Chemistry Reference
In-Depth Information
EXAMPLE 18.8
Predict how increasing the volume of the container will affect the equilibrium
state at high temperature of the reaction of nitrogen and oxygen to make nitro-
gen monoxide, NO:
N
2
(g)
O
2
(g) E 2 NO(g)
Solution
Because there are 2 mol of gas on each side of this equation, neither a shift to
the left nor a shift to the right will change the total pressure. The increase in
volume causes a reduction in pressure but it will not shift the equilibrium.
EXAMPLE 18.9
Predict how increasing the volume of the container will affect the equilibrium state
of the reaction of solid carbon and oxygen gas to make carbon monoxide gas:
2 C(s)
O
2
(g) E 2 CO(g)
Solution
The increase in volume causes a decrease in partial pressure of each of the
gases.
The carbon is a solid and has no partial pressure. Therefore, the equilibrium will
shift to the right to increase the number of moles of gas at equilibrium.
Snapshot Review
❒
If a stress is placed on a system at equilibrium, the equilibrium will
shift to reduce the stress. (If the system is not at equilibrium to begin
with, LeChâtelier's principle does not apply.)
ChemSkill Builder 17.5
A. (a) If ammonia is added to a system of ammonia and its elements at
equilibrium, which way will the equilibrium shift? (b) What will happen
to the temperature of the system because of this shift?
18.4
Equilibrium Constants
LeChâtelier's principle (Section 18.3) allows us to make qualitative predictions
about the effects of changes of conditions on an equilibrium system but does
not allow quantitative calculations. However, at equilibrium at a given temper-
ature, a certain ratio of concentration terms is very nearly constant for all solutes
and gases involved in any given reaction. (Solids and pure liquids are not
included in the ratio.) Because it is not exactly constant, we will use two or at
most three significant digits in equilibrium constant calculations. For the general
reaction
a
A
b
B E
c
C
d
D
