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LeChatelier's Principle

Written by tutor Kathie Z.

On this page you will find information related to chemical equilibrium in physical chemistry. Links to helpful pages will be at the end. Remember that your textbook publisher may have help links as well.

Definition: When a change is introduced to a system in equilibrium, the equilibrium shifts in the direction that relieves the change.

Discoverer: French chemist Henri Louis Le Chatelier (1850-1936) first published his principle regarding the chemical equilibrium of gases in 1884.

What is equilibrium? Equilibrium is when both the forward and reverse reactions are in balance, and are happening at the same rate.

How many types of equilibria are there? There are two main types of equilibria (which is the plural of equilibrium). Physical equilibrium is when the physical processes are in balance (represented by a 2-way arrow ↔ ) and do NOT go to completion (remember completion was represented by a one way arrow → ).

One example of physical equilibrium is pouring water into a jar and putting a lid on it. Some of the liquid water (l) will evaporate making water vapor (g). When the forward change (evaporation) and the reverse change (condensation) are happening at the same rate (simultaneously), it can be written as
          H2O (l) ↔ H2O (g)
Both these forward and reverse changes are physical changes (liquid to gas and back to liquid), so this equation with the ↔ represents physical equilibrium.

Chemical equilibrium is when a reaction is reversible and the conversion of reactants to products and the reverse: products to reactants happens at the same time (simultaneously).

A Chemical Equilibrium example you may have studied is when sulfur dioxide and oxygen gases are mixed in a closed container, the forward reaction will start to produce sulfur trioxide. But, as soon as some molecules of the sulfur trioxide are synthesized (formed) some of them will break down (decompose) into the original reactants. Eventually the system will establish equilibrium: the rate of synthesis (forward reaction) will equal the rate of decomposition (reverse reaction).
          2SO2(g) + O2 ↔ 2SO3(g)
In this example the forward and reverse changes are chemical, so this equation with the ↔ represents reversible chemical equlibrium.

Factors That Affect Chemical Equilibrium

There are three conditions which affect reaction rates and therefore the balance of the equilibrium. You can change the temperature, the pressure and/or the concentration of the reactants or products.

Le Chatelier's Principle states: when a change is introduced to a system in equilibrium, the equilibrium shifts in the direction that relieves the change. We'll use the the equation for synthesizing ammonia to explore the factors which affect chemical equilibrium and apply Le Chatelier's principle:

N2(g) + 3H2(g) ↔ 2NH3(g) + Heat

Temperature

First, notice that heat is a product in this reaction. That means that heat is given off (exothermic) as ammonia is synthesized in the forward reaction. So the reverse equation is endothermic and heat is absorbed when ammonia is decomposed.

As a result, according to Le Chatelier's principle, if you add heat to a balanced equation the system will want to shift in the direction that removes heat from the system (endothermic) which is the reverse direction. So by increasing the temperature you would remove ammonia, it would decompose.

Pressure

Since we start with a balanced equation, we can count each substance as a molecule. That means that we start with 1 molecule of nitrogen gas and 3 of hydrogen gas for a total of 4 molecules of reactants. They combine to make (synthesize) 2 molecules of ammonia and release heat. Remember that a change in pressure only applies to gases not to solids or liquids.

Now we apply Le Chatelier's principle as we increase the pressure on the system: the equilibrium will shift in the direction that decreases the pressure of the system. Which side will have less pressure? The side with more molecules or the side with less? Well, the side with less molecules will be less crowded so it will have less pressure! The products side has only 2 molecules while the reactants side has 4 molecules. Therefore when you increase the pressure the system wants to shift to the right and synthesize more ammonia.

Concentration

If you change the concentration of the reactants or the products, you will affect the equilibrium. To maximize the amount of ammonia synthesized, a manufacturer needs to remove the ammonia from the system. That is because as Le Chatelier's principle tells you, when you remove the ammonia the equilibrium shifts in the direction to produce more ammonia which is the forward reaction!

The most efficient ammonia plants operate at relatively low temperatures, high pressure and low ammonia concentrations to maximize the amount of ammonia synthesized. I wonder if the plant employees know they are using Le Chatelier's Principle?

Easy Rules for Le Chatelier's Principle

  1. Add = Away (shift away, the → points Away from what you Added)
  2. Take = Toward (shift toward, → points Toward what was removed)
  3. Heat: endothermic → ; exothermic ←
  4. Cool: endothermic ← ; exothermic →
  5. Increase pressure / Decrease volume → less gas side
  6. Decrease pressure / Increase volume → more gas side

Test Taking Tips for Balancing Chemical Equations:

        ✔ Never change the subscript in any formula, ONLY change the coefficient
        ✔ First, balance the atom that is most obviously unbalanced by placing a coefficient in front of the appropriate formula
        ✔ Make a T with the element in the center and the reactant coefficient on the left and the product coefficient on the right
        ✔ Count the number of atoms of each element on both sides of the equation, If they are not equal continue balancing.

There are several excellent examples of Le Chatelier's Principle on YouTube. Two of my favorite are:

and

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