Chemistry » Fundamental Equilibrium Concepts » Shifting Equilibria: Le Châtelier’s Principle

Summarizing Le Châtelier’s Principle

Key Concepts and Summary

Systems at equilibrium can be disturbed by changes to temperature, concentration, and, in some cases, volume and pressure; volume and pressure changes will disturb equilibrium if the number of moles of gas is different on the reactant and product sides of the reaction. The system’s response to these disturbances is described by Le Châtelier’s principle: The system will respond in a way that counteracts the disturbance. Not all changes to the system result in a disturbance of the equilibrium. Adding a catalyst affects the rates of the reactions but does not alter the equilibrium, and changing pressure or volume will not significantly disturb systems with no gases or with equal numbers of moles of gas on the reactant and product side.

Effects of Disturbances of Equilibrium and K
DisturbanceObserved Change as Equilibrium is RestoredDirection of ShiftEffect on K
reactant addedadded reactant is partially consumedtoward productsnone
product addedadded product is partially consumedtoward reactantsnone
decrease in volume/increase in gas pressurepressure decreasestoward side with fewer moles of gasnone
increase in volume/decrease in gas pressurepressure increasestoward side with more moles of gasnone
temperature increaseheat is absorbedtoward products for endothermic, toward reactants for exothermicchanges
temperature decreaseheat is given offtoward reactants for endothermic, toward products for exothermicchanges

Glossary

Le Châtelier’s principle

when a chemical system at equilibrium is disturbed, it returns to equilibrium by counteracting the disturbance

position of equilibrium

concentrations or partial pressures of components of a reaction at equilibrium (commonly used to describe conditions before a disturbance)

stress

change to a reaction’s conditions that may cause a shift in the equilibrium

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