will occur. Figure 20.20 shows a diagram of some of those conditions for
H 2 /O 2 mixtures.
Another example of interesting kinetics is the oscillating reaction.In these
reactions, an apparent oscillation in a concentration of an intermediate is seen
during the course of the reaction. This concentration oscillation can be seen as
a color-change cycle, the periodic formation of a gaseous product, or some
other measurable increase-and-decrease in the concentration of some species.
Oscillating reactions are rare but are particularly fascinating to chemists be-
cause of their seemingly unusual behavior.
Oscillating reactions might seem to violate the laws of thermodynamics,
which suggest that a reaction should proceed toward equilibrium and, once
there, not deviate from equilibrium conditions unless some external influence
is imposed. Oscillating reactions start from some nonequilibrium condition,
appear to pass through some equilibrium concentration of products, then con-
tinue to a different nonequilibrium concentration. At some point, the reaction
reverses and proceeds back toward the equilibrium amounts, again passing
through the equilibrium condition to some other extreme, then reverses again.
The analogy is a clock pendulum swinging back and forth, but the general un-
derstanding of chemical reactions is that they should proceed toward equilib-
rium and then stop, a dynamic equilibrium having been established.
One key in understanding oscillating reactions is that the oscillating concen-
tration is typically one of an intermediate, which may or may not be a final
product of the overall reaction. Another key is the idea that there are two (or
more) pathways that the reaction can take, and that the intermediate is a prod-
uct of one pathway and a reactant of another. Thus, when the intermediate’s20.9 Chain and Oscillating Reactions 717BPBPBPBPBPBPBPBPBPBPBPBPBPBP BPBPBPBPBPBPBPBBPBPBPBPBPBPBPBP BPFigure 20.19 Branching reactions increase the number of propagation reactions, which can lead
to an explosion. In the figure, P stands for a propagation and B represents a branching reaction.100000
400
Temperature (°C)600Pressure (mmHg)(a)
550ExplosionExplosionNo explosionExplosionNo explosion450 5001000100107000
500
Temperature (°C)800Pressure (mmHg)(b)
600 700600500400300200100Figure 20.20 Graphs of explosion limits for
gaseous mixtures. (a) Stoichiometric mixtures of
H 2 and O 2. (b) Stoichiometric mixtures of CO
and O 2.