Le-Chatelier's Principle: Comprehensive NEET Chemistry Notes

1. Le-Chatelier's Principle Overview

Le-Chatelier's principle is a key concept in chemical equilibrium, crucial for NEET preparation. It predicts how a system at equilibrium reacts when external changes are applied. The principle states:

"If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change."

In simpler terms, when a change is applied to a system in equilibrium (such as altering concentration, temperature, or pressure), the system adjusts itself to minimize the effect of that disturbance. Understanding this principle allows students to predict how equilibrium systems behave when altered, a frequent question type in NEET.

2. Factors Affecting Equilibrium According to Le-Chatelier's Principle

2.1 Effect of Concentration Change

When the concentration of reactants or products in an equilibrium system is changed, the system shifts to favor the reaction that consumes the added substance or replenishes the removed substance. This is the equilibrium's way of minimizing the disturbance caused by the change in concentration.

Example: Consider the reaction:
${\text{H}}_2(g)+{\text{I}}_2(g)\rightleftharpoons 2\text{HI}(g)$

• If ${\text{H}}_2$ is added to the system, the equilibrium shifts to the right to consume the excess ${\text{H}}_2$​ by forming more $\text{HI}$.
• Removing $\text{HI}$ shifts the equilibrium to the right to produce more $\text{HI}$.

NEET Problem-Solving Strategy:

In questions involving concentration changes, always remember that the system shifts toward the side that counteracts the change. Focus on identifying the species being added or removed and predict the direction of the shift accordingly.

2.2 Effect of Pressure Change

Le-Chatelier's principle explains that when pressure is increased, the equilibrium shifts toward the side with fewer gas molecules. Conversely, when pressure is decreased, the equilibrium shifts toward the side with more gas molecules.

Example: Consider the reaction:
$\text{CO}(g)+3{\text{H}}_2(g)\rightleftharpoons {\text{CH}}_4(g)+{\text{H}}_2\text{O}(g)$

• The forward reaction involves 4 moles of gaseous reactants and produces 2 moles of gaseous products. If pressure increases, the equilibrium shifts to the right (towards fewer moles of gas), favoring the production of ${\text{CH}}_4$​ and ${\text{H}}_2\text{O}$.

NEET Tip:

For pressure-related questions, count the moles of gas on each side. Shifts toward fewer moles of gas occur when pressure increases; shifts toward more moles occur when pressure decreases.

2.3 Effect of Temperature Change

Temperature affects equilibrium based on the enthalpy change ($\Delta H$) of the reaction. Le-Chatelier's principle provides the following insights:

• For exothermic reactions ($\Delta H<0$), increasing temperature shifts equilibrium to the left (reactants), while decreasing temperature shifts it to the right (products).
• For endothermic reactions ($\Delta H>0$), increasing temperature shifts equilibrium to the right, and decreasing temperature shifts it to the left.

Example: In the Haber process:
${\text{N}}_2(g)+3{\text{H}}_2(g)\rightleftharpoons 2{\text{NH}}_3(g);\Delta H=-92.38\text{kJ/mol}$

• As this is an exothermic reaction, increasing temperature shifts the equilibrium to the left, reducing ammonia production.

Real-life Application:

The Haber process is vital for ammonia production used in fertilizers. Controlling temperature and pressure maximizes ammonia yield in industrial setups.

2.4 Effect of Inert Gas Addition

Adding an inert gas at constant volume does not affect equilibrium because it doesn't change the partial pressures of reactants or products. However, adding an inert gas at constant pressure increases the total volume, which shifts the equilibrium toward the side with more moles of gas.

Common Misconception:

Adding an inert gas at constant volume does not shift the equilibrium. Only changes in the partial pressures of reacting species influence the position of equilibrium.

2.5 Effect of Catalysts

Catalysts speed up both the forward and reverse reactions equally, helping the system reach equilibrium faster. However, they do not affect the position of the equilibrium itself.

NEET Tip:

While catalysts do not alter the equilibrium position, understanding their role in industrial processes (such as the Haber process) is essential for NEET.

Quick Recap

• Le-Chatelier's Principle: Predicts how equilibrium systems adjust to external changes.
• Concentration Changes: The system shifts to consume added reactants/products or replenish removed ones.
• Pressure Changes: Shifts toward fewer moles of gas with increased pressure, more moles with decreased pressure.
• Temperature Changes: For exothermic reactions, increasing temperature shifts equilibrium left; for endothermic reactions, it shifts right.
• Inert Gas Addition: No effect at constant volume; shifts toward more moles of gas at constant pressure.
• Catalysts: Speed up the attainment of equilibrium but do not affect the position.

Practice Questions

1. In the reaction ${\text{N}}_2{O}_4(g)\rightleftharpoons 2{\text{NO}}_2(g)$, what happens to the equilibrium position when temperature is increased?
   Solution: Since the reaction is endothermic, increasing temperature shifts the equilibrium to the right, increasing the concentration of ${\text{NO}}_2$.
2. For the reaction $2{\text{SO}}_2(g)+{\text{O}}_2(g)\rightleftharpoons 2{\text{SO}}_3(g)$, predict the effect of decreasing pressure.
   Solution: Decreasing pressure shifts the equilibrium to the left, where there are more moles of gas.
3. The equilibrium constant $K_c$ for the reaction ${\text{H}}_2(g)+{\text{I}}_2(g)\rightleftharpoons 2\text{HI}(g)$ is 50 at 500 K. If ${\text{H}}_2$​ is added, what happens?
   Solution: The system shifts to the right, producing more $\text{HI}$ to consume the added ${\text{H}}_2$​.
4. In the reaction:
   ${\text{CaCO}}_3(s)\rightleftharpoons \text{CaO}(s)+{\text{CO}}_2(g)$
   What happens to the equilibrium when ${\text{CO}}_2$ is removed?
   Solution: The equilibrium shifts to the right to produce more ${\text{CO}}_2$ and $\text{CaO}$.

Self-Assessment Quiz

1. What effect does increasing pressure have on the equilibrium in the reaction ${\text{2SO}}_2(g)+{\text{O}}_2(g)\rightleftharpoons {\text{2SO}}_3(g)$?
2. How does the addition of a catalyst affect the equilibrium position?
3. Predict the effect of decreasing temperature on an exothermic reaction.
4. In which direction does the equilibrium shift when hydrogen gas is removed from the reaction ${\text{H}}_2(g)+{\text{I}}_2(g)\rightleftharpoons \text{2HI}(g)$?

Glossary of Key Terms:

• Equilibrium: The state in which the forward and reverse reactions occur at equal rates, resulting in constant concentrations of reactants and products.
• Endothermic Reaction: A reaction that absorbs heat from the surroundings.
• Exothermic Reaction: A reaction that releases heat to the surroundings.
• Catalyst: A substance that increases the rate of a chemical reaction without being consumed.

Final Recommendations

1. Incorporate Diagrams and Visual Aids: Add diagrams, such as pressure-volume graphs or enthalpy profiles, to illustrate the effects of changes in pressure, temperature, and concentration.
2. Expand Practice Questions: Include more varied NEET-style questions, especially assertion-reason questions and multiple-choice types.
3. Use Mnemonics for Key Concepts: Introduce mnemonics for remembering the effects of temperature and pressure changes on equilibrium systems, making it easier for students to recall.