Le Chatelier's Principle and Chemical Equilibrium Shifts

Le Chatelier's Principle

Le Chatelier's Principle: When a system undergoes a change in equilibrium concentration of the reacting species, pressure, or temperature, the system responds by reaching a new equilibrium that partially offsets the effect of the modification.

Altering Pressure at Constant Temperature

We can alter the pressure of a system in equilibrium at a constant temperature in the following ways:

• Adding or removing a species in a gaseous state: The effect amounts to changes in the concentration of one of the reacting species, and the reaction will shift as described in the previous section.
• Adding an inert gas: Keeping the volume constant increases the total pressure, but not the partial pressures; therefore, this addition will have no effect on the equilibrium. However, an inert gas may change the equilibrium when added at constant pressure, since it increases the volume of the mixture to make room.
• Changing the system volume: At a constant temperature, pressure and volume are inversely proportional; an increase in volume produces a decrease in pressure and vice-versa.

Effects of Volume Changes

We can change the volume to influence the system:

• If we increase the volume, pressure decreases. Since concentration is inversely proportional to the volume, concentrations fall; this change is offset by a shift toward the side where there is a greater number of moles of gas.
• If we decrease the volume, the pressure increases and concentrations increase. The system will compensate for this change by shifting toward the side where there are fewer moles of gas.

Temperature and Catalysts

Changing the Temperature

Providing or removing heat affects the system as follows:

• An increase in temperature favors the direction in which the reaction is endothermic.
• A decrease in temperature favors the direction in which the reaction is exothermic.

Addition of a Catalyst

The addition of a catalyst does not affect the equilibrium; it only ensures that the equilibrium state is attained earlier.

Ion Product and Solubility

Ion Product (Q): The product of the molar concentrations of ions in a given solution, each raised to the power of its corresponding stoichiometric coefficient.

• Q = K_sp: This represents a saturated solution in an equilibrium system.
• Q > K_sp: This represents a supersaturated solution; the excess salt will precipitate until Q = K_sp.
• Q < K_sp: This represents an unsaturated solution, meaning it can dissolve more solid.