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A dynamic equilibrium is a state in which the rates of the forward and reverse reactions in a chemical or physical system are equal, maintaining the concentrations of reactants and products at a constant value over time. In other words, despite the appearance of no change in the macroscopic properties, the system has a continuous exchange of matter at the molecular level.

There are a few important features of dynamic equilibrium to consider: - It is only achieved in a closed system. No reactants or products can enter or leave the system. - The concentrations of reactants and products remain constant, even if they are not equal. - The rate of the forward reaction is equal to the rate of the reverse reaction. - It is established when the system reaches its lowest energy state.

The state of equilibrium is considered dynamic because, at the molecular level, reactions are still occurring. Reactant molecules continue to collide and form product molecules, while product molecules likewise break apart to form reactants. However, because the rates of the forward and reverse reactions are equal, there is no net change in the concentrations of reactants and products. This constant back-and-forth at the molecular level is what makes the equilibrium dynamic. In conclusion, a state of chemical or physical equilibrium is described as dynamic because, although there appears to be no macroscopic change, the forward and reverse reactions continue to occur at equal rates, maintaining a constant exchange of matter at the molecular level.