RRE - Le Chatelier's Principle (Lesson)

A stress is any outside influence that can affect an equilibrium system. When you are stressed, what do you do? You do something to alleviate that stress! Chemical reactions in equilibrium do the same thing. A stress to chemical equilibrium is anything that takes that system out of equilibrium. We will discuss several types of stresses on this reaction in a state of equilibrium:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Adding or Removing a Reactant or Product

Adding a chemical to a system in equilibrium is a stress to that system because it moves it out of equilibrium. To re-establish equilibrium, the system must do something to remove the stress, in this case the added chemical.

Let's consider the addition of hydrogen as a stress to the system. To get rid of some of the extra hydrogen, the reaction shifts away from the hydrogen. The forward reaction converts hydrogen into ammonia, so the equilibrium is said to shift to the right.

Don't memorize that adding a chemical will cause the equilibrium to shift to the right because this is not always true. Instead, memorize that when a chemical is added, the equilibrium will shift away from that chemical. The reverse is true when a chemical is removed. Note that chemicals in the system can be removed directly or can be removed by the addition of another chemical that will react with it.

As we learn new stresses, we will add to this chart. So, go ahead - make your own copy and write in the other stresses.

Once you have established the direction of the shift, you can then determine the result of the shift. Here, the [N₂] will decrease because it is reacting with the extra hydrogen. The [NH₃] will increase. The concentrations of reactants and products will have changed when equilibrium is re-established, but the reaction quotient, Q will equal K_c again.

Question: N₂(g)+3H₂(g) ⇌2NH₃(g); Predict the direction of the shift when nitrogen is removed. When equilibrium is re-established, how will the concentrations of the hydrogen and ammonia be changed?

Solution: The system will shift to the left to replace the lost nitrogen. The concentration of the hydrogen will increase and the ammonia will decrease.

Changing the Volume of a Gaseous System

When the volume of mixture of gases is decreased, the pressure of the system increases. The system can relieve this stress if it is able to decrease the number of gas molecules in the container. To determine the direction of the shift, look at the number of moles of gas reactants and the number of moles of gas products. The shift will move toward the side with fewer gas particles. If the volume increases, the pressure decreases. So, the system will shift toward the side with more gas molecules. Remember that changes in volume or pressure only affect gases. Do not consider other reactants or products when determine the direction of the shift for these stresses.

In our example, there are 4 moles of gas reactants (1 mole N_2(g) + 3 moles H_2(g)). There are 2 moles of gas product (2NH_3(g)). So, if the volume of this system is decreased, the reaction will shift toward the right because that is where there are fewer gas particles.

When the volume is decreased this reaction shifts to the right.

Add these to your chart of stresses and shifts.

Question: H₂(g)+I₂(g)⇌2HI(g); Predict the direction of the shift when the volume is increased.

Solution: The system cannot shift since there are 2 reactant molecules and 2 product molecules.

Addition of an Inert Gas

When an inert gas is added to a system in equilibrium at constant volume, the total pressure will increase, but the total pressure does not have any effect on the equilibrium. Only changes in partial pressures that will cause a shift in the equilibrium. This is because the ratio of their moles to the volume of the container will not change. If the volume is allowed to increase in the process, the partial pressures of all gases would be decreased resulting in a shift towards the side with the greater number of moles of gas.

Add this to your chart of stresses and shifts.

Changing the Temperature of a System

To know how a reaction will change respond to temperature changes, you must know if the reaction is endothermic or exothermic. Our example reaction is exothermic.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = - 46kJ

The easiest way to deal with this is to rewrite the equation, including heat as a reactant or product. An exothermic reaction, negative Δ H, the reactions give off, or produce heat. So the reaction can be rewritten as:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + heat

Now you can treat the stress the same way you would treat the addition or removal of any reactant or product. Think of an increase in temperature as heat being added to the system. Recall that when something is added, the system shifts away from that addition.

You also need to be able to comment on the direction of a shift as a result of temperature change of the system in terms of the endothermic or exothermic reaction. Let's do this for our example above. The reaction was exothermic. The temperature of the system was increased and as a result, the system shifted to the left (or reverse reaction). This reverse reaction is endothermic. So, we can make the conclusion that when temperature increases, the system will shift in the direction of the endothermic reaction. If you don't remember this, just analyze the problem like we did in the example above. Then make the conclusion in terms of endothermic or exothermic directions.

Add these to your chart of stresses and shifts.

Changing the temperature of a system is the only stress that changes the value of the equilibrium constant, K_c.