14.6
Dynamic Equilibrium
A reversible chemical reaction represents a chemical process that proceeds in both forward (left to right) and reverse (right to left) directions. Whe…
A reversible chemical reaction is one in which the conversion of reactants into products and the conversion of products back into reactants occur simultaneously. A double arrow between the reactants and products signifies its reversible nature.
When the rate of formation of products, or the rate of the forward reaction, equals the rate of formation of reactants, or the rate of the reverse reaction, chemical equilibrium is achieved.
Chemical equilibrium is in fact a dynamic process. From the outside, the whole system appears static—however, at the molecular level, it is highly active.
Consider the thermal decomposition of phosphorus pentachloride.
When heated in a closed container, gaseous phosphorus pentachloride decomposes into a mixture of phosphorus trichloride and chlorine gas in a reversible reaction. As the reaction proceeds, the concentration of phosphorus pentachloride decreases while phosphorus trichloride and chlorine gas increase.
As a result, the rate of the forward reaction decreases and that of the reverse reaction increases.
Eventually, the rates of the forward and reverse reactions become equal, and the system achieves chemical equilibrium. In this state, the relative concentrations of phosphorus pentachloride, phosphorus trichloride, and chlorine no longer change.
Such an equilibrium state is also called dynamic equilibrium because both the forward and reverse reactions continue to occur but at the same rates. Since the reactants and products form at the same rates as they are consumed, their concentrations remain constant, though not necessarily equal.
A chemical equilibrium is homogeneous if all the reactants and products in a given reversible reaction are in the same phase — either gaseous or aqueous. In a heterogeneous equilibrium, reactants and products exist in different phases.
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Q1: What is a reversible chemical reaction?
A reversible chemical reaction is a process in which reactants convert to products and products simultaneously convert back to reactants. The double arrow notation signifies this reversible nature. In the thermal decomposition of phosphorus pentachloride, for example, gaseous PCl₅ decomposes into PCl₃ and Cl₂ while these products recombine to reform PCl₅, demonstrating how both forward and reverse reactions occur simultaneously.
Q2: How does chemical equilibrium become dynamic?
Chemical equilibrium is dynamic because both forward and reverse reactions continue occurring at equal rates. Although the system appears static from outside, molecular-level activity remains high. When the rate of product formation equals the rate of reactant formation, concentrations stabilize. Reactants and products form and are consumed simultaneously, maintaining constant but not necessarily equal concentrations.
Q3: Why do forward and reverse reaction rates eventually become equal?
As a reversible reaction proceeds, reactant concentration decreases, slowing the forward reaction rate. Simultaneously, product concentration increases, accelerating the reverse reaction rate. This continues until both rates match, achieving equilibrium. At this point, the rate of NO₂ formation equals its consumption rate, and N₂O₄ formation equals its consumption rate, stabilizing all concentrations.
Q4: What is the difference between homogeneous and heterogeneous equilibria?
A homogeneous equilibrium occurs when all reactants and products exist in the same phase—either gaseous or aqueous. A heterogeneous equilibrium involves reactants and products in two or more different phases. For example, the decomposition of N₂O₄ gas to NO₂ gas is homogeneous, while reactions involving solids and gases represent heterogeneous equilibria.
Q5: Why do concentrations remain constant at equilibrium?
At equilibrium, concentrations remain constant because reactants and products form at the same rates they are consumed. The forward reaction rate equals the reverse reaction rate, creating a balance. Although individual molecules continue reacting, the overall composition of the system does not change, making equilibrium a dynamic steady state rather than a static condition.
Q6: How does the concentration of reactants change during the approach to equilibrium?
Initially, reactant concentration is finite while product concentration is zero, so the forward reaction proceeds at a finite rate. As the reaction progresses, reactant concentration decreases while product concentration increases. This changing concentration affects reaction rates: the decreasing reactant concentration slows the forward reaction, while increasing product concentration speeds the reverse reaction until equilibrium is reached.
Q7: What happens to reaction rates when a system reaches equilibrium?
When a system reaches equilibrium, the forward and reverse reaction rates become equal. Although both reactions continue occurring, they proceed at identical rates, so no net change in concentrations occurs. This equal-rate condition defines the equilibrium state and explains why equilibrium concentrations from initial concentrations can be predicted using equilibrium expressions and constants.