Le Chatelier's Principle

Learning Targets

  • Understanding Le Chatelier's Principle: Aimed to outline the principles governing reactions at equilibrium and how they respond to various stresses.

  • Determining Reaction Direction: Identify the direction of the reaction (towards reactants or products) when subjected to specific changes:

    • Changing Concentrations: Addition or removal of reactants/products.

    • Changing Temperature: Impacts based on reaction type (exothermic or endothermic).

    • Changing Pressure: Adjustments via changes in volume.

Le Chatelier's Principle

  • Defined as: “If a system at equilibrium is disturbed by a change in temperature, pressure, or concentration of one of the components, the system will shift its equilibrium position to counteract the disturbance.”

  • Key Factors Influencing Equilibrium:

    • Changes in concentration.

    • Changes in temperature.

    • Changes in pressure.

Effects of Changes on the System

Concentration

  • Adding Component:

    • Reaction proceeds away from the side where the component was added.

    • Example: In the reaction N₂(g) + 3H₂(g) ↔ 2NH₃(g), adding nitrogen shifts to the right (towards products).

  • Removing Component:

    • Reaction proceeds towards the side where the component was removed.

    • Example: Removing ammonia shifts to the left (towards reactants).

  • Expressions:

    • Adding gaseous or aqueous reactants:

    • Reaction proceeds to the right:

    • Conditions: Q = \frac{[products]}{[reactants]}, K > Q

    • Adding gaseous or aqueous products:

    • Reaction proceeds to the left:

    • Conditions: Q < K

    • Removing gaseous or aqueous reactants:

    • Reaction shifts left: K < Q

    • Removing gaseous or aqueous products:

    • Reaction shifts right: K > Q

Temperature

  • Influence on Equilibrium Constant (K):

    • Endothermic Reactions: Heat is a reactant.

    • Increasing temperature increases K (more products formed).

    • Exothermic Reactions: Heat is a product.

    • Increasing temperature decreases K (more reactants formed).

  • Reaction Direction Based on Temperature Changes:

    • System moves to the side opposite to where heat is added and to the side with heat when removed.

  • Examples:

    • For the reaction N₂(g) + 3H₂(g) ↔ 2NH₃(g) + Heat:

    • Increasing temperature results in K decrease (shifts left).

    • Decreasing temperature results in K increase (shifts right).

Pressure

  • General Principles:

    • Increasing Pressure: Shifts to the side with fewer gaseous moles.

    • Decreasing Pressure: Shifts to the side with more gaseous moles.

    • The effect of pressure changes is examined by analysing the number of moles of gases on both sides of the equation.

    • Impact of Volume Changes:

    • Changing volume directly influences pressure; Boyle’s Law applies.

    • Inert Gas Addition: Does not affect the equilibrium position as it does not change the concentration of reactants or products.

  • Example:

    • In the reaction N₂(g) + 3H₂(g) ↔ 2NH₃(g):

    • Increased pressure → shifts to right (fewer moles).

    • Decreased pressure → shifts to left (more moles).

The Haber Process

  • Overview:

    • Reaction: N₂(g) + 3H₂(g) ↔ 2NH₃(g)

    • Important for ammonia production used in fertilizers.

  • Process Description:

    • Incoming N₂ and H₂ gases are reacted under high pressure.

    • Heated to approximately 500°C in the presence of a catalyst.

    • NH₃(g) is produced and then cooled to a liquid form.

    • Key Observations:

    • Percent of NH₃ increases with increasing pressure.

    • Percent of NH₃ decreases with increasing temperature.

Catalysts

  • Role in Reactions:

    • Catalysts lower activation energy for both forward and reverse reactions, enhancing the rate at which equilibrium is reached.

  • Equilibrium Effects:

    • Introduction of a catalyst does not shift the equilibrium position or change the equilibrium constant (K); it only accelerates the approach to equilibrium.

Graphical Analysis of Equilibrium Changes

General Graphs

  • Graphs in LeChat’s Analysis:

    • Variations observed during temperature and concentration changes can be documented graphically.

    • Left Graph: Represents gradual temperature changes.

    • Middle Graph: Represents sudden concentration changes.

    • Right Graph: Represents sudden changes in pressure or volume.

Example Scenarios

  • Example 1:

    • Given the equilibrium: H₂(g) + I₂(g) ⇌ 2HI(g), adding extra I₂ affects the curve behavior as the system responds to restore equilibrium.

  • Example 2:

    • Reaction: H₂(g) + I₂(g) ⇌ 2HI(g).

    • Time to reach equilibrium: 10 seconds; Kc calculated post-equilibrium.

    • Removal of HI at 20 seconds alters the system.

    • Increasing temperature at 35 seconds causes shifts based on exothermic or endothermic classification, resulting in shifts in Kc.

Application of Equilibrium Principles

  • Example 3: Identifying disturbances from a specified time graph.

    • Evaluating shifts related to specific disturbances (e.g., CO removal, temperature or pressure changes).

  • Example 4: Change due to temperature adjustments and the forward reaction rates until equalization.

  • Example 5: Addition of a catalyst at t = 10 minutes leads to increases in both forward and reverse rates, demonstrating equal acceleration effects on equilibrium.