Key concepts

1. What is Chemical Equilibrium?

• A reversible reaction occurs when products can react to form the original reactants.
• Chemical equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction.
• At equilibrium:
  • The concentration of reactants and products remains constant (but not necessarily equal).
  • The reaction appears to stop on a macroscopic level but continues on a microscopic level.

2. Types of Equilibrium Systems

2.1 Open vs. Closed Systems

• Open System: Reactants or products can escape (e.g., gas escaping from a reaction vessel).
• Closed System: No substances enter or leave, allowing equilibrium to be established.

2.2 Homogeneous vs. Heterogeneous Equilibrium

• Homogeneous Equilibrium:

  • All reactants and products are in the same phase (all gases, all liquids, or all solutions).
  • Example: $\text{N}_2(g) + 3\text{H}_2(g) ⇌ 2\text{NH}_3(g)$

• Heterogeneous Equilibrium:

  • Reactants and products exist in different phases (solids, liquids, and gases in the same reaction).
  • Example: $\text{CaCO}_3(s) ⇌ \text{CaO}(s) + \text{CO}_2(g)$

3. Characteristics of Dynamic Equilibrium

• Dynamic means the reaction is still occurring at a molecular level.
• The forward and reverse reactions happen at equal rates.
• No net change in the concentration of reactants and products.
• Can only be established in a closed system.

4. Factors Affecting Equilibrium (Le Châtelier's Principle)

A system at equilibrium will adjust to oppose any changes applied to it:

• Change in Concentration:
  • Increasing reactants → shifts equilibrium right (towards products).
  • Increasing products → shifts equilibrium left (towards reactants).
• Change in Temperature:
  • Endothermic reaction: Increased temperature shifts equilibrium right.
  • Exothermic reaction: Increased temperature shifts equilibrium left.
• Change in Pressure (for gases only):
  • Increasing pressure shifts equilibrium towards the side with fewer gas molecules.
  • Decreasing pressure shifts equilibrium towards the side with more gas molecules.
• Catalysts:
  • Do not change equilibrium position but help the system reach equilibrium faster.