In-Depth Notes on Equilibrium Concepts in Chemistry
Definition of Equilibrium
General Definition: A state of balance resulting from equal action of opposing forces.
Chemical Definition: A condition where the rate of the forward reaction equals the rate of the reverse reaction in a reversible process.
Dynamic Equilibrium
Concept: No observable changes at a macroscopic level, but changes occur at the molecular level.
Characteristics:
Process is reversible and is often referred to as dynamic equilibrium.
Example: At equilibrium, evaporation and condensation rates are equal in a sealed container of liquid.
The pressure and the liquid level remain constant despite ongoing molecular activity.
Evaporation and Condensation
Evaporation: Molecules transition from liquid to gas (shown by arrows pointing up).
Condensation: Molecules transition from gas to liquid (shown by arrows pointing down).
Behavior in a Sealed Container: Initially, more molecules evaporate than condense; over time, the rates equalize and dynamic equilibrium is achieved.
Chemical Reactions and Equilibrium
Chemical Equilibrium: Similar concept applies in reversible chemical reactions, where both forward and reverse processes occur at equal rates at equilibrium.
Equilibrium Arrow Notation: Indicated by a double-headed arrow in chemical equations, showing the reversible nature of the reaction.
Example of Chemical Equilibrium
Reaction Example:
7 moles of water (H₂O) + 7 moles of carbon monoxide (CO) produce hydrogen (H₂) and carbon dioxide (CO₂).
Progress: Moles of products and reactants change over time until a constant amount is reached, indicating equilibrium.
Equilibrium Constant Expression (K)
Definition: K is the equilibrium constant, a measure of the ratio of products to reactants at equilibrium.
General Form:
Where [C] and [D] are concentrations of products and [A] and [B] are concentrations of reactants.
Coefficients from the balanced equation become the exponents in the expression (lowercase letters).
Examples of Writing K Expressions
Example 1: Formation of Ammonia (NH₃) from Nitrogen (N₂) and Hydrogen (H₂):
Example 2: Ozone (O₃) converting to Oxygen (O₂):
Heterogeneous Equilibria
Definition: When solids or liquids are part of the equilibrium reaction but not included in the K expression.
Example of Exclusion:
Reaction: CH₄(g) + Cl₂(g) ⟷ CCl₄(l) + HCl(g)
K Expression:
CCl₄ excluded because it is a liquid.
Influence of K Value
Product-Favored Reactions: If K > 1, equilibrium favors products (more product at equilibrium).
Example: CO + Cl₂ producing COCl₂; K = 4.56 x 10⁹ implies greater quantity of products.
Reactant-Favored Reactions: If K < 1, equilibrium favors reactants (more reactant at equilibrium).
Example: CH₃COOH in water producing CH₃COO⁻ and H₃O⁺; K = 1.74 x 10⁻⁵ indicates more reactants.
Summary of Writing K Expressions Steps
Write products in the numerator and reactants in the denominator.
Exclude solids and liquids from the expression, only include gases and aqueous species.
Use stoichiometric coefficients as powers in the expression.
Practice Exercises
Example 1: From KClO₃ to KCl and O₂;
Example 2: From Fe + O₂ to Fe₂O₃ (all solids);
(not applicable).Example 3: From CS₂(g) + Cl₂(g) to S₂Cl₂(l);