Chemical Equilibrium: Le Chatelier's Principle & Kc
A-Level Chemistry Year 2 requires mastery of dynamic equilibrium and equilibrium constants. Le Chatelier's Principle states: if a change is imposed on a system at equilibrium, the system will shift in the direction that partially counteracts the change. Effects: (1) Concentration change—adding reactant shifts equilibrium right (toward products); removing product also shifts right. (2) Pressure change (for gases)—increasing pressure shifts equilibrium toward fewer moles of gas. (3) Temperature change—increasing temperature shifts equilibrium in the endothermic direction. Note: adding a catalyst does NOT shift equilibrium—it only increases the rate of reaching equilibrium. The equilibrium constant Kc = [products]^p / [reactants]^r (where p and r are stoichiometric coefficients). Kc is temperature-dependent only—it doesn't change with concentration or pressure. If Kc >> 1, equilibrium lies toward products; if Kc << 1, toward reactants. Kc calculations require expressing all concentrations at equilibrium, often using an ICE table (Initial, Change, Equilibrium) to find equilibrium concentrations from initial conditions and changes.