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 $$K = \frac{[C]^c [D]^d}{[A]^a [B]^b}$$ for the reaction $$aA + bB \rightleftharpoons cC + dD$$ .
Stress Factors: Changes in concentration, temperature, and pressure can disturb the equilibrium.

2. Key Concepts

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.

3. Examples

Example 1 (Basic)

Problem: Consider the equilibrium reaction $$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$$ . If the concentration of $$H_2$$ is increased, predict the direction of the shift.

Step-by-Step Solution:

Identify the stress: Increase in $$H_2$$ concentration.
Apply Le Châtelier’s Principle: The system will shift to reduce the added $$H_2$$ .
Shift direction: The equilibrium will shift to the right, producing more $$NH_3$$ .

Validation: The equilibrium constant expression is $$K = \frac{[NH_3]^2}{[N_2][H_2]^3}$$ . Increasing $$[H_2]$$ will cause the system to produce more $$NH_3$$ to re-establish equilibrium.

Example 2 (Intermediate)

Problem: For the reaction $$CO(g) + H_2O(g) \rightleftharpoons CO_2(g) + H_2(g)$$ , which is exothermic, predict the effect of increasing the temperature on the equilibrium.

Step-by-Step Solution:

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 $$CO$$ and $$H_2O$$ .

Validation: The equilibrium constant expression is $$K = \frac{[CO_2][H_2]}{[CO][H_2O]}$$ . Increasing temperature will favor the reverse reaction, reducing the concentrations of $$CO_2$$ and $$H_2$$ .

4. Problem-Solving Techniques

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.