Le Chatelier’s Principle helps predict how a reaction at equilibrium responds to changes in various factors. You’ve already seen how changes in concentration and temperature can shift a reaction. Now, we will focus on pressure for reactions involving gases.
Recall that pressure and volume have an inverse relationship: when the volume of a gas decreases (as when squeezing a balloon), the pressure of the gas increases. The opposite relationship is also true (when the volume increases, the pressure will decrease).
These pressure changes can cause an equilibrium shift for reactions involving gases. If the pressure is increased, then the equilibrium will shift in the direction that decreases the overall pressure. That is, it will shift in the direction that will produce fewer gas molecules. Conversely, if the pressure of the system is decreased, then the equilibrium will shift in the direction that increases the overall pressure, which is the direction that will produce more gas molecules.
It is important to note that only the number of gas particles matters when determining equilibrium shifts due to pressure changes. Aqueous, solid, or liquid substances will not be affected by pressure.
Click each stress in this chart to see how increasing or decreasing the volume/pressure affects the equilibrium for this reaction involving gases:
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g)
Notice there are 4 reactant gas molecules (1 N2 + 3 H2), while there are only 2 product gas molecules (2 NH3). This will be important for determining how pressure changes affect equilibrium for this reaction.
| Stress | Equilibrium Shift | Reasoning |
|---|---|---|
|
→ (towards products) |
There are fewer gas molecules on the product side, so when the pressure is increased, the reaction will favor the forward direction to produce fewer gas particles in order to lower the overall pressure and get back to the equilibrium ratio. |
|
|
← (towards reactants) |
There are more gas molecules on the reactant side, so when the pressure is decreased, the reaction will favor the reverse direction to produce more gas particles in order to make up for the pressure deficit and get back to the equilibrium ratio. |
Question
Why do changes in pressure affect the equilibrium of gases but not solids or liquids?
Solids and liquids are essentially incompressible, so changes in pressure do not significantly change their concentrations. Gases, however, are compressible, so increasing or decreasing pressure changes their concentration and can shift the equilibrium in the direction that reduces or increases the number of gas molecules.
Question
Explain why there is no shift in equilibrium when a catalyst is added.
Adding a catalyst does not shift the equilibrium because it speeds up both the forward and reverse reactions equally. This means the rates of the reactions increase, but the relative amounts of reactants and products at equilibrium stay the same.
