ACS Chemistry Practice Exam 2026 – The All-in-One Guide to Exam Success!

Le Chatelier's principle states that if a system at equilibrium experiences a change in conditions (such as concentration, temperature, or pressure), the system will adjust in a way that counteracts that change. When the pressure of a gas-phase equilibrium system is increased, the system will shift in the direction that reduces the number of gas molecules, thereby decreasing the pressure.

In reactions involving gas, if one side of the equilibrium has fewer moles of gas than the other, increasing the pressure will favor the side with the fewer moles of gas. This happens because the system minimizes the change by producing a shift that leads to a reduction in the total number of gas molecules, thus alleviating the increased pressure.

For example, if the equilibrium reaction has a shift to the side producing fewer gas molecules, that helps lower the overall pressure of the system. Conversely, if the side produces more gas molecules, that would exacerbate the pressure increase, which the system aims to avoid.

A shift towards the side with fewer gas molecules effectively stabilizes the system under the new conditions. Thus, this principle is crucial for understanding how chemical reactions respond to changes in pressure within a gaseous environment.