Concentrations usually are expressed in moles per liter for solutions, or in mole fractions or partial pressures for gases. The equilibrium constants expressed in partial pressures and moles per liter often are designated by and , respectively. In general, and will have different numerical values, but one can be calculated from the other using the ideal gas law.

Le Chatelier's principle is an important shortcut in predicting how a system at equilibrium will behave. It says that if an outside stress is applied to a chemical system at equilibrium, the system will shift in such a way to reduce that stress.

If the stress is a pressure increase, the reaction will shift in the direction that decreases the number of moles of reactants and products. If the stress is a dilution, then equilibrium shifts to increase the number of moles in the reaction mixture.

These adjustments do not change the numerical value of . If the temperature is raised, however, the reaction will shift in the direction in which heat is absorbed, and will change.

The equilibrium constant gives the ratio of products to reactants at equilibrium. If, for a given set of experimental conditions, the actual ratio



is less than , equilibrium has not yet been reached, and will not be reached until A and B have formed more C and D. Conversely, if the concentration ratio is numerically larger than , then the reaction is on the far side of equilibrium. Equilibrium then will be attained only after C and D have broken up spontaneously into more A and B.