Le Chatelier's Principle: Changes in Concentration and Pressure explained in Dynamic Equilibrium

Le Chatelier’s Principle:

A system in chemical equilibrium can be controlled or altered in an attempt to favour either the forward or backward reaction; (that is, the equilibrium position). Some of these changes include a change in concentration, pressure and temperature. The dynamics of these changes will be explained in this article.

You can read on the concept of dynamic equilibrium here.

According to French Chemist - Henry Louis Le Chatelier, an altered system (in terms of changing its concentration, pressure and temperature) will adjust itself by shifting equilibrium to the right or left in order to counteract or undo the change. This is the concept of Le Chatelier’s Principle.

Le Chatelier’s Principle state that when a system in dynamic equilibrium is subjected to any change or alteration, the system will adjust itself in order to counteract or revert the applied change.

Factors that affect Equilibrium

Below are the explanations of some factors that can change or alter equilibrium:

• A change in concentration

The equation below will be used to explain how a change in concentration affects dynamic equilibrium according to Le Chatelier’s principle:

2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

From the above equation, if the concentration of either or both reactants (2SO_2(g) + O_2(g)) are increased, the forward reaction will be favoured, thus shifting equilibrium to the right. As a result, more reactants are available for usability and this ultimately favours the forward reaction because more products are formed (and the amount of reactants decreases appropriately).

You can read on the differences between static and dynamic equilibrium here.

Similarly, if the concentration of the product (2SO_3(g)) is increased, the backward reaction is favoured as equilibrium shifts towards the formation of reactants. This indirectly increases the amount (or concentration) of the reactants; thus consequently reverting the system into dynamic equilibrium with time.

In summary:

2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

If the concentration of the reactants (2SO_2(g) + O_2(g)) increases or those of the products (2SO_3(g)) decreases then:

• The formation of more products will be favoured.

• The added reactants are converted to more SO₃.

• The reaction's equilibrium shift towards the right.

• Forward reaction is favoured.

If the concentration of the reactant decreases or those of the product increases:

• The formation of more reactants will be favoured.

• The added products (S0₃) will be converted to SO₂ and O₂ respectively.

• The reaction's equilibrium shift towards the left.

• Backward reaction is favoured.

Note: In practical terms, if the cost of 2SO_3(g) is high while those of the reactants are relatively cheap, then more 2SO_3(g) can be economically produced by adding more of the cost friendly reactants to the reaction.

Note: A change in the concentration of the reactants or products neither increases or decreases the equilibrium constant. It only shifts the position of equilibrium (either to the right or left). This is because the system counteracts the change by adjusting itself (according to Le Chatelier’s Principle). For this reason, the equilibrium value always remain constant regardless the amount of changes made on the concentration of the reactants, products or both.

You can read on salts in chemistry here.

• A change in pressure

A change in pressure only becomes effective when the reacting system includes at least one gaseous substance. This is true because pressure is effected when gas molecules hit the walls of their container (due to their random motion).

The more gaseous molecules you have inside a container, the higher its pressure. The less gaseous molecules, the lesser the pressure it effects inside its container.

Please read more on gases and their characteristics here.

A change in pressure will only shift the position of equilibrium depending on where the change emanates from; but eventually, the system reverts the change, thus bringing it back to a state of dynamic equilibrium in fulfillment to Le Chatelier’s principle.

Note: Similar to 'change in concentration', a change in pressure does not alter the value of equilibrium constant 

Consider the equation below:

2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

If the pressure of the reactants (2SO_2(g) + O_2(g)) increases or those of the products (2SO_3(g)) decreases, then:

• The formation of more products will be favoured.

• The added reactants are converted to SO₃.

• The reactions equilibrium shifts towards the right.

• Forward reaction is favoured.

If the pressure of the reactants decreases or those of the product increases, then:

• The formation of more reactants will be favoured.

• The added products (S0₃) will be converted to SO₂ and O₂ respectively.

• The reactions equilibrium shifts towards the left.

• Backward reaction is favoured.

You can read on physical change and chemical change here.

Note: Increasing the pressure on a gaseous reaction shifts the position of equilibrium towards the side with fewer molecules. Conversely, decreasing the pressure on a gaseous reaction shifts the position of equilibrium towards the side with more molecules.

The explanations so far on 'changes in concentration and pressure' on equilibrium is quite superficial. Le Chatelier’s principle only gives us a quick summary of the reaction procedure. The ideal explanations on equilibrium shifts (when changes are made on concentration, pressure and temperature) are explained in a mathematical expression via the equilibrium constant 'K'.