KEY2CHEM

Le Chatelier's Principle

When a chemical system at equilibrium is subjected to a stress (which could be a change in concentration, pressure/volume in a gas phase system, or temperature), the system’s equilibrium position will shift in the direction away from the stress. This allows the system to re-attain equilibrium.

Example 1.

The reaction $$\require{mhchem}\ce{A(aq) + B(aq) <=> C(aq) + D(aq)}$$ is at equilibrium. If more $$A$$ is added, what will happen to the equilibrium position?

A. It will shift to the right (toward products).

B. It will shift to the left (toward reactants).

C. It will not change.

Solution

A. It will shift to the right (toward products).

The stress (disturbance) in this system is increasing the concentration of $$A$$ (a reactant). The equilibrium position will shift to the right, to consume the excess $$A$$ and convert it into additional products. This will allow the system to achieve equilibrium again.

Example 2.

For the gas phase reaction $$\require{mhchem}\ce{2 NO(g) + O2(g) <=> 2 NO2(g)}$$, what is the effect of decreasing the volume of the reaction vessel on the equilibrium position?

A. It will shift to the right (toward products).

B. It will shift to the left (toward reactants).

C. It will not change.

Solution

A. It will shift to the right (toward products).

Decreasing the reaction vessel increases the pressure of the reaction mixture. This is a stress on the system, so the system will compensate by attempting to decrease pressure. In a gas phase reaction, this is done by decreasing the number of moles of gas. In the reaction below, this is a shift toward the product side (the side with fewer moles of gas).

Example 3.

The reaction $$\require{mhchem}\ce{2 NO2(g) <=> N2O4(g)}$$ is exothermic. What will happen to the equilibrium position if the temperature is increased?

A. It will shift to the right (toward products).

B. It will shift to the left (toward reactants).

C. It will not change.

Solution

B. It will shift to the left (toward reactants).

Since the reaction is exothermic, heat is produced as a product. As a thermochemical equation, the reaction can be written as $$\require{mhchem}\ce{2 NO2(g) <=> N2O4(g) + heat}$$. Adding heat (by increasing the temperature) will shift the equilibrium position to the left (toward reactants).