http://www.ck12.org Chapter 19. Equilibrium
19.2 Le Châtelier’s Principle
Lesson Objectives
- Explain the factors that stress a system at equilibrium, and use Le Châtelier’s principle to predict how the
system will respond to each stress. - Explain how a reaction is able to go to completion.
- Analyze how a change in the concentration of a reaction component can change the equilibrium position but
not the value of the equilibrium constant.
Lesson Vocabulary
- Le Châtelier’s principle
Check Your Understanding
Recalling Prior Knowledge
- How is equilibrium defined?
- What is an equilibrium constant, and how is it calculated?
A chemical system that is at equilibrium can be disrupted by changes in concentrations, temperature, or pressure. In
this lesson, you will learn how to analyze stresses to a system at equilibrium and predict how the system will respond
to that stress.
Factors Affecting an Equilibrium System
Equilibrium represents a balance between the reactants and the products of a chemical reaction. Changes to the
conditions of the system can disturb that equilibrium. When this occurs, the system reacts in such a way as to restore
the equilibrium. However, the position of equilibrium will be changed following the disturbance. In other words,
the response of the system involves a change in the amounts of the reactants and products. Some will increase and
some will decrease until equilibrium is reestablished.
Chemical equilibrium was studied by French chemist Henri Le Châtelier (1850-1936), and his description of how a
system at equilibrium responds to a change in conditions has become known asLe Châtelier’s principle:When a
chemical system that is at equilibrium is disturbed by a stress, the system will respond by attempting to counteract
that stress until a new equilibrium is established(Table19.1). Stresses to a chemical system include changes in
the concentrations of reactants or products, changes in the temperature of the system, or changes in the pressure of
the system. We will discuss each of these stresses separately. In each case, the change to the equilibrium position