Study Notes on Le Châtelier’s Principle and Equilibrium

15.9 Le Châtelier’s Principle: Disturbing and Restoring Equilibrium

• Definition of equilibrium: When a reaction is at equilibrium, concentrations of all reactants and products remain constant.

• Reaction changes with disturbance: If conditions are altered, concentrations will change until new equilibrium is achieved.

• New equilibrium concentrations differ, but equilibrium constant remains the same unless temperature changes.

• Le Châtelier’s principle: Guides predictions on how changes in conditions affect the position of equilibrium.

  • Principle states: If a system at equilibrium is disturbed, the position of equilibrium will shift to minimize the disturbance.

  • Disturbances involve making the system open.

A. Adding Reactants or Products

• After establishing equilibrium, adding reactants changes the concentrations if the substance is in the equilibrium constant expression (not solid or liquid).

• Effects of Adding/Removing Components:

  • Increasing the concentration of reactants: Shifts reaction to the right (toward products).

  • Reaction quotient (Q) < K, thus reaction proceeds to products.

  • Decreasing reactants: Shifts reaction to the left (toward reactants).

  • Q > K, thus reaction proceeds to reactants.

  • Increasing products: Shifts reaction to the left (toward reactants).

  • Q > K.

  • Decreasing products: Shifts reaction to the right (toward products).

  • Q < K.

• The value of equilibrium constant (K) remains unaffected at the new equilibrium position despite concentration changes.

B. Effect of Volume Change on Equilibrium

• Decreasing container volume:

  • Increases concentration of all gases, raising their partial pressures.

  • Total pressure inside the container increases.

• According to Le Châtelier’s principle:

  • Equilibrium shifts to decrease pressure by reducing the number of gas molecules in the vessel.

  • When volume decreases, equilibrium shifts to the side with fewer gas molecules.

  • At the new equilibrium position, partial pressures of gases result in unchanged equilibrium constant value.

Effects of Pressure Changes on Equilibrium

• Increasing container volume:

  • Decreases total pressure, shifting equilibrium toward the side with more gaseous molecules.

C. The Effect of Temperature Changes on Equilibrium Position

• Two distinct cases apply:

  1. Exothermic Reactions:

  • Definition: Reactions that release energy, where heat functions as a product.

  • Effect of temperature on equilibrium shifts:

    • Increasing temperature shifts equilibrium to reactants side (left).

    • Decreasing temperature shifts equilibrium to products side (right).

  • Adjustments to equilibrium constant (K):

    • Adding heat decreases product concentrations and increases reactant concentrations.

    • K value also decreases with increased temperature.

  1. Endothermic Reactions:

  • Definition: Reactions that absorb energy, where heat functions as a reactant.

  • Effect of temperature on equilibrium shifts:

    • Increasing temperature shifts equilibrium toward products side (right).

    • Decreasing temperature shifts equilibrium toward reactants side (left).

  • Adjustments to equilibrium constant (K):

    • Adding heat decreases reactant concentrations and increases products concentrations.

    • K value increases with increased temperature.

Effects on Equilibrium Constants with Temperature

• In exothermic reactions:

  • Heat increases results in a decrease in K.

  • Heat decreases results in an increase in K.

• In endothermic reactions:

  • Heat increases results in an increase in K.

  • Heat decreases results in a decrease in K.

D. The Effect of Catalysts

• Catalysts and Equilibrium:

  • Catalysts provide a more efficient reaction mechanism affecting both forward and reverse reactions equivalently.

  • They do not affect the position of equilibrium nor the equilibrium constant value.

15.5 Factors That Affect Chemical Equilibrium

• Le Châtelier’s principle clarifies that applying stress leads to equilibrium shifts to minimize this stress:

  • Examples of stressors include:

  • Addition/removal of reactants or products.

  • Volume change leading to concentration or partial pressure changes.

  • Temperature change.

Worked Example: Haber Process

• For the reaction: $N<em>2(g) + 3H</em>2(g) \rightleftharpoons 2NH_3(g)$ at 700 K, initial concentrations are:

  • [N2] = 2.05 M, [H2] = 1.56 M, [NH3] = 1.52 M.

• Adding N2 to make [N2] = 3.51 M:

  • Shift occurs to the right due to increased reactant concentration.

  • New K values show changes in reaction quotient.

Worked Example Strategy for Predicting Shift Directions

• Apply Le Châtelier’s principle to determine the effect of concentration changes:

  • Consider: $2H<em>2S(g) + O</em>2(g) \rightleftharpoons 2S(s) + 2H_2O(g)$ .

  • Assess scenarios:

  • Addition/removal of species represented in the expression.

Additional Worked Examples on Volume, Temperature, and Pressure

• Assess shift in equilibrium with changing volume and pressure based on mole counts.

• Recognize how temperature affects both endothermic/exothermic reactions similarly to volume and pressure shifts.

Examples and Predictions

• Given reactions where conditions change (volume, concentration, temperature), predict direction of equilibrium shifts and consequent changes to K values.

• Always remember; changes in volume and concentration do not affect K unless temperature is altered.