14.8
Le Chatelier's Principle: Changing Volume (Pressure)
For gas-phase equilibria, changes in the concentrations of reactants and products can occur with altered volume and pressure. The partial pressure, P,…
Le Châtelier’s principle can be used to predict how a system at equilibrium would respond to the stress of a change in volume or pressure.
The volume of a gas is inversely proportional to its pressure; therefore, for a system at equilibrium, a decrease in volume increases the pressure and disturbs the equilibrium. In response, the equilibrium position will shift in a direction to minimize the stress.
The ideal gas law states that the pressure of a gas is directly proportional to the number of moles. Thus, the direction of the shift needed to restore equilibrium is dependent on the number of moles of gas particles on either side of the reaction.
As more moles of gas results in a higher pressure, an increase in pressure shifts the equilibrium position to the side with fewer moles to lower the pressure. Likewise, a decrease in pressure shifts the equilibrium position to the side with more moles of gas.
Consider a chemical equilibrium, where one mole of gaseous phosphorus pentachloride decomposes into one mole of phosphorus trichloride and one mole of chlorine gas—two total moles of product.
If the piston is pushed down, the volume of the equilibrium system decreases, increasing the pressure. This disturbs the equilibrium and results in Q greater than K. Thus, the equilibrium position shifts towards the reactants, with fewer moles of gas particles, in order to lower the pressure and restore equilibrium.
Conversely, pulling the piston up increases the volume and decreases the pressure. In this case, Q becomes smaller than K. In order to raise the pressure, the equilibrium position shifts towards the products, the side with the most moles of gas, and restores equilibrium.
Increasing the pressure by adding an inert gas to an equilibrium mixture at constant volume, does not affect the equilibrium because the partial pressures of the gaseous reactants and products remain unchanged.
For equilibrium systems with equal numbers of moles of gaseous reactants and products, such as the reaction between iodine gas and chlorine gas to produce iodine monochloride, a change in the volume of the system will have no effect on the equilibrium.
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Q1: How does decreasing volume affect an equilibrium system?
Decreasing volume increases pressure, disturbing the equilibrium. The system responds by shifting toward the side with fewer moles of gas to minimize the pressure increase. This shift continues until equilibrium is restored. The direction depends on comparing mole counts of reactants versus products.
Q2: Why does adding an inert gas at constant volume not shift equilibrium?
Adding an inert gas increases total pressure but does not change the partial pressures of reactants or products. Since partial pressures determine equilibrium position, the system remains undisturbed. The equilibrium stays at its original position without shifting, regardless of the inert gas added.
Q3: What happens to equilibrium when volume increases in a gas-phase reaction?
Increasing volume decreases pressure, causing the equilibrium to shift toward the side with more moles of gas to raise the pressure and restore equilibrium. For reactions with equal moles on both sides, volume changes have no effect on equilibrium position. The shift direction depends on stoichiometry.
Q4: How does the ideal gas law relate pressure to moles in an equilibrium system?
The ideal gas law shows that pressure is directly proportional to the number of moles of gas. Therefore, when volume decreases, pressure increases proportionally to mole count. Systems respond by shifting toward fewer moles to lower pressure and restore equilibrium.
Q5: Why does stoichiometry determine how volume changes affect equilibrium?
Stoichiometry reveals the mole ratio of reactants to products. When volume changes, the system compares mole counts on each side. If reactants and products have equal moles, volume changes cause no shift. If moles differ, the equilibrium shifts to accommodate the volume change.
Q6: What is the relationship between partial pressure and concentration in gas equilibria?
Partial pressure of an ideal gas is proportional to its molar concentration. Changes in volume alter partial pressures of all species equally, producing the same concentration changes. This relationship determines whether the equilibrium shifts based on mole differences between reactants and products.
Q7: How does Le Chatelier's principle predict equilibrium shifts from pressure changes?
Le Chatelier's principle states the system shifts to minimize stress. When pressure increases from volume decrease, equilibrium shifts toward fewer moles. When pressure decreases from volume increase, equilibrium shifts toward more moles. The shift direction depends entirely on mole stoichiometry.