Increasing pressure affects equilibrium by shifting it to the side of the reaction with fewer moles of gas.
Le Chatelier's Principle and Pressure
Le Chatelier's principle states that if a change of condition is applied to a system in equilibrium, the system will shift in a direction that relieves the stress. In the context of pressure, an increase in pressure is a stress, and the system will shift to reduce that pressure. The easiest way for the system to reduce pressure is to decrease the number of gas molecules present.
How Pressure Relates to Moles of Gas
Pressure and the number of moles of gas are directly related (as demonstrated in the Ideal Gas Law, PV=nRT). Therefore, if you increase the pressure on a system at equilibrium, the reaction will shift to the side that has fewer moles of gas to try and reduce the overall pressure of the system. Conversely, if you decrease the pressure, the reaction will shift towards the side with more moles of gas.
Examples
Here are a couple of examples to illustrate this principle:
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Example 1: Haber-Bosch Process
N2(g) + 3H2(g) ⇌ 2NH3(g)
In this reaction, there are 4 moles of gas on the reactant side (1 mole of N2 and 3 moles of H2) and 2 moles of gas on the product side (2 moles of NH3). If you increase the pressure on this system, the equilibrium will shift to the right (towards the products) because there are fewer moles of gas on that side.
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Example 2: Dissociation of Dinitrogen Tetroxide
N2O4(g) ⇌ 2NO2(g)
In this reaction, there is 1 mole of gas on the reactant side (1 mole of N2O4) and 2 moles of gas on the product side (2 moles of NO2). If you increase the pressure, the equilibrium will shift to the left (towards the reactants) to reduce the pressure, as there are fewer moles of gas on the reactant side.
Important Considerations
- Only gases are considered. Changes in pressure only affect equilibria involving gases. Solids and liquids are considered incompressible, so changes in pressure have negligible effects on their concentrations.
- Equal Moles of Gas: If the number of moles of gas is the same on both sides of the reaction, a change in pressure will have little to no effect on the equilibrium position.
- Inert gases: Adding an inert gas doesn't change the partial pressures of the reactants and products, so it doesn't shift the equilibrium, even though the total pressure has changed. It's the change in partial pressures of the reacting gases that matters.
Conclusion
In summary, increasing the pressure on a system at equilibrium involving gases will shift the equilibrium towards the side of the reaction with fewer moles of gas, in accordance with Le Chatelier's principle. This helps to minimize the increase in pressure and re-establish equilibrium.
