Many chemical reactions are reversible — products can react to reform reactants. When the rate of the forward reaction equals the rate of the reverse reaction, the system reaches equilibrium. At this point, the concentrations of all species remain constant (but not necessarily equal), and the system appears to be static even though reactions continue to occur at the molecular level. This dynamic equilibrium is a central concept in chemistry.
The Equilibrium Constant (K)
For the reaction aA + bB <=> cC + dD, the equilibrium constant Kc = [C]^c [D]^d / [A]^a [B]^b, where the brackets represent equilibrium concentrations in mol/L. A large K (much greater than 1) means the reaction favors products. A small K (much less than 1) means it favors reactants. K is temperature-dependent but independent of the initial concentrations. The Equilibrium Constant Calculator computes Kc from equilibrium concentrations.
Le Chatelier's Principle
If you disturb a system at equilibrium, it will shift to counteract the change. Adding more reactant shifts the equilibrium toward products. Adding more product shifts it toward reactants. Increasing temperature favors the endothermic direction. Increasing pressure (by decreasing volume) favors the side with fewer moles of gas. This principle is incredibly useful for predicting how changes in conditions affect equilibrium position and yields.
ICE Tables
ICE (Initial, Change, Equilibrium) tables are a systematic way to solve equilibrium problems. You write down the initial concentrations, express the changes in terms of x (using the stoichiometric coefficients), and express the equilibrium concentrations in terms of x. Then plug these into the equilibrium expression and solve for x. This approach works for many common equilibrium problems and is worth practicing until it becomes second nature.
Equilibrium calculations connect to many other areas of chemistry. Acid-base equilibrium determines pH, solubility equilibrium determines how much of a salt will dissolve, and gas-phase equilibrium governs many industrial chemical processes. Understanding equilibrium gives you a framework for predicting how chemical systems respond to changes — a skill that applies far beyond textbook problems.