Dynamic Equilibrium and Le Chatelier's Principle

Dynamic equilibrium is a fundamental concept in AQA A Level Physical Chemistry. It occurs when forward and reverse reactions proceed at the same rate in a closed system at constant temperature. This page introduces Le Chatelier's principle, which is crucial for predicting how equilibrium systems respond to changes.

Definition: Dynamic equilibrium is the point in a reaction when the forward and reverse reactions occur at exactly the same rate in a closed system at a constant temperature.

Le Chatelier's principle states that if a reaction at equilibrium is subjected to a change in concentration, pressure, or temperature, the position of equilibrium will move to counteract the change. This principle only applies to homogeneous equilibria, where all species are in the same physical state.

Highlight: Catalysts have no effect on the position of equilibrium but can help reach equilibrium faster.

The page explains how changes in concentration, pressure, and temperature affect equilibrium:

1. Concentration changes: Increasing reactant concentration shifts equilibrium to the right, while increasing product concentration shifts it to the left.

2. Pressure changes: Only affects gas equilibria. Increasing pressure favors the side with fewer gas molecules.

3. Temperature changes: Increasing temperature favors the endothermic direction, while decreasing temperature favors the exothermic direction.

Example: In an exothermic reaction, increasing temperature will shift the equilibrium to the left (towards reactants) to absorb the added heat.

This information is crucial for answering AQA A Level Chemistry Equilibria Exam Questions and understanding Chemical equilibrium concentration pressure temperature changes graph.

Practical Application: Copper Complex Equilibrium

This page demonstrates the application of equilibrium principles using the reaction of [Cu(H₂O)₆]²⁺ with concentrated hydrochloric acid (HCl). This example is excellent for understanding Dynamic equilibrium a Level Chemistry and Le Chatelier's principle AQA A Level Chemistry.

The equilibrium reaction is:

[Cu(H₂O)₆]²⁺ (aq) + 4Cl⁻ (aq) ⇌ [CuCl₄]²⁻ (aq) + 6H₂O (l)

Vocabulary: [Cu(H₂O)₆]²⁺ is a blue copper aqua complex, while [CuCl₄]²⁻ is a green-yellow copper chloride complex.

The color change in this reaction allows for visual monitoring of the equilibrium position:

• Blue solution: equilibrium lies to the left (more reactants)
• Green-yellow solution: equilibrium lies to the right (more products)

The page explains how concentration and temperature changes affect this equilibrium:

1. Concentration changes:

   • Adding concentrated HCl shifts the equilibrium right, turning the solution greener.
   • Adding distilled water shifts the equilibrium left, turning the solution bluer.

2. Temperature changes:

   • Heating the mixture shifts the equilibrium right (endothermic direction), producing more green [CuCl₄]²⁻.
   • Cooling the mixture shifts the equilibrium left (exothermic direction), producing more blue [Cu(H₂O)₆]²⁺.

This practical example is invaluable for understanding Kc Chemistry A Level Questions and Equilibria A Level Chemistry notes.