Le Châtelier’s Principle - Chemistry

1. Fundamental Concepts

Definition: Le Châtelier’s Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium will shift to counteract the change.
Equilibrium Constant (K): A value that describes the ratio of products to reactants at equilibrium, given by for the reaction .
Stress Factors: Changes in concentration, temperature, and pressure can disturb the equilibrium.

Three main factors affecting equilibrium: concentration, pressure, temperature. Catalysts affect reaction rate only and do not shift equilibrium.
Concentration: Increasing reactant concentration or decreasing product concentration shifts equilibrium forward; the reverse shifts it backward.
Pressure: Only effective for reactions involving gases with unequal moles of gas on both sides. Increasing pressure shifts equilibrium toward the side with fewer gas moles.
Temperature: Increasing temperature shifts equilibrium in the endothermic direction; decreasing temperature shifts it in the exothermic direction.

Problem: Consider the equilibrium reaction . If the concentration of is increased, predict the direction of the shift.

Validation: The equilibrium constant expression is . Increasing will cause the system to produce more to re-establish equilibrium.

Problem: For the reaction , which is exothermic, predict the effect of increasing the temperature on the equilibrium.

Identify the stress: Increase in temperature.
Apply Le Châtelier’s Principle: For an exothermic reaction, increasing temperature shifts the equilibrium to the left (towards reactants).
Shift direction: The equilibrium will shift to the left, producing more and .

Validation: The equilibrium constant expression is . Increasing temperature will favor the reverse reaction, reducing the concentrations of and .

ICE Table Method: Use Initial, Change, Equilibrium (ICE) tables to track changes in concentrations.
Le Châtelier’s Principle Application: Identify the type of stress (concentration, temperature, pressure) and predict the direction of the shift based on the principle.
Graphical Analysis: Plot the changes in concentration over time to visualize the shift in equilibrium.