R2.3.4 Le Châtelier’s principle

Le Châtelier's Principle and Its Applications

Imagine you’re sitting in a crowded room when someone opens a window, letting in a cold draft.
You might instinctively reach for a jacket or move to a warmer spot to counteract the chill.
This natural response to restore comfort mirrors how chemical systems respond to disturbances.

When a system at equilibrium experiences a change in concentration, pressure, or temperature, it adjusts to minimize the disturbance and reestablish equilibrium.

The Principle: Systems Resist Change

Definition

Le Châtelier's Principle

Le Châtelier's Principle states:

"If a dynamic equilibrium is disturbed by a change in the reaction conditions, the system adjusts to counteract the disturbance and restore a new equilibrium."

The “disturbances” can include:

Concentration changes: Adding or removing reactants or products.
Pressure changes: Altering the volume or overall pressure in systems involving gases.
Temperature changes: Heating or cooling the system.

Effect of Concentration Changes

How Does Concentration Affect Equilibrium?

When the concentration of a reactant or product changes, the system shifts to reduce the impact of this disturbance.

Adding Reactants: The system shifts toward the products to consume the excess reactants.
Removing Reactants: The system shifts toward the reactants to replenish the loss.
Adding Products: The system shifts toward the reactants to reduce the excess products.
Removing Products: The system shifts toward the products to compensate for the removal.

Example

Consider the synthesis of ammonia:

$N_{2} (g) + 3 H_{2} (g) ⇌ 2 {NH}_{3} (g)$

At equilibrium, if more hydrogen ( $H_{2}$ ) is added, the forward reaction is favored, producing more ammonia ( ${NH}_{3}$ ) to counteract the disturbance.
The equilibrium shifts to the right.

Common Mistake

Many students incorrectly assume that adding more of a substance will always increase its equilibrium concentration. Remember, the system adjusts to partially counteract the change, so the final concentration may still differ from the initial equilibrium state.

Effect of Pressure Changes

How Does Pressure Affect Equilibrium?

Pressure changes only affect systems with gaseous reactants or products.

Hint

The direction of the shift depends on the number of gas molecules on each side of the reaction.

Increasing Pressure: The system shifts toward the side with fewer gas molecules to reduce pressure.
Decreasing Pressure: The system shifts toward the side with more gas molecules to increase pressure.

Example

In the same ammonia synthesis reaction:

$N_{2} (g) + 3 H_{2} (g) ⇌ 2 {NH}_{3} (g)$

The left-hand side has 4 moles of gas ( $1 N_{2} + 3 H_{2}$ ), while the right-hand side has 2 moles of gas ( $2 {NH}_{3}$ ).

Increasing the pressure shifts the equilibrium to the right, favoring the formation of ammonia.

Tip

Pressure changes haveno effecton systems where the number of gas molecules is the same on both sides of the equation. For example, in $H_{2} (g) + I_{2} (g) ⇌ 2 HI (g)$ , pressure changes do not shift equilibrium.

Effect of Temperature Changes

How Does Temperature Affect Equilibrium?

Temperature changes affect both the position of equilibrium and the value of the equilibrium constant ( $K$ ) because temperature influences reaction spontaneity.

Exothermic Reactions ( $Δ H < 0$ ):
- Heat is a product.
- Increasing temperature shifts equilibrium toward the reactants, reducing $K$ .
Endothermic Reactions ( $Δ H > 0$ ):
- Heat is a reactant.
- Increasing temperature shifts equilibrium toward the products, increasing $K$ .

Example

For the exothermic synthesis of ammonia:

$N_{2} (g) + 3 H_{2} (g) ⇌ 2 {NH}_{3} (g) + Heat$

Raising the temperature causes the equilibrium to shift left (toward reactants), reducing ammonia yield.
Conversely, lowering the temperature favors the forward reaction, increasing ammonia yield.

Analogy

Think of heat as an ingredient in the reaction.
In exothermic reactions, the system "produces" heat, similar to how it produces products.
Adding more heat (raising temperature) is like adding too much of one ingredient—it disrupts the balance, and the system compensates by shifting toward the reactants.

Note

Temperature is theonly factorthat changes the value of $K$ . Concentration and pressure changes only shift the equilibrium position but leave $K$ unchanged.

Summary of Effects on Equilibrium

Change in Condition	Shift in Equilibrium	Effect on $K$
Increase in Reactant Concentration	Toward products	No change
Increase in Product Concentration	Toward reactants	No change
Increase in Pressure (gases)	Toward fewer gas molecules	No change
Increase in Temperature (Exothermic)	Toward reactants	Decreases
Increase in Temperature (Endothermic)	Toward products	Increases

Applications of Le Châtelier's Principle

Industrial Example: The Haber Process

The Haber process for ammonia synthesis is a classic example of Le Châtelier's Principle in action:

$N_{2} (g) + 3 H_{2} (g) ⇌ 2 {NH}_{3} (g) + Heat$

To maximize ammonia yield:

High Pressure: Favors the side with fewer gas molecules (ammonia).
Moderate Temperature: Balances the need for a reasonable reaction rate (higher at higher temperatures) with the equilibrium position (favoring ammonia at lower temperatures).
Removal of Ammonia: Continuously removing ammonia shifts equilibrium to the right, producing more ammonia.

Theory of Knowledge

The Haber process highlights both the benefits and ethical dilemmas of scientific advancements. While it enables large-scale fertilizer production to sustain global food supplies, it also facilitated the production of explosives during wartime. How should scientists balance the potential benefits and harms of their discoveries?

Effect of Changes on the Equilibrium Constant ( $K$ )

Key Insight: $K$ Depends Only on Temperature

Changes in concentration or pressure shift the equilibrium position but do not change $K$ .
Changes in temperature alter $K$ because they affect the relative favorability of the forward and reverse reactions.

Example

For the ammonia synthesis reaction at 475 K, $K = 0.59$ .
If the temperature is increased, the equilibrium shifts left (favoring reactants), and $K$ decreases.
However, doubling the pressure does not change $K$ , even though the equilibrium position shifts toward the products.

Reflection and Broader Implications

Self review

Why does increasing the temperature of an exothermic reaction decrease the equilibrium constant $K$ ?
How does Le Châtelier's Principle explain the effect of removing a product from a reaction mixture at equilibrium?
Why do changes in pressure not affect equilibrium in reactions where the number of gas molecules is the same on both sides?

Theory of Knowledge

Consider the broader applications of equilibrium principles.
How might Le Châtelier's Principle inform our understanding of environmental systems, such as the balance of carbon dioxide in the atmosphere?
What ethical considerations arise when we attempt to manipulate such equilibria?

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R2.3.4 Le Châtelier’s principle

Le Châtelier's Principle and Its Applications

The Principle: Systems Resist Change

Effect of Concentration Changes

How Does Concentration Affect Equilibrium?

Effect of Pressure Changes

How Does Pressure Affect Equilibrium?

Effect of Temperature Changes

How Does Temperature Affect Equilibrium?

Summary of Effects on Equilibrium

Applications of Le Châtelier's Principle

Industrial Example: The Haber Process

Effect of Changes on the Equilibrium Constant (K)

Key Insight: K Depends Only on Temperature

Reflection and Broader Implications

You've read 2/2 free chapters this week.

Introduction to Le Châtelier's Principle

Effect of Changes on the Equilibrium Constant ( $K$ )

Key Insight: $K$ Depends Only on Temperature