8.8: Le Châtelier’s Principle

 Learning Objectives

• Define Le Châtelier's principle.
• Predict the direction of shift for an equilibrium under stress with changes in concentrations.

Once equilibrium is established, the reaction is over, right? Not exactly. An experimenter has some ability to affect the equilibrium.

Chemical equilibria can be shifted by changing the conditions that the system experiences. We say that we "stress" the equilibrium. When we stress the equilibrium, the chemical reaction is no longer at equilibrium, and the reaction starts to move back toward equilibrium in such a way as to decrease the stress. The formal statement is called Le Châtelier's principle: If an equilibrium is stressed, then the reaction shifts to reduce the stress.

There are several ways to stress an equilibrium. One way is to add or remove a product or a reactant in a chemical reaction at equilibrium.

When additional reactant is added, the equilibrium shifts to reduce this stress: it makes more product.

When additional product is added, the equilibrium shifts to reactants to reduce the stress: it makes more reactant.

If reactant or product is removed, the equilibrium shifts to make more reactant or product, respectively, to make up for the loss.

To make more product, the rate of the forward reaction increases. To make more reactant, the rate of the reverse reaction increases.

Example \(\PageIndex{1}\)

Given this reaction at equilibrium:

\[N_{2}+3H_{2}\rightleftharpoons 2NH_{3}\nonumber \]

In which direction—toward reactants or toward products-—does the reaction shift if the equilibrium is stressed by each change?

1. H₂ is added.
2. NH₃ is added.
3. NH₃ is removed.

Solution

1. If H₂ is added, there is now more reactant, so the reaction will shift toward products to reduce the added H₂. (Rate of forward reaction increases/shifts right)
2. If NH₃ is added, there is now more product, so the reaction will shift toward reactants to reduce the added NH₃. (Rate of reverse reaction increases/shifts left)
3. If NH₃ is removed, there is now less product, so the reaction will shift toward products to replace the product removed. (Rate of forward reaction increases/shifts right)

Exercise \(\PageIndex{1}\)

Given this reaction at equilibrium:

\[CO(g)+Br_{2}(g)\rightleftharpoons COBr_{2}(g)\nonumber \]

In which direction—toward reactants or toward products—does the reaction shift if the equilibrium is stressed by each change?

1. Br₂ is removed.
2. COBr₂ is added.

Answers

1. toward reactants (Rate of reverse reaction increases/shifts left)
2. toward reactants (Rate of reverse reaction increases/shifts left)

It is worth noting that when reactants or products are added or removed, the value of the K_eq does not change. The chemical reaction simply shifts, in a predictable fashion, to reestablish concentrations so that the K_eq expression reverts to the correct value.

A catalyst is a substance that increases the speed of a reaction. Overall, a catalyst is not a reactant and is not used up, but it still affects how fast a reaction proceeds by lowering the activation energy. When a catalyst is used in an equilibrium equation the lowered activation energy speeds up both the forward and reverse reactions. The overall result is that a catalyst has no effect on an equilibrium or on the value of K_eq.

Chemistry is Everywhere: Equilibria in the Garden

Hydrangeas are common flowering plants around the world. Although many hydrangeas are white, there is one common species (Hydrangea macrophylla) whose flowers can be either red or blue, as shown in the accompanying figure. How is it that a plant can have different colored flowers like this?

Figure \(\PageIndex{1}\) Garden Equilibria © Thinkstock. This species of hydrangea has flowers that can be either red or blue. Why the color difference?

Interestingly, the color of the flowers is due to the acidity of the soil that the hydrangea is planted in. An astute gardener can adjust the pH of the soil and actually change the color of the flowers. However, it is not the H⁺ or OH⁻ ions that affect the color of the flowers. Rather, it is the presence of aluminum that causes the color change.

The solubility of aluminum in soil, and the ability of plants to absorb it, is dependent upon the acidity of the soil. If the soil is relatively acidic, the aluminum is more soluble, and plants can absorb it more easily. Under these conditions, hydrangea flowers are blue, as Al³⁺ ions interact with anthocyanin pigments in the plant. In more basic soils, aluminum is less soluble, and under these conditions the hydrangea flowers are red. Gardeners who change the pH of their soils to change the color of their hydrangea flowers are therefore employing Le Chatelier's principle: the amount of acid in the soil changes the equilibrium of aluminum solubility, which in turn affects the color of the flowers.

Key Takeaways

• Le Chatelier's principle addresses how an equilibrium shifts when the conditions of an equilibrium are changed.
• Catalysts do not affect the position of an equilibrium; they help reactions achieve equilibrium faster.