The equilibrium response to stress
Le Chatelier's principle states that a system at equilibrium opposes any change imposed on it. If you increase pressure on a gas-phase reaction, the equilibrium shifts toward the side with fewer moles to reduce pressure. If you add heat to an endothermic reaction, equilibrium shifts right to consume the added heat. If you remove a product, equilibrium shifts right to replenish it. The system does not eliminate the stress but adjusts to minimize its effect.
The principle applies to pressure, temperature, concentration, and volume changes. It does not apply to the addition of an inert gas at constant volume, because inert gases do not participate in the reaction and do not change partial pressures of the reactants.
Predicting shift direction
For pressure, count moles of gas on each side. The equilibrium favors the side with fewer moles when pressure increases. For temperature, recall that all reactions are either exothermic (release heat) or endothermic (absorb heat). Increasing temperature favors the endothermic direction; decreasing temperature favors the exothermic direction. For concentration, adding a species shifts equilibrium away from that species; removing a species shifts equilibrium toward that species.
Industrial and biological applications
The Haber process exploits Le Chatelier by running at high pressure to favor ammonia formation (fewer moles on the product side). Refrigeration uses Le Chatelier to shift phase equilibrium and cool air. Living cells use Le Chatelier to drive reactions forward by continuously removing products; the glycolytic pathway depends on this principle to drive glucose through phosphorylation steps. Understanding the principle is essential for optimizing reactions and processes.