realize, we're going in the forward direction here with this forward-facing arrow and we use k ₁ to signify this forward direction. Now, 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. 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 to make products. So that's the most common theme 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 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. 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 the reverse direction to make up for what we just lost. So in that way, their levels maintain the same number over time. 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 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.

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.

• A. Choice A
• B. Choice B
• C. Choice C
• D. Choice D
• E. Choice E

Here’s what students ask on this topic:

What is chemical equilibrium and how is it achieved in a reaction?

Chemical equilibrium is a state in a chemical reaction where the concentrations of reactants and products remain constant over time. This occurs because the forward and reverse reactions happen at the same rate. In a reversible reaction, reactants convert to products and products convert back to reactants simultaneously. When the rate of the forward reaction equals the rate of the reverse reaction, the system reaches equilibrium. At this point, the amounts of reactants and products do not change, even though molecular activity continues. This balance can be visualized in concentration plots where both reactants and products plateau.

How can you tell if a reaction has reached chemical equilibrium?

A reaction has reached chemical equilibrium when the concentrations of reactants and products remain constant over time. This can be observed in a concentration vs. time plot where the lines representing the amounts of reactants and products plateau. At equilibrium, the forward and reverse reactions occur at the same rate, so there is no net change in the concentrations of reactants and products. Although molecular activity continues, the overall amounts of reactants and products stay the same, indicating that equilibrium has been achieved.

What is the significance of the equilibrium constant (K) in chemical reactions?

The equilibrium constant (K) quantifies the ratio of the concentrations of products to reactants at equilibrium for a given reaction. It is a crucial parameter that indicates the extent to which a reaction proceeds. A large K value (K > 1) suggests that the reaction favors the formation of products, while a small K value (K < 1) indicates that reactants are favored. The equilibrium constant is specific to a particular reaction at a given temperature and provides insight into the reaction's dynamics and the relative stability of reactants and products.

Why do most chemical reactions not go to completion?

Most chemical reactions do not go to completion because they are reversible, meaning the products can revert back to reactants. In such reactions, the forward and reverse processes occur simultaneously. As the reaction progresses, the rate of the forward reaction decreases while the rate of the reverse reaction increases until they become equal. At this point, the system reaches chemical equilibrium, and the concentrations of reactants and products remain constant. This dynamic balance prevents the complete conversion of reactants into products, resulting in some amount of reactants always being present.

What happens to the concentrations of reactants and products at equilibrium?

At equilibrium, the concentrations of reactants and products remain constant over time. This occurs because the forward and reverse reactions happen at the same rate, leading to no net change in the amounts of reactants and products. Although molecular activity continues, the overall concentrations do not vary. This state of balance is visually represented in concentration plots where the lines for reactants and products plateau, indicating that their levels have stabilized and equilibrium has been achieved.