Dynamic Equilibrium

Equilibrium refers to the state of a system where the concentration of the reactant is equal to the concentration of products. The concentration of both reactant and product remain the same with change in time and there is no change in properties of the system. We can say that at equilibrium state rate of forward reaction (Rf) is equal to the rate of backward reaction (Rb). At this stage, the number of moles of a substance produced per second in the forward reaction is equal to the number of moles of substance that disappear per second in the backward reaction. 

Define Dynamic Equilibrium

Dynamic equilibrium exists only in reversible reactions. In the case of dynamic equilibrium, there is an equal rate of transition between both reactants and products i.e. there is no net change. Reactants and products formed at such a rate that their concentration remains the same. In this equilibrium ratio of products and reactants remains the same, but in them, there is a movement of reaction i.e from reactants to products. From this equilibria, we can represent equilibrium constant with the help of constant forward and backward reactions. Systems in which dynamic equilibrium is present are examples of systems in steady states.

Examples of Dynamic Equilibrium

If we take a new bottle of soda the concentration of carbon dioxide in the liquid phase has a fixed value. But if we poured out half of the liquid and again sealed the bottle, then in this case carbon dioxide will leave the liquid phase at their rate keep on decreasing and at the same time partial pressure of carbon dioxide in the gas phase keeps on increasing and they reach the equilibrium state. At that point, due to thermal motion, a molecule of CO2 may leave the liquid phase, but within a very short time, another molecule of CO2 will pass from the gas to the liquid, and vice versa. When equilibrium is reached, the rate of transfer of carbon dioxide from liquid to gas is the same as the rate of transfer of carbon dioxide from gas to liquid. Here the concentration of carbon dioxide in the liquid phase is given by Henry’s law. 

Henry’s Law

Henry’s law states that the solubility of a gas in a liquid medium is directly proportional to the partial pressure of that gas above the liquid.

This relationship can be written as: c=kp, where k is a temperature-dependent constant, p is the partial pressure and c is the concentration of the dissolved gas in the liquid. 

We can derive Henry’s law by setting the chemical potentials of carbon dioxide in the two phases to be equal to each other. As equality between chemical potential defines the state of chemical equilibrium. 

Existence of dynamic equilibrium is also in a single-phase system. This can occur in acid-base equilibrium, such as in the case of dissociation of acetic acid, in aqueous solution.

Reaction: CH3CO2H ----- CH3CO2− + H+

From the above-given reaction we can write dissociation constant of the reaction: 

KC = (CH3COOH-) (H+) / (CH3CO2H)

The existence of dynamic equilibrium is also in gaseous phase. Example of this existence is nitrogen dioxide dimerization.

2NO2  ⇄  N2O4    

Therefore Kp= k N2O4/ (NO2 )2

Difference Between Static and Dynamic Equilibrium

Static equilibrium is a condition where the reaction occurring in a system is completely stopped and the movement between reactants and products is also stopped so there are no such chemical reactions. As the forces acting between them in a chemical reaction cancel each other, so due to this process reaction becomes static in nature.  

Some of the major difference between static and dynamic equilibrium is mentioned in the given table:

So the resultant force acting between static equilibria and dynamic equilibrium is zero.