14.6
Equilibrio Dinámico
Una reacción química reversible representa un proceso químico que procede tanto en dirección de avance (de izquierda a derecha) como de retroceso (de…
Una reacción química reversible es aquella en la que la conversión de reactivos en productos y la conversión de productos nuevamente en reactivos ocurren simultáneamente. Una flecha doble entre los reactivos y los productos indica su naturaleza reversible. Cuando la velocidad de formación de los productos, o la velocidad de reacción directa, es igual a la velocidad de formación de los reactivos, o la velocidad de la reacción inversa, se logra el equilibrio químico.
El equilibrio químico es de hecho un proceso dinámico. Desde el exterior, todo el sistema parece estático;sin embargo, a nivel molecular, es muy activo. Considere la descomposición térmica del pentacloruro de fósforo.
Cuando se calienta en un recipiente cerrado, el pentacloruro de fósforo gaseoso se descompone en una mezcla de tricloruro de fósforo y cloro gaseoso en una reacción reversible. A medida que avanza la reacción, la concentración de pentacloruro de fósforo disminuye mientras que el tricloruro de fósforo y el gas cloro aumentan. Como resultado, la velocidad de la reacción directa disminuye y la de la reacción inversa aumenta.
Finalmente, las velocidades de las reacciones directa e inversa se igualan y el sistema alcanza el equilibrio químico. En este estado, las concentraciones relativas de pentacloruro de fósforo, tricloruro de fósforo, y cloro ya no cambian. Este estado de equilibrio también se llama equilibrio dinámico porque las reacciones directa e inversa continúan ocurriendo, pero a la misma velocidad.
Dado que los reactivos y productos se forman a la misma velocidad a la que se consumen, sus concentraciones permanecen constantes, aunque no necesariamente iguales. Un equilibrio químico es homogéneo si todos los reactivos y productos en una reacción reversible dada están en la misma fase, ya sea gaseosa o acuosa. En un equilibrio heterogéneo, los reactivos y productos existen en diferentes fases.
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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.