http://www.ck12.org Chapter 19. Chemical Equilibrium
The video below shows three sealed glass tubes containing N 2 O 4 and NO 2. When one tube is placed in hot water,
the equilibrium favors the brown NO 2. When another tube is placed in ice cold water, the equilibrium lies in favor
of the colorless N 2 O 4.
http://www.youtube.com/watch?v=tlGrBcgANSY (0:55)
MEDIA
Click image to the left for more content.Pressure
Changing the pressure of an equilibrium system in which gases are involved is also a stress to the system. A change
in the pressure on a liquid or a solid has a negligible effect. We will return again to the equilibrium for the Haber-
Bosch process. Imagine the gases are contained in a closed system in which the volume of the system is controlled
by an adjustable piston as shown inFigure19.6.
FIGURE 19.6
(A) A mixture of nitrogen, hydrogen, and
ammonia in equilibrium. (B) When the
pressure is increased on the equilibrium
mixture, the forward reaction is favored
because that results in a reduction of the
total moles of gas present. (C) Fewer
moles of gas will exert a lower total pres-
sure, so the stress is partially relieved by
such a shift.On the far left, the reaction system contains primarily N 2 and H 2 , with only one molecule of NH 3 present. As
the piston is pushed inwards, the pressure of the system increases according to Boyle’s Law. This is a stress to
the equilibrium. In the middle image, the same number of molecules are now confined to a smaller space, so the
pressure has increased. According to Le Châtelier’s principle, the system responds in order to relieve the stress. In
the image on the right, the forward reaction has been favored, in which each one molecule of N 2 combines with
three molecules of H 2 to form one molecule of NH 3. The overall result is a decrease in the number of gas molecules
in the entire system. This decreases the pressure and counteracts the original stress of a pressure increase. When
the pressure is increased by decreasing the available volume, system responds by favoring the reaction direction that
produces fewer gas molecules. In this case, it is the forward reaction that is favored.
A decrease in pressure on the above system could be achieved by pulling the piston outward, increasing the container
volume. The equilibrium would respond by favoring the reverse reaction, in which NH 3 decomposes to N 2 and H 2.
This is because the overall number of gas molecules would increase, and so would the pressure. When the pressure
of a system at equilibrium is decreased by providing more total volume, the reaction that produces more total moles