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How does increased pressure affect the equilibrium between nitrogen tetroxide and nitrogen dioxide?

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The equilibrium between nitrogen tetroxide ( $N_{2} O_{4}$ ) and nitrogen dioxide ( $N O_{2}$ ) is represented by the reversible reaction:

$2 N O_{2} ⇌ N_{2} O_{4}$

Because the dissociation of one molecule of $N_{2} O_{4}$ into two molecules of $N O_{2}$ results in an increase in the number of gas molecules, the system is sensitive to changes in pressure. According to the principles of chemical equilibrium, when the external pressure on this system is increased, the system acts to relieve the stress by reducing its total volume. It achieves this by favoring the formation of the dimer ( $N_{2} O_{4}$ ), thereby reducing the total number of gas molecules present.1

Conversely, if the external pressure is reduced, the equilibrium shifts in favor of the monomer ( $N O_{2}$ ) to increase the number of molecules and occupy more volume. Because systems cannot respond instantaneously to these changes, the process of reaching a new equilibrium concentration is known as chemical relaxation.1