The Foundations of Chemistry

The data from the table can be substituted into the reaction quotient expression, Q,as
a check. Even though the reaction is initiated by different relative amounts of reactants
in the two cases, the ratios of equilibrium concentrations of products to reactants (each
raised to the first power) agree within roundoff error.

FACTORS THAT AFFECT EQUILIBRIA

Once a reacting system has reached equilibrium, it remains at equilibrium until it is
disturbed by some change of conditions. The guiding principle is known as LeChate-
lier’s Principle(Section 13-6).

If a change of conditions (stress) is applied to a system at equilibrium, the system

shifts in the direction that reduces the stress to move toward a new state of equi-

librium.

The reaction quotient, Q,helps us predict the direction of this shift. Three types of changes
can disturb the equilibrium of a reaction.

1.Changes in concentration

2.Changes in pressure or volume (for reactions that involve gases)

3.Changes in temperature

We now study the effects of these types of stresses from a qualitative, or descriptive,
point of view. In Section 17-8 we expand our discussion with quantitative examples.

Changes in Concentration

Consider the following system starting at equilibrium.

AB 34 CD Kc

[

[

C

A

]

]

[

[

D

B]

]



When more of any reactant or product is addedto the system, the value of Qchanges, so
it no longer matches Kc, and the reaction is no longer at equilibrium. The stress due to
the added substance is relieved by shifting the equilibrium in the direction that consumes
some of the added substance, moving the value of Qback toward Kc. Let us compare the
mass action expressions for Qand Kc. If more A or B is added, then Q Kc, and the
forward reaction occurs more rapidly and to a greater extent than the reverse reaction
until equilibrium is reestablished. If more C or D is added, Q Kc, and the reverse reac-
tion occurs more rapidly and to a greater extent until equilibrium is reestablished.

Adding or removing reactants or products changes the value of Q.It does not change

the value of Kc.

We can understand LeChatelier’s Principle in the kinetic terms we used to introduce
equilibrium. The rate of the forward reaction is proportional to the reactant concentra-
tions raised to some powers,

Ratefkf[A]x[B]y

17-6

LaChatelieris pronounced “le-SHOT-

lee-ay.”

Remember that the valueof an

equilibrium constant changes only

with temperature.

For reactions involving gases at

constant temperature, changes in

volume cause changes in pressure,

and vice versa.

17-6 Factors that Affect Equilibria 721

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 16.11, Disturbing a Chemical

Equilibrium (1): LeChatelier’s Principle.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 16.13, Disturbing a Chemical

Equilibrium (3): Addition or Removal of

a Reagent.