Question

If there is no change in concentration, why is the equilibrium state considered dynamic?

Answer 1

A Dynamic Equilibrium occurs when rate of forward reaction is equal to rate of backward reaction. In simple words, a reaction is at dynamic equilibrium when reactants are converted to products and products are converted to reactants at equal and constant rates.

Complete step by step answer:
Dynamic Equilibrium exists only when there is reversible reaction occurs. When rate of forward reaction is equal to rate of backward reaction at equal rate that means there is no net change and concentration neither changes then the reaction will be at dynamic equilibrium.
The equilibrium does not indicate whether a reaction is in stage or dynamic equilibrium because it is simply the concentrations of products divided by concentrations of reactants.
For Example,
In a Single-phase System, let take an example of acid-base equilibrium :
1. Dissociation of acetic acid (in aqueous solution)
$C{H_3}COOH \rightleftarrows C{H_3}CO{O^ - } + {H^ + }$
At equilibrium, the acid dissociation constant $({K_C})$:
${K_C} = \dfrac{{\left[ {C{H_3}CO{O^ - }} \right]\left[ {{H^ + }} \right]}}{{\left[ {C{H_3}COOH} \right]}}$
In this case, forward reaction involves the release of proton from molecules of acetic acid and backward reaction involves the formation of acetic acid molecules by accepting a proton.
In a reaction, Equilibrium is attained when the sum of chemical potentials of the species on the left-hand side of the equilibrium expression is equal to the sum of chemical potentials of the species on the right-hand side.
At the same time the rates of forward and backward reactions are equal to each other. Equilibria involving the formation of chemical complexes are also dynamic equilibria and concentrations are governed by the stability constants of complexes.

Note:
In a simple reaction, such as isomerization,
$A \rightleftarrows B$
Here, A is converted to B as forward reaction and B is converted to A as backward reaction. If both reactions are elementary reactions ,
Then rate of reaction is given by,
$\dfrac{{d\left[ A \right]}}{{dt}} = - {k_f}{\left[ A \right]_t} + {k_b}{\left[ B \right]_t}$