Lesson 3: LeChâtelier’s Principle

LeChâtelier’s Principle (LCP) states that if a system at equilibrium is “stressed,” the system will respond to decrease the effects of that stress. A stress could be a change in any of the factors that affect equilibrium: concentration, pressure, or temperature.

Increasing the concentration of a substance on one side of a chemical equation will shift the reaction toward the other side of the equation.

The physical properties of gases are all essentially the same. The Ideal Gas Law relates the pressure (P), volume (V), moles of gas (n), and temperature (T). It is written as the following:

PV = nRT

For our lessons, we’re only concerned about P, V, and n. As V increases, P decreases, and vice versa. As n increases, P increases, and the opposite is true as well.

Increasing pressure in a chemical reaction will shift the reaction toward the side with the least moles of gas. Decreasing pressure will shift the reaction toward the side with the most moles of gas.

When thinking about the effects of temperature on equilibrium, it’s helpful to think of heat as being part of the chemical equation. It would be placed on the products side of the equation in an exothermic reaction (ΔH < 0), and it would be on the reactants side in an endothermic reaction (ΔH > 0). When the temperature of a reaction increases, the system shifts away from the side with the heat. When the temperature of a reaction decreases, the system shifts toward the side with the heat.

Side note about temperature: temperature changes the equilibrium constant (K). This may affect the math related to the equilibrium, but the method stated above is all we need to find the qualitative details of the change.