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.

• 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.

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.

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.

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:

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.

Fig. 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:

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

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.