Le Chatelier's Principle

Introduction to Le Chatelier's Principle

  • Le Chatelier's Principle explains how a system at equilibrium reacts to external changes.

  • The principle states that if an equilibrium system is subjected to a change, it will adjust to counteract that change.

Basic Concept

  • Consider a system at equilibrium represented by a straight line:

    • If a stress is applied (e.g., increase or decrease), the system shifts to restore balance.

    • Examples:

      • Increase: If the line is raised, the system tries to bring it back down.

      • Decrease: If the line is lowered, the system attempts to bring it back up.

Dynamic vs. Static Equilibrium

  • Equilibrium in chemical reactions isn’t static; it's dynamic.

  • In dynamic equilibrium, the forward and reverse reaction rates are equal, resulting in constant concentrations of products and reactants.

Chemical Reaction Example

  • Example Reaction: A + 2B ⇌ C + D

    • A and B are reactants, C is a product (gas), and D is a product (liquid).

    • Coefficients are 1 (A), 2 (B), 1 (C), 1 (D).

Applying Stress to the System

  • Increasing Concentration of Reactant B:

    • Shifts the equilibrium to the right (towards products) to decrease B concentration.

  • Decreasing Concentration of Reactant B:

    • Shifts the equilibrium to the left (towards reactants) to increase B concentration.

  • Increasing Concentration of Product C:

    • Shifts the equilibrium to the left (towards reactants) to decrease C concentration.

  • Decreasing Concentration of Product C:

    • Shifts the equilibrium to the right (towards products) to increase C concentration.

Effects of Changing Solid and Liquid Concentrations

  • Addition of Reactant A:

    • No effect on equilibrium position as A is a solid.

  • Addition of Product D:

    • Similar to A, adding a liquid does not affect equilibrium since it's not a gas or dissolved in a solution.

Actions Affecting Equilibrium Shifts

  1. Increasing Reactants (e.g., B or A): Shifts to the right.

  2. Decreasing Reactants: Shifts to the left.

  3. Increasing Products: Shifts to the left.

  4. Decreasing Products: Shifts to the right.

  5. Adding a Catalyst: Speeds up both forward and reverse reactions without affecting equilibrium position.

Specific Problems

Problem 1: Shifting Direction Analysis (N₂ + 3H₂ ⇌ 2NH₃)

  • Increasing Nitrogen Gas ( 2): Shifts right (eliminated choice A).

  • Removing NH3: Shifts right to increase NH3 (eliminated choice B).

  • Adding a Catalyst: No shift (eliminated choice C).

  • Removing H2: Shifts left to produce more H2 (correct choice D).

Problem 2: Decreasing CO Concentration (CO + 3H₂ ⇌ CH₄ + H₂O)

  • Adding CH4: Shifts left (not valid).

  • Removing H2: Shifts left (not valid).

  • Increasing H2: Shifts right (valid choice C).

  • Adding Inert Gas: No effect (disregarded).

Problem 3: Removing O2 (2SO₂ + O₂ ⇌ 2SO₃)

  • Removes O2: Shifts left to increase O2 concentration and partial pressure of SO2 will increase.

  • Statement checks:

    • 1st statement: The reaction will shift to the right — (false)

    • 2nd statement: The reaction will shift to the left — (true)

    • 3rd statement: The concentration of SO3 will increase — (false)

    • 4th statement: The partial pressure of SO2 will increase — (true)

Effects of Volume Changes on Equilibrium

  • Increasing Volume: Decreases pressure; system shifts to the side with more gas moles to increase pressure.

  • Decreasing Volume: Increases pressure; shifts to the side with fewer gas moles to decrease pressure.

Summary of Relationships

  • Pressure and Volume: Inversely related (increased volume decreases pressure and vice versa).

  • Mole Relationships: Shifting direction affects concentrations:

    • Increase in pressure leads to a shift toward fewer moles of gas.

    • A shift towards a product indicates an increase in product concentrations and a decrease in reactants at equilibrium.