Le Chatelier’s Principle- Complete Guide With Examples

What does Le Chatelier’s Principle State?

Henri-Louis Le Châtelier, a French industrial chemist came up with Le Chatelier’s principle, which states that If a system at equilibrium is disturbed by a change in temperature, pressure, or a component concentration, the system will shift its equilibrium position so as to counteract the effect of the disturbance. This means that if there is a change that takes the system away from its’ equilibrium position, it will react in such a way as to come back and restore its equilibrium.

There are three ways a system can be disturbed. These are: changes in concentration, changes in temperature, and changes in pressure/volume.

Effect of Changes In Concentration On The Equilibrium

1. When concentration is increased, equilibrium shifts to the opposite side of the substance whose concentration increased. For example, let’s say we have an equation A (aq) + B(s) ⇌ C(g). If we increase A, where does the equilibrium shift? A is on the left side, so equilibrium will shift to the right side to get rid of A.
2. When concentration is decreased, equilibrium shifts to the same side of the substance whose concentration decreased. For example, let’s say we have an equation A (aq) + B(s) ⇌ C(g). If we decrease A, where does the equilibrium shift? A is on the left side, so equilibrium will shift to left side to make more A.

Changes in solids (s) and liquids(l) DO NOT shift the equilibrium. What happens if we add more B to our equation? No change because B is a solid.

Effect of Changes In Pressure and Volume On The Equilibrium

First of all, volume and pressure are inversely proportional. When volume increases, pressure decreases and vice versa. A question may ask about changes in volume or pressure.
1. When volume is decreased, pressure is increased and equilibrium shifts to the side with less GAS moles. How do we know which side has less moles of gas? We need to look at the coefficients in the equation in front of gaseous (g) substances. For example, let’s say we have an equation A (aq) + B(s) ⇌ C(g). Only C is a gas in this equation as shown by the symbol (g). In the case of our equation, there are 0 moles of gas on the left and 1 on the right. If volume is decreased and pressure increases equilibrium will shift to the left.
2. When volume is increased, pressure is decreased and equilibrium shifts to the side with more gas moles.

Effect of Changes In Temperature On The Equilibrium

In order to determine the effect of changing temperature, we must first figure out what side the temperature is on. If the reaction is endothermic, heat is on the left side. If the reaction is exothermic, heat is on the right side. How do we know whether a reaction is endothermic or exothermic. Either the question will tell us or we will be given ΔH (change in enthalpy). If ΔH is negative, the reaction is exothermic and we should put heat on the right side of the equation. If ΔH is positive, the reaction is endothermic and we should put heat on the left side of the equation.
Once this has been determined, we can treat temperature as one of the reactants or products, depending on which side it is on.

Let’s say we have an equation A (aq) + B(s) ⇌ C(g) with ΔH= -500kJ/mol and we are asked where the equilibrium will shift if the temperature increases. First, you need to figure out what side heat should be on. Since ΔH is negative, the reaction is exothermic and heat should be on the product side. My new equation will be A (aq) + B(s) ⇌ C(g) + heat. Since heat is on the product side, I can treat it as one of the products. If temperature increases, the equilibrium should shift to the opposite side, which is to the left in this case.

Other Factors

There are a couple more things that we need to know about this topic before we can get to practice. These are: addition of catalyst and addition of an inert gas.

Effect of Catalyst On The Equilibrium

A catalyst is a substance that speeds up the rate of a reaction by lowering the activation energy. A catalyst DOES NOT shift or affect the equilibrium.

Effect of Addition of Inert Gas On The Equilibrium

This is a more complex topic that may not appear in your class. You may be asked how the equilibrium will be shifted if an inert (not reactive) gas that is not in your equation is added at either constant volume or constant pressure. Let’s take a look at both of these scenarios.

Inert gas is added under constant volume.
If a new gas is added and the volume of the reaction does not change, then this means the pressure of the reaction mixture does not change either (total pressure increases ). Adding inert gas at constant volume, does not shift the equibrium.

Inert gas is added under constant pressure.
If a new gas is added but the pressure remains constant, this means that the partial pressures of reaction gases decrease. Total pressure is equal to the sum of the partial pressures of all the gases in the mixture. Therefore, we can treat this as a decrease of pressure, and the equilibrium will shift to the side with more moles of gas.

Other Tips When It Comes to LeChatelier’s Principle

If equilibrium shifts to the right, this means products will increase and reactants will decrease.
If equilibrium shifts to the left, this means reactants increase and product decrease.
Equilibrium constant only changes with the changes of temperature.

Le Chatelier’s Principle Practice

4OH^–(aq) + 2Cu²⁺(aq)⇌ O₂(g) + 2H₂O (l) + 2Cu(s) ΔH = +244.3 kJ/mol

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References:
General Chemistry by Ebbing and Gammon
Chemistry The Central Science by Brown, LeMay, Bursten, Murphy, Woodward, Stoltzfus