Le Châtelier's Principle and Chemical Equilibrium

The Reaction Quotient ($Q$)

If a system is not at equilibrium, we can use the Reaction Quotient ($Q$) to determine which direction the reaction will shift to reach equilibrium. $Q$ uses the same mathematical formula as $K$, but it is calculated using the concentrations at any given moment, not necessarily at equilibrium.

• If $Q > K$: The ratio of products to reactants is too high. The reaction will proceed in the reverse direction to reach equilibrium.
• If $Q < K$: The ratio of products to reactants is too low. The reaction will proceed in the forward direction.
• If $Q = K$: The system is already at equilibrium.

Le Châtelier's Principle

Le Châtelier's principle provides a way to predict how a system at equilibrium will respond to external disturbances. It states that if a system at equilibrium is disturbed, the position of equilibrium will shift to minimize that disturbance.

1. Concentration Changes

Changing the concentration of a component is one of the most direct ways to shift equilibrium.

• Adding a reactant: This increases the forward reaction rate, causing the equilibrium to shift toward the products.
• Removing a product: This creates a "deficit" that the system tries to fill, shifting the equilibrium toward the products.

Exam Tip: Adding more of a solid or a liquid will not shift the equilibrium because their concentrations remain constant.

2. Volume and Pressure Changes

For reactions involving gases, changing the volume of the container affects the pressure and concentration.

• Decreasing Volume: This increases the concentration and partial pressure of all gases in the container. To minimize this pressure increase, the system shifts toward the side with fewer moles of gas.
• Increasing Volume: This decreases pressure, causing the system to shift toward the side with more moles of gas.

3. Temperature Changes

Temperature is unique because it is the only factor that actually changes the value of the equilibrium constant ($K$). To predict the shift, treat "heat" as if it were a chemical species in the reaction equation.

• Exothermic Reactions: These release heat. You can think of heat as a product.
  • Increasing temperature (adding heat) shifts the equilibrium toward the reactants.
• Endothermic Reactions: These absorb heat. You can think of heat as a reactant.
  • Increasing temperature (adding heat) shifts the equilibrium toward the products and increases the value of $K$.

| Toward Reactants | Decreases $K$ | | Increase Temp (Endothermic) | Toward Products | Increases $K$ | | Add Catalyst | No Shift | No Change |