Practical Application: Using LeChatelier's Principle

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Understanding how the equilibrium will shift in a reaction can be confusing, but thankfully we have Le Chatelier's Principle to help us out. This lesson will explore this principle and then go through some practice problems.

Le Chatelier's Principle

Let's start out with some peanut butter and jelly chemistry. Take a look at the equation:


PBJ


In this reaction, peanut butter and jelly combine to form a peanut butter and jelly sandwich, which we abbreviated PBJ. The peanut butter and jelly are reactants and the PBJ is a product.

Note the double arrows, which show the reaction is at equilibrium, meaning the reaction can move in both the forward and in the reverse direction, is occurring at the same time, and at the same rate.

In other words, imagine a bunch of peanut butter and jelly combining to make PBJ sandwiches. At the same time, some of the PBJ sandwiches are being dismantled and returned to peanut butter and jelly.

Understanding equilibrium can be confusing, even with PBJ. Fortunately, there are some rules to help us, including Le Chatelier's Principle, which says that if the reaction experiences some change, whether that's a temperature change, a pressure change, or a concentration change, the equilibrium will shift to counteract that change.

For example, let's add some extra peanut butter to the reactant side of our reaction. What do you think will happen?

For starters, since there's more peanut butter, the chances some peanut butter will find some jelly increases. This results in the equilibrium being pushed to the product side, resulting in more peanut butter and jelly sandwiches. But, since peanut butter reacts with jelly, this will reduce the amount of jelly.

PBJ chemistry is fun, but let's explore this whole thing with real reactants and products.

Concentration

From the PBJ example, we have some inkling on how concentration will impact a reaction. If the concentration of a reactant or a product is increased, then the equilibrium will shift to the other side of the reaction. For example, if a reactant is increased, the equilibrium will shift towards the products.

If a reactant or a product is decreased, the equilibrium will shift to balance that out. For example, if a reactant is reduced, the equilibrium will shift towards the reactant side.

Problem 1: Concentration

For the following reaction:


Problem 1


A. What happens to the equilibrium if there is an increase in the concentration of H2

B. What happens to the equilibrium if there is a decrease in the concentration of H2

  • Step 1: Determine if the concentration was increased or decreased. For part A, it was increased on the reactant side and for part B, it was decreased on the reactant side.
  • Step 2: Use Le Chatelier's Principle to determine how it impacts equilibrium. For part A, since it was increased on the reactant side, the equilibrium will shift toward the product side. For part B, we know the concentration was reduced on the reactant side, so the equilibrium will shift towards the reactant side.

Temperature

In temperature problems, determining how the equilibrium will shift is dependent upon if the reaction is endothermic, where energy is absorbed, or exothermic, where energy is released. For simplicity, note that:

  • Exothermic: reactant ---> product + heat
  • Endothermic: reactant + heat --> product

You can think of 'heat' as an additional product or reactant, and when the temperature is increased, imagine that the 'heat' part of the equation is increasing. Think back to the concentration problems we just did, and you can determine how the equilibrium will shift (i.e. increasing the temperature will increase the 'concentration' of heat).

Remember, we know that increasing a reactant or product will make the equilibrium shift to the other side of the reaction. For example, if we increase the temperature in an exothermic reaction, the equilibrium will shift to the reactants. If the temperature is reduced, you can think of the 'heat' being reduced and therefore shifting the equilibrium to balance that. For example, if the temperature is reduced in an endothermic reaction, the equilibrium will shift towards the reactant.

Problem 2: Temperature

For the following reaction:


Problem 2


A. What will happen if the temperature is increased?

B. What will happen if the temperature is decreased?

  • Step 1: Determine if the reaction is endothermic or exothermic. Here, heat is on the product side so the reaction released heat, or is exothermic.
  • Step 2: For part A, since it is exothermic, an increase in temperature will cause the equilibrium to shift towards the reactants. For part B, since the temperature is decreasing in an exothermic reaction, the equilibrium will shift towards the product.

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