The Foundations of Chemistry

When a new equilibrium condition is established, (1) the rates of the forward and

reverse reactions are again equal; and (2) Kcis again satisfied by the new concen-

trations of reactants and products.

Practical applications of changes of this type are of great economic importance. Removing
a product of a reversible reaction forces the reaction to produce more product than could
be obtained if the reaction were simply allowed to reach equilibrium.

EXAMPLE 17-8 Applying a Stress to a System at Equilibrium

Given the following reaction at equilibrium in a closed container at 500°C, predict the effect
of each of the following changes on the amount of NH 3 present at equilibrium: (a) forcing
more H 2 into the system; (b) removing some NH 3 from the system.

N 2 (g)3H 2 (g) 34 2NH 3 (g)

Plan

We apply LeChatelier’s Principle to each part of the question individually.

Solution

(a) Adding a substance favors the reaction that uses up that substance (forward in this case).

More NH 3 is formed.

(b) Removing a substance favors the reaction that produces that substance (forward in this case).

More NH 3 is formed.

Note that at the new equilibrium, the concentration[NH 3 ] would still be less than at the orig-
inalequilibrium.

Changes in Volume and Pressure

Changes in pressure have little effect on the concentrations of solids or liquids because
they are only slightly compressible. Changes in pressure do cause significant changes in
concentrations of gases, however. Such changes therefore affect the value of Qfor reac-
tions in which the number of moles of gaseous reactants differs from the number of moles
of gaseous products. For an ideal gas,

PVnRT or P

V

n

(RT)

The term (n/V) represents concentration, that is, mol/L. At constant temperature, n, R,
and Tare constants. Thus, if the volume occupied by a gas decreases, its partial pressure
increases and its concentration (n/V) increases. If the volume of a gas increases, both its
partial pressure and its concentration decrease.
Consider the following gaseous system at equilibrium.

A(g) 34 2D(g) Kc

[

[

D

A

]

]

2



17-6 Factors that Affect Equilibria 723

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 16.14, Disturbing a Chemical

Equilibrium (4): Volume Changes.

Reaction proceeds to the right

Increasing Q

QK

(a)

Reaction proceeds to the right

Increasing Q

QK

(b)