Chapter 15 · Chemical Equilibrium
15.9The Effect of a Volume Change on Equilibrium
9 min · two checks
Predict
You shrink the volume of N₂ + 3 H₂ ⇌ 2 NH₃, increasing the pressure. Which way does it shift?
The idea
- Predict the shift when volume of a gas equilibrium changes.
- Recognize when a volume change does nothing.
Decreasing the volume of a gas mixture raises the pressure and raises every gas concentration. The equilibrium shifts toward the side with fewer moles of gas, which consumes some of that pressure increase. Increasing the volume shifts toward more moles of gas. Count only gases. If both sides have the same number of gas moles, a volume change does not shift the position. H₂ + I₂ ⇌ 2 HI is the teaching example of “no shift.”
Adding an inert gas at constant volume does not change the concentrations of the reacting gases, so it does not shift the equilibrium, even though the total pressure rises. Adding an inert gas at constant pressure expands the volume, which is a real volume change. Read the problem for which quantity is held constant. In solution, the same counting idea applies to dissolved particles when you dilute.
Keep these
- Higher pressure (smaller volume) favors fewer gas moles.
- Equal gas moles on both sides: no shift.
- Inert gas at constant volume: no shift.