13.3 Shifting Equilibria: Le Châtelier’s Principle

Chemical equilibrium is a dynamic balance between forward and reverse reactions. Factors like concentration, temperature, and pressure can shift this balance. Understanding these influences helps predict and control reaction outcomes in various chemical processes.

The reaction quotient (Q) compares to the equilibrium constant (K) to predict shifts. Temperature affects exothermic and endothermic reactions differently. Pressure impacts gas-phase equilibria, while catalysts speed up reactions without changing the equilibrium position.

Factors Affecting Equilibrium

Dynamic Equilibrium and Reaction Quotient

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• Dynamic equilibrium occurs when forward and reverse reaction rates are equal
• Reaction quotient (Q) measures the relative amounts of products and reactants at any point in a reaction
• Comparing Q to the equilibrium constant (K) predicts the direction of equilibrium shift:
  • If Q < K, the reaction will shift towards products
  • If Q > K, the reaction will shift towards reactants
  • If Q = K, the system is at equilibrium

Effects of concentration on equilibrium

• Increasing the concentration of a reactant
  • Shifts the equilibrium to the right, towards the products (forming more products)
  • Increases the rate of the forward reaction until a new equilibrium is established to counteract the change
• Decreasing the concentration of a reactant
  • Shifts the equilibrium to the left, towards the reactants (forming more reactants)
  • Increases the rate of the reverse reaction until a new equilibrium is established to counteract the change
• Increasing the concentration of a product
  • Shifts the equilibrium to the left, towards the reactants (consuming more products)
  • Increases the rate of the reverse reaction until a new equilibrium is established to counteract the change
• Decreasing the concentration of a product
  • Shifts the equilibrium to the right, towards the products (forming more products)
  • Increases the rate of the forward reaction until a new equilibrium is established to counteract the change

Temperature changes in chemical equilibria

• Exothermic reactions ($\Delta H < 0$) release heat to the surroundings
  • Increasing temperature shifts the equilibrium to the left, towards the reactants (consumes heat)
  • Decreasing temperature shifts the equilibrium to the right, towards the products (releases heat)
• Endothermic reactions ($\Delta H > 0$) absorb heat from the surroundings
  • Increasing temperature shifts the equilibrium to the right, towards the products (absorbs heat)
  • Decreasing temperature shifts the equilibrium to the left, towards the reactants (releases heat)
• Temperature changes affect the equilibrium constant ($K$)
  • For exothermic reactions, increasing temperature decreases $K$ (favors reactants)
  • For endothermic reactions, increasing temperature increases $K$ (favors products)

Pressure impacts on gas-phase equilibrium

• Increasing pressure
  • Shifts the equilibrium towards the side with fewer moles of gas (reduces volume)
  • Favors the reaction that decreases the total number of gas molecules (forming fewer gas molecules)
• Decreasing pressure
  • Shifts the equilibrium towards the side with more moles of gas (increases volume)
  • Favors the reaction that increases the total number of gas molecules (forming more gas molecules)
• Pressure changes have no effect on equilibrium if
  • The total number of moles of gas is the same on both sides of the reaction (no net change in gas molecules)
  • The reaction involves only solid or liquid phases (incompressible, volume unaffected by pressure)

Catalysts and Equilibrium

• A catalyst increases the rate of both forward and reverse reactions equally
• Catalysts do not affect the position of equilibrium or the equilibrium constant
• Catalysts help a system reach equilibrium faster without changing the final equilibrium composition