Textbook 17.6

Reaction Conditions and Equilibrium: Le Châtelier's Principle

• Equilibrium Definition: Once a reaction reaches equilibrium, it remains until conditions change.

• Le Châtelier's Principle: States that if a system at equilibrium is disturbed, it shifts to counteract the disturbance and reattain equilibrium.

Key Concepts in Le Châtelier's Principle

1. Disturbance of a System:

   • At equilibrium, the reaction quotient (Q) equals the equilibrium constant (K). A disturbance occurs when conditions change, resulting in Q not equal to K.

   • Common disturbances include:

     • Change in concentration

     • Change in pressure (via volume change)

     • Change in temperature

2. Shifts in Equilibrium Position:

   • Shift to the Right: Conversion of reactants to products.

   • Shift to the Left: Conversion of products back to reactants.

Changes in Concentration

Impact of Concentration Changes

• An increase in concentration of a component causes the system to shift in a direction that consumes that component.

• A decrease in concentration results in the system producing more of that component.

• Pure solids and liquids do not impact Q.

Example: PC13 and Cl2 Reaction

Reaction: PC13(g) + Cl2(g) ⇌ PCl5(g)

• Initial Equilibrium: Q = Kc

Effects of Concentration Changes

1. Adding Reactant (Cl2):

   • Increases [Cl2], hence decreases Qc (Qc < Kc).

   • Reaction shifts to the right, producing more PCl5 until equilibrium is restored.

   • New equilibrium concentrations: higher [Cl2] and [PCl5], lower [PC13].

2. Removing Reactant (PC13):

   • Decreases [PC13], increasing Qc (Qc > Kc).

   • Reaction shifts to the left, producing more PC13 until equilibrium is restored.

3. Adding/Removing Product (PCl5):

   • Adding PCl5 shifts equilibrium left to consume it.

   • Removing PCl5 shifts equilibrium right to produce more.

Effects of Pressure (Volume Changes)

• Changes in pressure significantly affect gaseous equilibrium systems:

  • Changing Volume:

    1. Volume Reduces (Pressure Increases): Reaction shifts to the side with fewer gas moles.

    2. Volume Increases (Pressure Decreases): Reaction shifts to the side with more gas moles.

• Inert Gas Addition: Has no effect on equilibrium position as it does not participate in the reaction.

Effects of Temperature Change

• Temperature Impact on K:

  • Only temperature changes alter the value of K.

  • Exothermic Reaction: Heat released, shift left with temperature increase (decreases K).

  • Endothermic Reaction: Heat absorbed, shift right with temperature increase (increases K).

The van't Hoff Equation

• Relationship: K2 = K1 * e^((ΔH°/R)(1/T1 - 1/T2))

  • K1 and K2 are equilibrium constants at T1 and T2.

  • ΔH°: change in enthalpy of reaction;

  • R: gas constant;

  • Demonstrates how temperature affects K, depending on reaction being exothermic or endothermic.

Catalyst Effects

• Catalysts speed up reactions by lowering activation energy but do not affect the equilibrium position or the value of K.

Summary of Key Points

• Disturbances to a system will cause shifts to maintain equilibrium.

• Changes in concentrations impact equilibrium and how K remains constant despite these alterations.

• Temperature alters K and affects equilibrium direction depending on reaction type.

• Catalysts enhance reaction rates without altering equilibrium coordinate.

Industrial Applications: Ammonia Synthesis

• Haber Process: Produces ammonia (NH3) with emphasis on utilizing Le Châtelier's principle:

  • Gaseous Reaction: N2(g) + 3H2(g) ⇌ 2NH3(g);

  • Exothermic heat release (

  • Maximizing yield through removal of NH3, high pressure, and reduced temperature.

  • Compromise conditions optimize yield and reaction rate using a catalyst.