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E-1
EXPERIMENT E:
Le CHÂTELIER'S PRINCIPLE AND
INTERACTIVE CHEMICAL EQUILIBRIA
OBJECTIVES
The concept of chemical equilibrium will be explored in this experiment. You will examine the effect of
changing concentrations on the position of equilibrium for a number of simple chemical systems.
Most systems found in nature involve the simultaneous interaction of several equilibria. Changes to one
component can affect another, which in turn might affect yet another, giving rise to a cascade of effects. In
this experiment, you will examine several such interactive equilibria and in the report you will explain your
observations using Le Châtelier's Principle.
THEORETICAL CONSIDERATIONS
Part I. Le Châtelier's Principle and Position of Equilibrium
Chemical reactions do not generally go to completion. In many cases, the conversion of reactants to
products is incomplete, even if the reaction proceeds for a long time. When a condition is reached in which
the concentrations of the products and reactants do not change with time, a state of chemical equilibrium
exists. This does not mean that all chemical reactions have stopped, but rather that the rate of product
formation equals the rate of reactant formation. The position of chemical equilibrium can be altered by
changing the concentration of one or more of the reactants or products. The position can also be altered by
changing the temperature or pressure of the system. An important generalization, with regard to chemical
equilibrium, is Le Châtelier's Principle. It states that, when an external stress is applied to a chemical
system at equilibrium, the system responds by shifting the position of equilibrium in the direction
that minimizes the effects of the external stress. In other words, the stress created by the addition (or
removal) of species is relieved by a shift in the position of equilibrium in the direction that offsets the
stress.
To illustrate Le Châtelier's Principle, we will consider the dissociation of acetic acid in water. When acetic
acid (CH3COOH) is dissolved in water, an equilibrium is set up among undissociated acetic acid molecules
(CH3COOH
(aq)
), acetate ions (CH3COO
-
(aq)
), and hydronium ions (H3O
+
(aq)
):
CH3COOH
(aq)
+ H2O
(l)
CH3COO
-
(aq)
+ H3O
+
(aq)
(1)
