Le Chatelier's Principle Examples Cheat Sheet

Key Facts

• Le Chatelier's Principle states that if a stress is applied to a system at equilibrium, the system shifts in the direction that reduces that stress.
• For $aA + bB \rightleftharpoons cC + dD$, the equilibrium constant is $K = \frac{[C]^c[D]^d}{[A]^a[B]^b}$.
• Adding a reactant shifts equilibrium toward products, while removing a reactant shifts equilibrium toward reactants.
• Adding a product shifts equilibrium toward reactants, while removing a product shifts equilibrium toward products.
• Increasing pressure by decreasing volume shifts a gas equilibrium toward the side with fewer moles of gas.
• Decreasing pressure by increasing volume shifts a gas equilibrium toward the side with more moles of gas.
• For an exothermic reaction written as $\text{reactants} \rightleftharpoons \text{products} + \text{heat}$, increasing temperature shifts equilibrium toward reactants.
• A catalyst speeds up the forward and reverse reactions equally, so it does not change $K$ or the equilibrium position.

Vocabulary

Chemical equilibrium

A state in which the forward and reverse reaction rates are equal, so the concentrations of reactants and products remain constant.

Le Chatelier's Principle

A rule that predicts how an equilibrium system shifts when concentration, pressure, volume, or temperature is changed.

Equilibrium constant

The value $K$ that compares product concentrations to reactant concentrations at equilibrium for a specific temperature.

Stress

A change to an equilibrium system, such as adding a substance, removing a substance, changing pressure, or changing temperature.

Exothermic reaction

A reaction that releases heat, so heat can be treated as a product in equilibrium shift predictions.

Endothermic reaction

A reaction that absorbs heat, so heat can be treated as a reactant in equilibrium shift predictions.