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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. As
well,
the effect of temperature on a system
will be studied.
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)
If we now add more acetate ions (CH3COO
-
(aq)
) to a solution of acetic acid, Le
Châtelier's Principle predicts
that the system responds in a way
that counteracts the disturbance or stress (the increased [CH3COO
-
(aq)
]),
hence, some CH3COO
-
(aq)
and H3O
+
(aq)
will react to form more CH3COOH
(aq)
and H2O
(l)
. The result is that
