Effect of Pressure on Equilibrium (Change in Partial Pressure of Gases)

A(g) + B(g) ↔ AB(g) At equilibrium if 1 mole of each gas exits in a 1 liter container. Then Kc = 1 Now let’s look at the effect of adjusting the volume without adding or removing any of the products or reactants. This means a “Q” situation will result and the system will then adjust to reach “K”. a. Let’s change the volume to 0.5 L. Note that less volume this will increase the pressure. Since Qc is less than Kc, the system will shift to make Qc = Kc. This means more product will be formed. (A shift to have less moles of gas in the container)

A(g) + B(g) ↔ AB(g) b. Now let’s try decreasing the pressure by increasing the volume while not adding or removing and products or reactants. Let’s change the volume to 2 liters. Now note that Qc is greater than Kc. so the system will shift to make Qc = Kc. To make Qc smaller, the system will have to shift toward more reactants.

Summary: Note that changing the volume results in changing the concentrations of each gaseous species and thus results in a change in their relative partial pressures and a shift away from equilibrium if there is a difference