How does nitrogen tetroxide dissociate into nitrogen dioxide?

Nitrogen tetroxide (${\text{N}}_2{\text{O}}_4$), a colourless dimer, dissociates into two molecules of brown nitrogen dioxide (${\text{NO}}_2$) gas through a reversible chemical equilibrium reaction:12

$${\text{N}}_2{\text{O}}_4\rightleftharpoons 2{\text{NO}}_2$$

At ambient temperature and pressure, this mixture exists in a steady state where approximately 80 percent of the molecules are dimers. The dissociation process is sensitive to external conditions:1

• Pressure: Because the dissociation of one ${\text{N}}_2{\text{O}}_4$ molecule into two ${\text{NO}}_2$ molecules doubles the number of particles and increases the gas volume, the system shifts toward the dimer form under increased pressure to reduce volume. Conversely, reduced pressure favors the formation of monomers.1
• Temperature: Changes in temperature alter the equilibrium ratio, as the system requires time to reach a new balance—a process known as chemical relaxation.1

The rate of this dissociation is finite, meaning the system cannot respond instantaneously to rapid fluctuations in temperature or pressure, such as those caused by sound waves passing through the gas.1