7.7: Reversible Reactions & Equilibrium
- Page ID
- 51449
Skills to Develop
- Describe the three possibilities that exist when reactants come together.
- Describe what is occurring in a system at equilibrium.
Introduction
Think for a minute about sitting down to a table to eat dinner. There are three possibilities that could happen when you eat dinner. You could (1) finish your entire dinner, (2) you could not want any of it and leave it all on your plate, or (3) you could eat some of it and leave some of it. Reactions have the same possibilities. Reactions also do not always proceed all the way from start to finish. You may have reactions that (1) go to completion so that at the end the reaction vessel contains all products and only products. Some reactions (2) may not start at all so at the end the reaction vessel contains all reactants and only reactants. And some reactions (3) may start but not go to completion, that is, the reaction might start but not go completely to products. In this last case, the reaction vessel would contain some reactants and some products. In this section, we are going to take a closer look at the third type of reaction.
Reversible Reactions and Equilibrium
Consider the hypothetical reaction: \(\text{A} + \text{B} \rightarrow \text{C} + \text{D}\). If we looked at this reaction using what we have learned, this reaction will keep going, forming \(\text{C}\) and \(\text{D}\) until \(\text{A}\) and \(\text{B}\) run out. This is what we call an "irreversible reaction" or a "reaction that goes to completion".
Some reactions, however, are reversible, meaning the reaction can go backwards in which products react to form reactants, so that: \(\text{A} + \text{B} \leftarrow \text{C} + \text{D}\). The direction of the arrow shows that \(\text{C}\) and \(\text{D}\) are reacting to form \(\text{A}\) and \(\text{B}\). What if the two reactions, the forward reaction and the reverse reaction, were occurring at the same time? What would this look like? If you could peer into the reaction, you would be able to find \(\text{A}\), \(\text{B}\), \(\text{C}\), and \(\text{D}\) particles. \(\text{A}\) and \(\text{B}\) would react to form \(\text{C}\) and \(\text{D}\) at the same time that \(\text{C}\) and \(\text{D}\) are reacting to form \(\text{A}\) and \(\text{B}\). If the forward and reverse reactions are happening at the same rate, the reaction is said to be at equilibrium or dynamic equilibrium. At this point, the concentrations of \(\text{A}\), \(\text{B}\), \(\text{C}\), and \(\text{D}\) are not changing (or are constant) and we would see no difference in our reaction container, but reactions are still occurring in both directions. It is important to point out that having constant amounts of reactants and products does NOT mean that the concentration of the reactants is equal to the concentration of the products. It means they are not changing. These reactions appear to have stopped before one of the reactants has run out.
Chemists use a double-headed arrow, \(\rightleftharpoons\), to show that a reaction is at equilibrium. We would write the example reaction as: \(\text{A} + \text{B} \rightleftharpoons \text{C} + \text{D}\). The arrow indicates that both directions of the reaction are happening.
Another way to think about reversible and irreversible reactions is to compare them to two types of games of tag. Reversible reactions are in many ways like a traditional game of tag: The "it" person can become "not it" and somebody who is "not it" is tagged and becomes "it". In this way it is a reversible change. It is also like a reaction at equilibrium, because overall no change is occurring. There is always a constant number of "it" people and "not it" people in the game. Also, having constant numbers of "it" and "not it" people in our game does not mean that the number of "it" people (reactants) is equal to the number of "not it" people (products). Furthermore, this is similar to equilibrium in that this game never truly ends (unless everybody gets tired of playing). The game could go on forever. We could write this as the following reversible reaction:
\[\text{"It"} \rightleftharpoons \text{"Not it"}\]
Irreversible reactions(those that only go in one direction from reactants to products and cannot reach a state of equilibrium) is more like a game of sharks and minnows. In sharks and minnows almost everybody starts out as a minnow. Once tagged, they become a shark. However, the difference here is that once you are a shark you are always a shark; there is no way to go back to becoming a minnow. The game continues until everybody has been tagged and becomes a shark. This is similar to irreversible reactions in that the reactants turn into products, but can't change back. Furthermore, the reaction will proceed until the reactants have been used up and there isn't any more left. We could write the reaction as:
\[\text{Minnow} \rightarrow \text{Shark}\]
Lesson Summary
- There are a few possible ways a reaction can go: It can go to completion (\(\text{reactants} \rightarrow \text{products}\)); it can occur but not go to completion. Instead is would reach chemical equilibrium (\(\text{reactants} \rightleftharpoons \text{products}\)).
- Chemical equilibrium occurs when the number of particles becoming products is equal to the number of particles becoming reactants.
- A dynamic equilibrium is a state where the rate of the forward reaction is equal to the rate of the reverse reaction.
Vocabulary
- Equilibrium: A state that occurs when the rate of the forward reaction is equal to the rate of the reverse reaction.