Chemists often want to know what actions they can take to control the relative amounts of reactants or products in an equilibrium system. LeChatelier’s Principle gives us a way to make qualitative predictions about equilibrium changes.
LeChatelier’s Principle
When a stress is applied to a system in equilibrium, the system undergoes a change in direction to counteract the stress.
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.
When H2 is added, this reaction shifts to the right (away from the H2).
N2(g) + 3H2(g) ⇌ 2NH3(g)
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 and write in the other stresses.
| Stress | System will shift |
|---|---|
| Addition of a chemical | Away from that chemical |
| Removal of a chemical | Toward that chemical |
| Decrease in volume | |
| Increase in volume | |
| Addition of inert gas | |
| Increase in temperature | |
| Decrease in temperature | |
| Addition of catalyst |
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 Kc again.
A shift to the right increases [products] and decreases [reactants].
A shift to the left increases [reactants] and decreases [products].
THINK ABOUT IT!
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 hydrogen and ammonia be changed?
Click to reveal the answer.
The system will shift to the left to replace the lost nitrogen.
The concentration of hydrogen will increase and 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 N2(g) + 3 moles H2(g)). There are 2 moles of gas product (2NH3(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
4 moles gas → 2 moles gas
1N2(g) + 3H2(g) ⇌ 2NH3(g)
Add these to your chart of stresses and shifts.
| Stress | System will shift |
|---|---|
| Addition of a chemical | Away from that chemical |
| Removal of a chemical | Toward that chemical |
| Decrease in volume | Toward side with fewer gas molecules |
| Increase in volume | Toward side with more gas molecules |
| Addition of inert gas | |
| Increase in temperature | |
| Decrease in temperature | |
| Addition of catalyst |
THINK ABOUT IT!
H2(g) + I2(g) ⇌ 2HI(g)
Predict the direction of the shift when the volume is increased.
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.
| Stress | System will shift |
|---|---|
| Addition of a chemical | Away from that chemical |
| Removal of a chemical | Toward that chemical |
| Decrease in volume | Toward side with fewer gas molecules |
| Increase in volume | Toward side with more gas molecules |
| Addition of inert gas | No shift |
| Increase in temperature | |
| Decrease in temperature | |
| Addition of catalyst |
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 D 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.
In our example, the heat is a product. When the temperature of the system is increased, the system will shift away from the heat. So this will shift to the left.
added
N2(g) + 3H2(g) ⇌ 2NH3(g) + heat
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.
| Stress | System will shift |
|---|---|
| Addition of a chemical | Away from that chemical |
| Removal of a chemical | Toward that chemical |
| Decrease in volume | Toward side with fewer gas molecules |
| Increase in volume | Toward side with more gas molecules |
| Addition of inert gas | No shift |
| Increase in temperature | Toward the direction of the endothermic reaction
(Away from the side with heat – when heat is written in as a reactant or product) |
| Decrease in temperature | Toward the direction of the exothermic reaction
(Toward from the side with heat – when heat is written in as a reactant or product) |
| Addition of catalyst |
Changing the temperature of a system is the only stress that changes the value of the equilibrium constant, Kc.
How does the value of Kc change as temperature changes?
To figure this out, write out the equilibrium expression for our sample reaction. Also, write the equation with heat included in the appropriate place. Then, think about how an increase in temperature or a decrease in temperature will affect Kc.
Click to reveal the answer.
N2(g) + 3H2(g) ⇌ 2NH3(g) + heat
When temperature increases, this reaction shifts to the left. As a reasult, [N2] inc,
[H2] inc, and [NH3] dec. “Plug” this in to the equilibrium expression. Since the numerator dec and the denominator inc, Kc will decrease.
Adding a Catalyst
A catalyst affects both the forward and reverse reactions equally. So catalysts have no effect on an equilibrium system.
Add this to your chart of stresses and shifts.
| Stress | System will shift |
|---|---|
| Addition of a chemical | Away from that chemical |
| Removal of a chemical | Toward that chemical |
| Decrease in volume | Toward side with fewer gas molecules |
| Increase in volume | Toward side with more gas molecules |
| Addition of inert gas | No shift |
| Increase in temperature | Toward the direction of the endothermic reaction
(Away from the side with heat – when heat is written in as a reactant or product) |
| Decrease in temperature | Toward the direction of the exothermic reaction
(Toward from the side with heat – when heat is written in as a reactant or product) |
| Addition of catalyst | No shift |
PRACTICE MAKES PROGRESS!!
PCl3(g) + Cl2(g) ⇌PCl5(g)
ΔH = -88kJ
In which direction will the equilibrium shift and how will the [Cl2] be affected if:
a) the temperature is lowered?
b) the volume of the container is increased?
c) PCl5 is added?
Click here for the answer.
a) shift right [Cl2 decreases
b) shift left [Cl2] increases
c) shift left [Cl2] increases