Le Chatelier's Principle

Le Chatelier's Principle states that if a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the system will adjust itself to counte\fract that change and establish a new equilibrium.

Key Concepts

• Equilibrium: A state where the rates of the forward and reverse reactions are equal.
• Dynamic Nature: Even at equilibrium, reactions continue to occur, but there is no net change in concentrations.
• Stress Factors: Changes in concentration, temperature, or pressure that can shift the position of equilibrium.

Mathematical Representation

For a general reaction: ${aA} + bB \rightleftharpoons cC + {dD}$ The equilibrium constant, $K_c$, is given by: $K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$

Effects of Changes

1. Change in Concentration:

   • Adding a reactant shifts equilibrium to the right (producing more products).
   • Removing a product shifts equilibrium to the left (producing more reactants).

2. Change in Temperature:

   • For an exothermic reaction, increasing the temperature shifts equilibrium to the left (favoring reactants).
   • For an endothermic reaction, increasing the temperature shifts equilibrium to the right (favoring products).

3. Change in Pressure:

   • Increasing the pressure shifts equilibrium towards the side with fewer moles of gas.
   • Decreasing the pressure shifts equilibrium towards the side with more moles of gas.

Example

Consider the equilibrium reaction: ${N}_{2}(g) + 3 {H}_{2}(g) \rightleftharpoons 2 {NH}_{3}(g)$

• If we increase the concentration of ${H}_{2}$, the system will shift to the right to produce more ${NH}_{3}$.
• If we decrease the temperature of an exothermic reaction, it will also shift to the right to produce more products.

Key Questions

• How does changing the concentration of reactants affect the position of equilibrium?

• What happens to the equilibrium when the temperature is increased in an exothermic reaction?
• How does a change in pressure influence gaseous equilibria?

Summary

Le Chatelier's Principle provides a framework for predicting how changes in conditions affect chemical equilibria, allowing chemists to manipulate reactions for desired outcomes.