# What is the difference between chemical equilibrium and dynamic equilibrium?

Is chemical equilibrium and dynamic equilibrium the same thing? Both point to the reversible reactions and no net changes in the concentrations of product and reactant.

In chemistry, and in physics, a dynamic equilibrium exists once a reversible reaction occurs. Substances transition between the reactants and products at equal rates, meaning there is no net change. Reactants and products are formed at such a rate that the concentration of neither changes.

In a chemical reaction, chemical equilibrium is the state in which both reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of the system. Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction. The reaction rates of the forward and backward reactions are generally not zero, but equal. Thus, there are no net changes in the concentrations of the reactant(s) and product(s). Such a state is known as dynamic equilibrium.

Chemical equilibrium is a type of dynamic equilibrium, but not every dynamic equilibrium is a chemical equilibrium.

In a chemical equilibrium there is no change on the macroscopic scale. That means that if you look at the system it seems like nothing is happening, but at molecular scale there are reactions going on and the rate of forward reaction = rate of backward reaction.

There is another example of dynamic equilibrium (although not the best one, as it is not a closed system): Imagine a bath into which water is flowing in at constant rate. Water is also flowing out of the bath at the bottom with the same rate. This is an example of dynamic equilibrium, which has nothing to do with chemistry. If you look at the bath, the water level is constant, so on the macro level nothing is happening, but there is flow of water, so on micro level water from the tap is replacing water flowing out of the bath.

A chemical equilibrium concerns chemical reactions. There should be at least a forward- and backward reaction between two species but more complex systems with multiple individual reactions may occur. The important observation is that there is no macroscopic change to the chemical constituents of the system, i.e. the concentrations of all reaction partners remain the same despite all the individual reactions happening. Chemical equilibria can always be written with reaction equations such as below

$$\ce{H2SO4 + 2 H2O <=> HSO4- + H3O+ + H2O <=> SO4^2- + 2 H3O+}$$

Before I get to dynamic equilibria, it is important that I briefly note equilibria in general. In equilibria, there must be some kind of balance but there need not be any movement. For example, in a tug-of-war two groups of people are pulling the same rope in different directions. When there is no macroscopic movement, it means that both sides are pulling with the same force. This is an equilibrium. However, even if you go down to a microscopic level, you will still notice no movement (i.e. the rope would seem (mostly) stationary). Thus, this type of equilibrium is classified as a static equilibrium: there is no change to the overall state because there is no change to any individual states.

In a dynamic equilibrium, the macroscopic state still remains the same but if you care to look at the individual microscopic parts you will notice them switching between states. For example, consider two sealed rooms, one filled with nitrogen and one with oxygen separated by a door. Once that door is opened, gases can flow back and forth. After a certain time, the concentrations in both rooms of both gases will have equilibrised (say to a $$4:1$$ ratio if that is the ratio of room sizes). Individual gas molecules will still flow back and forth through the open door, but as their concentrations are identical on both sides the flow rate for individual gases is the same and the constant back and forth will not affect the overall ratio of the rooms. This is a dynamic equilibrium, because the individual molecules move back and forth (microscopic change) while the overall state remains the same. However, it cannot be written as a chemical equation and thus it is not a chemical equilibrium.

• Discussing both the macroscopic and the atomic level is critical to defining a dynamic equilibrium in chemistry. – Karsten Theis Nov 19 '19 at 13:58