Why can a slow sound wave keep the nitrogen tetroxide and nitrogen dioxide reaction near equilibrium?

When a sound wave propagates through a gas, it creates a sinusoidal variation in pressure and temperature. The ability of the nitrogen tetroxide and nitrogen dioxide reaction to maintain chemical equilibrium depends on the frequency of these oscillations relative to the reaction's relaxation time.

A slow sound wave allows the reaction to remain at equilibrium because the pressure and temperature changes occur gradually enough for the dissociation and association rates to keep pace with the shifting conditions. In this state, the extremes of the monomer-dimer ratio coincide perfectly with the extremes of the pressure and temperature variations.

Conversely, if the pressure fluctuates too rapidly, the reaction cannot adjust quickly enough to follow the changes, and the ratio of monomers to dimers remains effectively constant. At intermediate frequencies, a relaxation effect occurs, where the readjustment of the equilibrium lags behind the pressure variations, leading to sound absorption and dispersion.1