14.8
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 pr
For gas-phase equilibria, changes in the concentrations of reactants and products can occur with altered volume and pressure. The partial pressure, P,…
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