Le Chatelier's Principle Study Notes

Definition: Le Chatelier’s Principle is a fundamental concept in chemistry used to predict the behavior of a reaction at equilibrium when subjected to various changes. The principle states that:
- General Statement: "When a system at equilibrium is subjected to a change, the system will respond to minimize the effect of the change."
- This implies that if changes are made to a system at equilibrium, the system will attempt to counteract the changes imposed on it.

The equilibrium position can shift to either the right or the left depending on the intervention:
- Shift to the Left: Leads to an increase in the concentration of reactants.
- Shift to the Right: Leads to an increase in the concentration of products.

Removal of Product:
- Effect: Shifts equilibrium position to the right
- Result: Increases yield of products.
Removal of Reactant:
- Effect: Shifts equilibrium position to the left
- Result: Increases yield of reactants.
Addition of Reactant:
- Effect: Shifts equilibrium position to the right
- Result: Increases yield of products.
Important Note: Changes in the concentration of a reactant or product will not affect the value of the equilibrium constant (K).

Increase in Pressure:
- Effect: Favors the side of the reaction with fewer gaseous molecules.
- Reason: Higher pressure results in increased frequency of collisions among gas particles, which in turn increases reaction rates.
Decrease in Pressure:
- Effect: Favors the side of the reaction with a greater number of gaseous molecules.
- Reason: Lower pressure results in particles being farther apart, leading to reduced collision frequency and reaction rates.

Consideration of Enthalpy: The enthalpy change of the reaction is vital when assessing how temperature affects the reaction.
Exothermic Reactions:
- Heat is released as a product.
- Adding Heat: Shifts the equilibrium position to the left.
- Removing Heat: Shifts the equilibrium position to the right.
Endothermic Reactions:
- Heat is absorbed as a reactant.
- Adding Heat: Shifts the equilibrium position to the right.
- Removing Heat: Shifts the equilibrium position to the left.

Temperature's Exclusive Role: Only temperature affects the equilibrium constant (K).
Exothermic Reactions:
- Increasing temperature shifts the equilibrium position to the left, which causes the equilibrium constant (K) to decrease.
Endothermic Reactions:
- Increasing temperature shifts the equilibrium position to the right, which leads to an increase in the equilibrium constant (K).

Catalysts increase the rate at which a reversible reaction reaches equilibrium but do not alter the value of the equilibrium constant.

Effect on Hydrogen Iodide Production:
- Reaction: $H_{2}(g) + I_{2}(g) <br>ightleftharpoons 2HI(g)$
- Change: Increase in concentration of hydrogen ( $H_{2}$ ).
- Prediction: Yield of hydrogen iodide (2HI) will increase, but equilibrium constant remains unchanged.
Effect of Volume Decrease on Ammonia:
- Reaction: $N_{2}(g) + 3H_{2}(g) <br>ightleftharpoons 2NH_{3}(g)$
- Change: Decrease in reaction vessel volume at constant temperature.
- Prediction: Equilibrium position will shift towards the right (generating more ammonia, $NH_{3}$ ), resulting in an increased yield of ammonia, no change to the equilibrium constant.
Effect of Temperature Increase on Reaction:
- Reaction: $CO(g) + H_{2}O(g) <br>ightleftharpoons H_{2}(g) + CO_{2}(g)$
- Change: Temperature of the system is increased (given that $ext{ΔH}$ is negative).
- Prediction: Equilibrium position will shift to the left (resulting in decreased yield of products, $H_{2}$ and $CO_{2}$ ), no change to the equilibrium constant.
Effect of Adding a Catalyst:
- Observation: Adding a catalyst to a reaction at equilibrium will not alter the equilibrium position or the equilibrium constant but will increase the rate at which equilibrium is achieved.