Chemical reactions are happening all around us, from the digestion of food in our bodies to the combustion of fuel in car engines. Some of these reactions are reversible, meaning they can proceed in both the forward and backward directions. In a reversible reaction, the system can reach a state where the forward reaction and the reverse reaction occur at the same rate. This state is called chemical equilibrium. One of the fundamental principles that helps us understand how equilibrium systems respond to changes is Le Chatelier’s Principle.
Le Chatelier’s Principle is named after the French chemist Henri Louis Le Chatelier, who introduced it in 1884. This principle explains how a system at equilibrium reacts to external changes, such as changes in concentration, pressure, temperature, or volume. It states:
"If a dynamic equilibrium is disturbed by changing the conditions, the system responds in a way that counteracts the change and re-establishes equilibrium."
Let’s explore this concept in detail, breaking it into simple and understandable parts.
Before diving into Le Chatelier’s Principle, let’s quickly review the idea of equilibrium. Consider a reversible reaction:
A + B⇋C + D
In the forward reaction, A and B combine to form C and D. In the reverse reaction, C and D break down to reform A and B. At equilibrium:
Even though the reaction appears to "stop," it is still dynamic—both reactions continue happening at the same rate.
When a system at equilibrium experiences a change, the system "shifts" to counteract that change and restore balance. This shift can either favor the forward reaction or the reverse reaction. Let’s examine the effects of different changes:
If you change the concentration of any reactant or product in a system at equilibrium, the system will shift to reduce the effect of that change.
In short, the system "moves" in the direction that helps counteract the change in concentration.
Pressure changes affect equilibrium systems involving gases. According to Le Chatelier’s Principle, the system will shift to minimize the effect of pressure changes.
Increasing Pressure: If you increase the pressure by reducing the volume of the container, the system will shift toward the side with fewer gas molecules. This reduces the total pressure.
For example:
Here, 4 gas molecules (1 N₂ + 3 H₂) react to form 2 gas molecules (2 NH₃). If pressure increases, the equilibrium shifts to the right (fewer molecules) to reduce pressure.
Temperature changes can have a significant effect on equilibrium because they influence the heat of the reaction. Every reaction is either exothermic (releases heat) or endothermic (absorbs heat).
A catalyst speeds up both the forward and reverse reactions equally. It does not affect the position of equilibrium but helps the system reach equilibrium faster. Therefore, adding a catalyst has no impact on the shift of equilibrium.
Think of equilibrium as a balanced seesaw. When you add or remove weight from one side, the seesaw tilts. The system (seesaw) adjusts to restore balance by moving in the opposite direction of the disturbance.
Le Chatelier’s Principle has many practical applications in industries and daily life:
This process is crucial for producing fertilizers. The reaction is:
To maximize ammonia production:
This process produces sulfuric acid by oxidizing sulfur dioxide:
High pressure and a catalyst help shift equilibrium to favor sulfur trioxide (SO₃).
The carbon dioxide-bicarbonate equilibrium in our blood maintains pH:
When CO₂ levels increase (e.g., during exercise), the equilibrium shifts to produce more HCO₃⁻ and H⁺, maintaining pH balance.
In the preservation of food, changing conditions like temperature and pressure can help slow down microbial reactions by shifting their equilibria.
While Le Chatelier’s Principle is incredibly useful, it has limitations:
The principle of Le Chatelier can be used to predict the behaviour of a system in response to changes in pressure, temperature, or concentration.
According to Le Chatelier's principle, if a reaction at equilibrium is subjected to a change in parameters such as temperature, pressure, or the concentration of reactants and products, the reaction equilibrium shifts in the direction of the change.