Dynamic Equilibrium

• Dynamic equilibrium refers to a state in a closed system where the rate of forward and backward reactions is equal. Hence, the concentrations of reactants and products remain constant over time.
• The principle of a dynamic equilibrium is applicable only to reversible reactions that occur in a closed system.
• In a dynamic equilibrium, the concentrations of the reactants and products do not change over time. This, however, does not mean their amounts are equal.
• The concept of dynamic equilibrium is explained by the collision theory - particles need to collide with each other for a reaction to happen. In a closed system, the number of collisions resulting in the forward reaction becomes equal to the ones resulting in the reverse reaction.
• Le Chatelier’s Principle is critical to understanding dynamic equilibrium. It states that if a system in equilibrium is subjected to a change, the system adjusts itself to offset that change, i.e., it tries to restore equilibrium.
• Changes that can disturb a dynamic equilibrium include changes in concentration, temperature or pressure. The system will respond by favouring the direction that counteracts the change.
• An increase in the concentration of reactants will cause the system to favour the forward reaction to consume the excess reactants and restore equilibrium. Similarly, increasing the concentration of products will favour the reverse reaction.
• If a reaction is exothermic in the forward direction (releases heat), then increasing the temperature will cause the system to favour the reverse, endothermic reaction (which absorbs heat). The opposite is true if the forward reaction is endothermic.
• For reactions involving gases, an increase in pressure favours the direction with fewer gas molecules, while a decrease in pressure favours the direction with more gas molecules.
• Catalysis does not affect the position of the equilibrium, it only speeds up the rate at which equilibrium is reached.
• The expression for the equilibrium constant (Kc or Kp) depends on the stoicheiometry of the reaction, and gives a quantitative measure of the position of equilibrium.
• Chemical equilibrium can be quantitative, which is represented by the equilibrium constant, or qualitative, where only the direction of change is considered.
• The constant for equilibrium, K, is given by the concentrations of the products divided by the concentrations of the reactants, each raised to the power of their stoichiometric coefficients. The concentration of solids and liquids is typically not considered in equilibrium constant expressions.