Chemical equilibrium represents a dynamic state in a closed chemical system where the rate of the forward reaction is exactly equal to the rate of the reverse reaction. This balance ensures that the concentrations of reactants and products remain constant over time. However, this balance is sensitive to external changes, and altering the container’s volume is one way to disturb the system. For reactions that involve gases, a change in volume introduces a mechanical stress that the chemical system must accommodate to re-establish a stable state.
Chemical Equilibrium and Le Chatelier’s Principle
The behavior of a system at equilibrium when conditions change is described by Le Chatelier’s Principle. This 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. This concept applies to disturbances, including alterations to concentration, temperature, and pressure or volume.
The system does not necessarily return to the exact same concentrations as before, but it finds a new equilibrium position that partially counteracts the imposed change. When discussing volume, the principle predicts that the chemical reaction will adjust itself to manage the resulting pressure change. For gaseous reactions, a volume change is a specific type of mechanical stress that directly impacts the density of the molecules.
The Relationship Between Volume and Pressure
The physical foundation for the effect of volume change on equilibrium is the inverse relationship between volume and pressure. Assuming a constant temperature and a fixed amount of gas, the pressure a gas exerts is inversely proportional to the volume it occupies. When the volume is decreased, the pressure inside the container increases.
Conversely, when the volume is expanded, the total pressure exerted by the gas molecules decreases. This relationship arises from the kinetic theory of gases, where pressure results from gas molecules colliding with the container walls. A smaller volume forces molecules into a confined space, leading to more frequent collisions and a higher pressure. This instantaneous change in pressure is the stress the chemical system must then address.
Predicting the Direction of the Shift
The chemical system relieves a pressure stress by shifting the reaction toward the side that contains a different total number of moles of gas. If the volume is decreased, leading to an increase in pressure, the system shifts toward the side of the reaction with the fewer total moles of gas. This shift reduces the total number of gas particles in the container, thus lowering the resulting pressure and partially counteracting the initial stress.
If the volume is increased, resulting in a decrease in pressure, the equilibrium shifts toward the side with the greater total moles of gas. By producing more gas molecules, the system attempts to restore some of the lost pressure. The direction of the shift is determined by comparing the stoichiometric coefficients of the gaseous species on the reactant side versus the product side.
Example Reaction
Consider the general reaction: \(A(g) + B(g) \rightleftharpoons C(g)\). The reactant side has a total of two moles of gas, while the product side has one mole of gas. If the volume is decreased, the pressure increases, and the system shifts to the product side (to the right). If the volume is increased, causing the pressure to drop, the system shifts toward the reactant side (to the left).
When Volume Changes Have No Effect
A change in volume does not always result in a shift in the equilibrium position; this occurs under specific circumstances. Reactions involving only solids and liquids are generally unaffected by volume changes because these states are largely incompressible. The concentration of a substance that is a solid or a liquid is independent of the container volume, meaning the equilibrium expression remains unchanged.
Another scenario where volume has no effect is when the total number of moles of gas on the reactant side is exactly equal to the total number of moles of gas on the product side. In this case, there is no way for the system to favor one side over the other to reduce or increase the number of gas molecules.
Furthermore, adding an inert gas, such as argon, to a system at equilibrium does not cause a shift if the volume is held constant. While the total pressure inside the container increases, the partial pressures of the reacting gases remain the same because their concentrations have not changed. Since the equilibrium constant depends only on the partial pressures of the reacting species, the equilibrium position is undisturbed.