Chemistry » Chemical Equilibrium » What Is Chemical Equilibrium?

# Dynamic Equilibrium

## Dynamic Equilibrium

We will use the same reversible reaction that we used in an earlier example:

$$\color{blue}{\text{H}_{2}{\text{(g)}} + {\text{I}}_{2}{\text{(g)}} \rightleftharpoons {\text{2HI(g)}}}$$

The $$\color{orange}{\text{forward}}$$ reaction is: $$\color{orange}{\text{H}_{2}{\text{(g)}} + {\text{I}}_{2}{\text{(g)}} \to {\text{2HI(g)}}}$$

The $$\color{purple}{\text{reverse}}$$ reaction is: $$\color{purple}{\text{2HI(g)} \to {\text{H}}_{2}{\text{(g)}} + {\text{I}}_{2}{\text{(g)}}}$$

When the rate of the forward reaction and the rate of the reverse reaction are equal, the system is said to be in $$\color{blue}{\text{equilibrium}}$$. The figure below shows this. Initially (time = $$\text{0}$$), the rate of the $$\color{orange}{\text{forward reaction}}$$ is high (fast). With time, the rate of the $$\color{orange}{\text{forward reaction decreases}}$$. As the reaction gets closer to equilibrium the rate of decrease levels out until the forward reaction has a constant rate.

Initially the rate of the $$\color{purple}{\text{reverse reaction}}$$ is low (slow). As the reaction proceeds with time, the rate of the $$\color{purple}{\text{reverse reaction increases}}$$. As the reaction progresses the rate of increase levels out until the reverse reaction has a constant rate.

At this point the forward and reverse reaction rates are equal and this is called $$\color{blue}{\textbf{equilibrium}}$$.

The change in rate of $$\color{orange}{\text{forward}}$$ and $$\color{purple}{\text{reverse}}$$ reactions in a closed system.

Although it is not always possible to observe any macroscopic changes, this does not mean that the reaction has stopped. The forward and reverse reactions continue to take place and so microscopic changes still occur in the system. This state is called dynamic equilibrium.

### Definition: Dynamic equilibrium

There is a dynamic equilibrium in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction. The amounts of reactants and products remain constant.

In the liquid-gas phase equilibrium demonstration, dynamic equilibrium was reached when there was no observable change in the level of the water in the second beaker even though evaporation and condensation continued to take place.