264
Chemical Equilibrium in Gases
atm"
1 ) that is just the reciprocal of the other (1.78 atm). This reciprocity will
always be the case.
CATALYSTS
A catalyst affects only the rate of a chemical reaction; it has no effect on A" e or
on the position of equilibrium at a given temperature. You cannot, therefore,
increase the yield of a chemical reaction at a given temperature by adding a
catalyst to the reaction mixture. Catalysts are, however, of great practical
value because they may make an impractically slow reaction reach equilibrium at a practical rate, or may permit such a reaction to go at a practical rate
at a lower temperature, where a more favorable equilibrium position exists.
SUMMARY
The principle of Le Chatelier summarizes the conclusions that may be drawn
from the illustrative examples in this chapter: "Whenever a stress is placed on a
system at equilibrium, the equilibrium position shifts in such a way as to relieve
that stress." If the stress is an increase in the partial pressure (concentration) of
one component, the equilibrium shifts toward the opposite side in order to use
up part of the increase. If the stress is an increase in the total pressure, the
stress may be partially relieved by a shift toward the side with the smaller
number of gaseous moles; if there are the same number of gaseous moles on
each side, no shift will occur, and no stress will be relieved. If the stress is an
increase in temperature, the stress is partly relieved because, for an endothermic reaction, the equilibrium constant increases and the equilibrium shifts to
the right; for an exothermic reaction, the equilibrium constant decreases and the
equilibrium shifts to the left. A catalyst places no stress on the system and
causes no shift in the equilibrium position.
PROBLEMS A
1. In the following table, columns A, B, and C refer to these gaseous equilibria:
A: N 2 O 4 <=* 2NO 2
A// is positive
B: CO 2 + H 2 ?± CO + H 2 O
A// is positive
C: SO 2 + iO 2 ?± SO 3
A// is negative
For each reaction, tell the effect on K e and on the equilibrium position of each
change listed in the table. Assume that each change affects the reaction mixture only
after it has already once reached equilibrium. Use the following symbols in complet-
Chemical Equilibrium in Gases
atm"
1 ) that is just the reciprocal of the other (1.78 atm). This reciprocity will
always be the case.
CATALYSTS
A catalyst affects only the rate of a chemical reaction; it has no effect on A" e or
on the position of equilibrium at a given temperature. You cannot, therefore,
increase the yield of a chemical reaction at a given temperature by adding a
catalyst to the reaction mixture. Catalysts are, however, of great practical
value because they may make an impractically slow reaction reach equilibrium at a practical rate, or may permit such a reaction to go at a practical rate
at a lower temperature, where a more favorable equilibrium position exists.
SUMMARY
The principle of Le Chatelier summarizes the conclusions that may be drawn
from the illustrative examples in this chapter: "Whenever a stress is placed on a
system at equilibrium, the equilibrium position shifts in such a way as to relieve
that stress." If the stress is an increase in the partial pressure (concentration) of
one component, the equilibrium shifts toward the opposite side in order to use
up part of the increase. If the stress is an increase in the total pressure, the
stress may be partially relieved by a shift toward the side with the smaller
number of gaseous moles; if there are the same number of gaseous moles on
each side, no shift will occur, and no stress will be relieved. If the stress is an
increase in temperature, the stress is partly relieved because, for an endothermic reaction, the equilibrium constant increases and the equilibrium shifts to
the right; for an exothermic reaction, the equilibrium constant decreases and the
equilibrium shifts to the left. A catalyst places no stress on the system and
causes no shift in the equilibrium position.
PROBLEMS A
1. In the following table, columns A, B, and C refer to these gaseous equilibria:
A: N 2 O 4 <=* 2NO 2
A// is positive
B: CO 2 + H 2 ?± CO + H 2 O
A// is positive
C: SO 2 + iO 2 ?± SO 3
A// is negative
For each reaction, tell the effect on K e and on the equilibrium position of each
change listed in the table. Assume that each change affects the reaction mixture only
after it has already once reached equilibrium. Use the following symbols in complet-