What is the effect of compression on the composition of a gas-phase equilibrium?

In a gas-phase equilibrium, the effect of compression can significantly influence the composition of the mixture. When the volume of the gas is decreased (compression), the pressure of the gas increases. According to Le Chatelier's principle, a system at equilibrium will respond to a change in pressure by shifting the equilibrium position to counteract that change.

For reactions involving gases, if the number of moles of gas on one side of the equilibrium reaction is different from the other side, an increase in pressure (or a decrease in volume) will favor the side of the reaction with fewer moles of gas. This means that the equilibrium will shift to the side with fewer gas molecules, effectively decreasing the overall number of gas molecules present in the system. As a result, it can be concluded that the number of gas molecules decreases as the equilibrium shifts in response to compression.

This fundamental concept of gas behavior in equilibrium systems showcases the interplay between pressure, volume, and the amount of reactants and products present, emphasizing that changes in pressure can lead to shifts in chemical equilibrium, ultimately altering the composition of the gas mixture.