How do dynamic and static equilibrium differ?

Dynamic balance

Equilibria are basically divided into two classes: static equilibria and dynamic equilibria.

Static equilibria are so named because, in principle, nothing changes once the equilibrium has been reached. If a ball rolls down the mountain and remains in a hollow, it causes it to lie there until someone nudges it or takes it out of the hollow.

Dynamic equilibria are always in motion - hence the term "dynamic". This is not always apparent to the outside observer, but if he looks closely, he sees that the state of equilibrium is dynamic.

Examples of dynamic equilibria

If you let water flow from the tap into the sink and as much water flows out per unit of time as flows in, then the water level in the basin no longer changes. A balance has been established between inflow and outflow: inflow = outflow.

If there are initially many dissolved particles on the left side of a permeable membrane and a few on the right side, then there is a concentration equalization through diffusion. When the concentration equilibrium is achieved, a dynamic equilibrium has been established: per unit of time, exactly as many particles diffuse from left to right as from right to left: Diffusion to the left = diffusion to the right. For the outside observer, however, the concentrations on the left and right remain the same; he could take this dynamic equilibrium for a static one.

When the number of prey in an ecosystem increases, so do the predators. This means that more prey is eaten and the number of prey animals decreases. However, this has the consequence that the predators have less to eat, their number also decreases. As a result, fewer prey animals are killed and the number of prey animals increases again. And so it goes on and on; in the ideal case, the number of prey animals no longer changes in the time average, and the number of predators also remains constant in the time average. Here, too, a dynamic equilibrium has been established, albeit somewhat more complex.

Dynamic equilibria in chemistry

Dynamic equilibria play a role not only in physics (wash basin example) or in biology (diffusion and predator-prey example), but also in chemical reactions.

When you dissolve carbon dioxide in water, only a very small fraction of the dissolved carbon dioxide reacts with the water appropriately

to hydrated hydrogen carbonate ions and protons. The equilibrium of this reaction is said to be far to the left. However, there is a dynamic equilibrium, because per unit of time just as many reactant particles react with one another to form product particles, as do product particles react back to form reactant particles. Or to put it more simply: vTo = vBack, where vTo is the speed of the forward reaction and vBack the speed of the reverse reaction.

Another example is the formation of esters from carboxylic acids and alcohols. In chemistry lessons, this type of reaction is often treated using ethyl acetate as an example. Here, too, a dynamic equilibrium develops:

At room temperature exactly 67% of the acetic acid always reacts with the alcohol to form the ester, 33% of the acetic acid is not consumed in the reaction.

If you start the same reaction "on the other side", ie if you take the ester and let it react with water at room temperature, the same equilibrium is established; after some time, the mixture of substances again contains 67% ester and 33% acid and alcohol.