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Le Chatelier’s principle is also called equilibrium law. They predict the effect of some changes on a system in chemical equilibrium (especially the change in temperature or pressure). Henry Louis Le Chatelier, the French chemist after which this principle is named after.
Le Chatelier state, equilibrium adjusts the forward and backward reactions in such a way as to accept the changes affecting the equilibrium conditions.
Effect of Concentration Changes on Equilibrium and Product Formation
As per Le Chatelier’s principles, product formation is the only way to rise equilibrium to accept more reactants. The forward reaction is favored when the concentration of the reactant is raised. The equilibrium of the reaction shift towards the use of reactants in the reaction and which in turn falls the concentration of the reactants.
Likely, the addition of product shall rise the backward reaction to fall the product concentration. The backward reaction is favored when the concentration of the reactant falls along with the equilibrium of the reaction shift towards the production of reactants and the concentration of the reactants will be more.
Example:
Think about, a reaction between oxygen and sulfur dioxide to produce sulfur trioxide.
If the concentration of the reactant rises at that time, equilibrium will shift towards the fall in the concentration of the reactants. More favored for the forward reaction. A few of the SO2 or O2 form SO3. The equilibrium of the reaction moves towards the right.
If the concentration of the reactant lessens then equilibrium will shift towards the growth in the concentration of the reactants. More favored for the backward reaction. Some of the SO3 would change to SO2 or O2. Equilibrium of the reaction move towards the left.
If the product falls then the equilibrium of the reaction shift to rises the concentration of the sulfur trioxide. Rise in the forward reaction rate. Little of the SO2 or O2 to form SO3. Equilibrium of the reaction move towards the right.
If the product is right then the equilibrium of the reaction shifts to fall the concentration of the sulfur trioxide. Rise in the reverse reaction rate. Some of the SO3 would change to SO2 or O2. Equilibrium of the reaction move towards the left.
Effect of Change of Volume, Pressure, and Inert Gas on Equilibrium and Product Formation
Kp = Kc (RT)Δn = Kc (p/v)Δn
Change of volume, pressure and inert gases does not affect the reactions of liquids and solids. They may have resulted in gaseous reactions and that too only when the difference in the sum of the number of reactant and product molecules (∆n) is not zero.
When ∆n = 0,
According to Chatelier’s principles, there will be no effect on Equilibrium and Product Formation on changing the volume, pressure, or inert gas.
When ∆n = +ve,
A growth in pressure or lessen in volume will fall the formation of the product. Fall of pressure or rise of volume shall have the reverse effect of increasing the product formation. Inert gases do not participate in the reaction and shall raise the volume or pressure only.
At some pressure, the addition of inert gas rises the volume, so rise the product formation.
At the same volume, the addition of inert gas rises the pressure, so falls the product formation.
PCl5 ⇌ PCl3 + PCl2
In the decay of phosphorus pentachloride ∆n = +1.
A rise of pressure or fall in volume falls the decaying of PCl5
At some pressure, the addition of inert gas rises the PCl5 formation.
At the same volume, the addition of inert gas falls into the PCl5 formation.
When ∆n = -ve,
According to Chatelier’s principle, a rise of pressure or a fall in volume will rise the formation of the product.
At some pressure, the addition of inert gas rises the volume, so fall the product formation.
At the same volume, the addition of inert gas rises the pressure, so rises the product formation
N2 + 3H2 ⇌ 2NH3
In the formation of ammonia ∆n = -2. The rise of pressure or fall in volume rises the formation of ammonia.
At some pressure, the addition of inert gas falls into ammonia formation.
At the same volume, the addition of inert gas rises ammonia formation.
Result of Change of Temperature on Equilibrium and Product Formation
The individual reaction in the equilibrium can be either endothermic or exothermic. Likewise, at equilibrium net energy involved may make the opposite reactions either endothermic or exothermic.
According to Le Chatelier’s Principles,
- In exothermic equilibrium, a rise in temperature fall product formation, and a fall in temperature rise product formation.
- In endothermic reactions, a rise in temperature rises the product formation and a fall in temperature falls the product formation
According to Van’t Hoff’s equation, for an exothermic equilibrium, ∆H will be negative. A temperature rise shall fall K2 or a fall in temperature rises K2. The reverse is true for an endothermic reaction.
Example: Consider a reaction,
N2(g)+ 3H2(g) ⇋ 2NH3(g) – ΔH=−92kJ
Rise in Temperature
- This prefers an endothermic reaction because it takes energy.
- An endothermic reaction is a reverse reaction and it is favored.
- The yield of the product (NH3) falls.
Fall in Temperature
- This prefers an exothermic reaction because it gives energy.
- An exothermic reaction is a forward reaction and it is favored.
- The yield of the product (NH3) rises.
Result of Catalyst on Equilibrium and Product Formation
A catalyst is a substance that changes the rate of reactions (rise or fall) without quantitatively taking part in the reaction. According to reversible reactions, the change of reaction rate is the same for both forward and backward reactions.
The ratio of the reaction rates remains constant and so is the equilibrium same. By Le Chatelier’s principles, the presence of the catalyst may speed up or delay the attainment of equilibrium but will not affect the equilibrium concentration.
Conclusion
Le Chatelier’s principle can predict the effects of the change in temperature, pressure, and concentration in a chemical reaction. It is very much applicable in industrial fields. It states if the conditions in equilibrium are changed then the system will counteract the changes and reform the reaction.
