Dynamic Equilibrium

Imagine you're at the grocery store on a busy Saturday morning. You walk up to the front of the store, take a cart and enter the building. As you pass through the doors, someone else walks out of the store with their cart full of groceries. A frenzied-looking mom rushes in behind you just as an elderly man shuffles out; two teenagers walk in and head straight to the aisle of soda as a young couple exits, laden down with bulging bags of fresh produce and loaves of bread. Shoppers are constantly entering and leaving, but overall, the number of people in the store remains the same. This is an example of dynamic equilibrium.

Dynamic Equilibrium Dynamic Equilibrium

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Contents
Table of contents
    • This article is about dynamic equilibrium in chemistry.
    • We'll define dynamic equilibrium and looks at its properties before comparing static and dynamic equilibria.
    • After that, we'll explore some examples of dynamic equilibrium.

    Dynamic equilibrium definition

    Some reactions are reversible. The reactants react to form the products, as you'd expect, and this is called the forward reaction. However, the products also react to form the reactants, and this is known as the backward reaction. If you leave the species involved in a reversible reaction alone in a closed system for a certain period of time, eventually they'll reach a point of stability. This is known as a dynamic equilibrium.

    A dynamic equilibrium is the state of a reversible reaction in which the rate of the forward reaction equals the rate of the backward reaction and the concentrations of reactants and products remain the same.

    Let's take a closer look at that. Consider the following reaction:

    \[A + B \leftrightharpoons C\]

    In the forward reaction, the reactants (A and B) react to form the product (C). In the backward reaction, the product (C) reacts to form the reactants (A and B).

    If you mix A and B in a sealed container, they rapidly react together to form C. The rate of the forward reaction is initially very high, and lots of A and B turn into C. This causes the concentrations of A and B to decrease whilst the concentration of C increases. However, as the concentration of C increases, the backward reaction starts to occur: C turns back into A and B. This causes the concentration of C to decrease whilst the concentrations of A and B increase.

    You can see that there are two reactions taking place at the same time: A and B turning into C, and C turning back into A and B. The rates of these two reactions are initially very different, but over time, they become equal. Both reactions are still ongoing, but they merely happen at the same rate. This means that overall, the concentrations of A, B and C remain constant. We call this point a dynamic equilibrium.

    Dynamic Equilibrium graphs showing the rate of reaction and concentration at dynamic equilibrium StudySmarter

    Fig. 1: Graphs showing rate of reaction and concentration at dynamic equilibrium. The first graph shows that at dynamic equilibrium, the rate of the forward and backward reactions does not change and is the same for both. The second graph shows that at dynamic equilibrium, the concentration of the product and reactants, although different from each other, stay stable across time.

    We can apply this to our grocery store analogy. When the store opens, there are initially lots of people outside of the store and none inside. People rush inside as the doors open, representing a high rate of the forward reaction. The number of people inside the store increases, whilst the number of people outside the store decreases.

    After some time, people start leaving. They've got their groceries, and now they just want to go home. This represents the backward reaction. Eventually, the number of people leaving the store equals the number of people entering the store. At this point, the rates of the forward and backward reactions are equal and the concentrations of people inside and outside the store remain the same. We have reached a state of dynamic equilibrium.

    It is important to note that a dynamic equilibrium can be reached from either direction. It doesn't matter whether you start with just the reactants or just the products, or perhaps a mixture of both - give the system enough time, and you'll eventually end up with a dynamic equilibrium.

    Dynamic equilibrium properties

    To summarize, a dynamic equilibrium is characterized by two main properties:

    • The rate of the forward reaction and the rate of the backward reaction are equal.
    • The concentrations of reactants and products do not change.

    Dynamic vs static equilibrium

    We've learned that in a dynamic equilibrium, the rates of the forward and backward reactions are the same and the concentrations of products and reactants remain unchanged. However, both reactions are still ongoing - hence the use of the word dynamic. We can say that on a microscopic level, the system changes, but on a macroscopic scale, the system remains unchanged. This is similar to equal numbers of shoppers entering and leaving the grocery store at the same time.

    Static equilibria are a little different. In static equilibrium, the concentrations of products and reactants still don't change, but this is because there are no chemical reactions taking place - neither the forward nor the backward reaction take place. On both a microscopic and a macroscopic level, the system remains unchanged. This is analogous to the grocery store after opening hours are over. The number of people inside and outside of the store stays the same, but this is because no one is entering or leaving. We can say that neither the forward nor the backward reaction occur.

    This table summarises the differences between dynamic and static equilibria:

    Type of equilibriumDynamicStatic
    Concentrations of speciesStay the sameStay the same
    Rate of reactionRate of forward reaction equals rate of backward reactionBoth reactions are ongoingNeither reaction occurs
    SystemClosedOpen or closed

    Dynamic equilibrium example

    Let's now look at some examples of dynamic equilibria.

    If you place grey-black iodine crystals in a beaker and add a lid, you eventually form a dynamic equilibrium. Solid iodine crystals break down into purple iodine vapour whilst at the same time, purple gaseous iodine solidifies into grey-black iodine crystals.

    A beaker with iodine molecules in solid and gas state. Dynamic equilibrium, StudySmarterFig. 2: Iodine - an example of dynamic equilibrium.

    Another example of dynamic equilibrium is the Haber process, used in the industrial production of fertilizers. Here, nitrogen and hydrogen molecules react to form ammonia in a reversible reaction. Here's the equation:

    \[N_2 (g) + 3H_2 (g) \leftrightharpoons 2NH_3(g)\]

    Non-equilibrium dynamics

    Non-equilibrium dynamics is a branch of thermodynamics used to model systems that don't follow equilibrium conditions. It builds on equilibrium variables to predict the behaviour of systems that don't lie in dynamic equilibrium and has many real-world applications. These include modelling transport systems and the composition of ecosystems.

    That's the end of this article. By now you should be able to explain what we mean by dynamic equilibrium and give its characteristic properties. You should also be able to compare dynamic and static equilibria and give examples of dynamic equilibrium.

    Dynamic Equilibrium - Key takeaways

    • A dynamic equilibrium is the state of a reversible reaction in which the rate of the forward reaction equals the rate of the backward reaction and the concentrations of reactants and products stay the same.
    • Dynamic equilibria only occur in a closed system.
    • At dynamic equilibrium, both the forward and backward reactions are ongoing. In contrast, in static equilibrium, neither reaction occurs.
    • Examples of dynamic equilibria include many industrial reversible reactions such as the Haber process.
    Frequently Asked Questions about Dynamic Equilibrium

    What is dynamic equilibrium?

    A dynamic equilibrium is a state of a reversible reaction in which the rate of the forward reaction equals the rate of the backward reaction and the concentrations of reactants and products remain the same. 

    When does a given chemical system reach dynamic equilibrium?

    A sealed chemical system reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the backward reaction and the concentrations of products and reactants remain the same.

    Why is chemical equilibrium referred to as a dynamic equilibrium?

    Chemical equilibrium is referred to as dynamic equilibrium because both reactions are ongoing. In contrast, in static equilibrium, neither reaction occurs.

    What describes two phases in dynamic equilibrium? 

    An example of two phases in dynamic equilibrium are iodine crystals in a sealed beaker. Some of the solid iodine vaporizes into purple iodine gas. At the same time, iodine gas solidifies into grey-black iodine crystals.

    Which change is an example of maintaining dynamic equilibrium?

    A change that is a good example of maintaining dynamic equilibrium is the dissolution of sodium chloride in water. This reaction reaches dynamic equilibrium when the rate of dissolution of NaCl into Na+ + Clions, equals the rate of formation of NaCl.

    Test your knowledge with multiple choice flashcards

    True or false? Irreversible reactions can reach dynamic equilibrium.

    Dynamic equilibrium occurs  _____.

    True or false? A static equilibrium can occur in an open system.

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