Chemical Equilibrium and Le Chatelier's Principle
Chemical equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction in a closed system, and the concentrations of reactants and products remain constant. The equilibrium expression for aA + bB β cC + dD is Kc = [C]αΆ[D]α΅ / [A]α΅[B]α΅ β pure solids and liquids are excluded from the expression. Le Chatelier's Principle states that if a stress is applied to a system at equilibrium, the system shifts to partially relieve that stress. For concentration changes: adding a reactant shifts equilibrium toward products (right); removing a product also shifts right. For pressure changes (gas-phase equilibria only): increasing pressure shifts toward the side with fewer moles of gas. For temperature changes: this is the most nuanced β treat heat as a reactant (endothermic) or product (exothermic). Increasing temperature for an exothermic reaction (heat is a product) shifts equilibrium left, decreasing Kc. Adding a catalyst has no effect on the equilibrium position or K values β it only increases the rate at which equilibrium is reached. HSC markers specifically look for the direction of shift and its justification using Le Chatelier's reasoning.