Reversible Reactions and Dynamic Equilibrium - SS2 Chemistry Lesson Note

Reversible reactions are chemical reactions in which the conversion of reactants to products can occur in both forward and reverse directions. They are denoted by a double arrow (↔︎) instead of a single arrow (→). Reversible reactions can be represented by a chemical equation that includes reactants, products, and coefficients. At the molecular level, in a reversible reaction, the reactant molecules collide and can either form products or revert to reactants.

Dynamic equilibrium is a state in a reversible reaction where the forward and reverse reactions occur at the same rate, resulting in no net change in the concentrations of reactants and products over time. In dynamic equilibrium, the system appears to be static macroscopically, but at the molecular level, there is continuous movement and conversion between reactants and products.

The concept of equilibrium is governed by the principle of Le Chatelier's principle, which states that if a system at equilibrium is subjected to a change in conditions, the system will respond to counteract that change and establish a new equilibrium. Factors that can shift the equilibrium include changes in temperature, pressure (for gaseous reactions), concentration of reactants or products, and the addition or removal of a catalyst. The equilibrium constant (K) is a quantitative expression that relates the concentrations of reactants and products at equilibrium. It is determined by the ratio of the concentrations of the products to the concentrations of the reactants, each raised to the power of their stoichiometric coefficients.

The value of the equilibrium constant, K, indicates the extent of the reaction and whether reactants or products are favoured at equilibrium. If K is large, the products are favoured, whereas if K is small, the reactants are favoured. The reaction quotient (Q) is a similar expression to the equilibrium constant but can be calculated at any point during the reaction, not just at equilibrium. By comparing Q to K, it is possible to determine whether the reaction is at equilibrium or if it will shift to reach equilibrium.

Understanding reversible reactions and dynamic equilibrium is essential in fields such as chemical engineering, industrial processes, and environmental science. It helps in optimising reaction conditions, predicting reaction outcomes, and understanding how systems achieve a balance between reactants and products.