What is static and dynamic equilibrium?

Everything in this world is in equilibrium in one or the other way. Nature believes in equilibrium and no different in chemistry as well. The equilibriums in chemistry are dynamic in nature. To understand this let's take two examples. Think about two people in a tug of war. Both are pulling in opposite directions. Assume that these two are pulling with same force from both sides. Will the rope move? No. The rope will be stationary and there will be equal and opposite forces acting on both sides of the rope. This balanced condition of force is called equilibrium. Since the rope is at rest even after the application of force on it, we can call this equilibrium as static equilibrium.

In chemistry there is an example with graphite and diamond. Graphite is more stable than diamond at normal conditions. As a result diamond always have a tendency to go back to graphite and vice versa. But at room temperature this will never happen due to the high activation energy. So we can say that there exist an equilibrium between graphite and diamond. Since there is no change on both sides we can call this equilibrium as static equilibrium.

Let's take another example to understand dynamic equilibrium. A man on a small boat is in the river doing fishing. All of a sudden the boat got a small crack and the water started flooding in. But this fellow is intelligent and started pouring out the water with a mug. It is found that the boat is not sinking anymore. What actually happened here? Here, the amount and fastness of the water coming in has matched with the amount and fastness of the man's pouring out. As a result the amount of water inside the boat remained constant and the boat did not sink. See, there is a change happening on both sides. Water is coming in from one side and pouring off from other side but the amount of water inside the boat remained constant. We can say that here the forces and speed of changes are in balanced condition and the process is in equilibrium. This sort of equilibrium in which a change is happening on both sides but the amount of substance is constant are called dynamic equilibrium.

In chemistry we can take an example of H2 and I2 reacting to form HI reversibly. Initially in a closed container if we take H2 and I2, these two will start reacting to form HI. But later on the HI will do the backward reaction to form H2 and I2. Now both forward and backward reaction is occurring at the same time. After some time the concentration of all these reactants and products will become constant and at that time the rate of forward and backward reactions will be equal. This condition is called chemical equilibrium. Here, the reaction is happening on both sides but the amount of reactant and product is not changing. So we can conclude that chemical equilibriums are dynamic in nature.