Equilibrium Notes 2025 Filled In

UNIT 4: Chemical Equilibrium

Overview of Key Topics

  • Reversible Reactions

  • Interpreting Equations

  • Physical Equilibrium

  • Dynamic Equilibrium

  • Conditions for Equilibrium

  • Le Chatelier's Principle

  • Entropy Change vs Enthalpy Change

  • The Equilibrium Constant (Keq)

  • Calculating Keq

Reversible Reactions

  • Definition: A reaction in which products can revert back to reactants.

  • Most reactions do not fully convert reactants to products; they reach a stopping point due to reversibility.


  • Example:H2O(liquid) ⇌ H2O(gas) (Evaporation and Condensation)

Interpretation of Reversible Reactions

  • The double arrow (⇌) indicates that both forward and reverse reactions occur at the same time.

  • The relative sizes of the arrows indicate the rates of reaction; a longer arrow indicates a faster reaction.

Example of Rates

  • If NaCl(s) ⇌ NaCl(aq), determine the rates of the forward and reverse reactions by comparing the lengths of the arrows.

Interpreting Chemical Equations






  1. Write interpretations of the following reactions:a. H2O(liquid) ⇌ H2O(gas)b. N2O4 ⇌ 2 NO2c. H2(g) + I2(g) ⇌ 2 HI(g)d. PCl5(g) ⇌ PCl3(g) + Cl2(g)e. COCl2(g) ⇌ CO(g) + Cl2(g)

  2. Identify physical vs chemical changes



  3. Word Descriptions to Equations: a. Dinitrogen tetroxide consumption > nitrogen dioxide formationb. Formation of PCl5 = Formation of PCl3 + Cl2c. Consumption of HI > consumption of H2 + I2

Physical Equilibrium

Vapour-Liquid Equilibrium

  • Situation 1: Uncovered beaker with water

  • Situation 2: Covered beaker with water

  • Heating in Situation 2 leads to a temperature increase affecting the equilibrium conditions of the liquid-gas phase.

Solution Equilibrium

  • Example with KCl in water at different temperatures to differentiate between unsaturated, saturated, or supersaturated solutions.

  • Equilibrium Definition: At equilibrium, the concentrations of reactants and products are constant.

Dynamic Equilibrium

Definitions

  • Macroscopic: Reactants and products remain constant; macroscopically observable effects.

  • Microscopic: Continuous formation of products from reactants and vice versa.

Analogy

  • Compare to a shopping scenario where equilibrium is achieved when the number of customers on each floor of a store stabilizes.

Key Features of Equilibrium

  • Reactions are constantly occurring, but no visible changes in concentrations.

  • All systems adjust to disturbances, maintaining equilibrium.

Conditions for Equilibrium

  1. Constant temperature.

  2. Constant macroscopic properties.

  3. Closed system.

  4. System adapts when conditions change.

Compare & Contrast:

  • Chemical vs Physical Equilibrium

    • Differences and similarities between stages and behaviors in each.

Graphical Representation of Equilibrium

  • Rate vs. Time and Concentration vs. Time Graphs:

    • Shows actual changes until equilibrium is reached.

    • Equilibrium is attained when rates of forward and reverse reactions are equal.

Le Chatelier's Principle

  • How equilibrium responds to changes (stresses) such as concentration, temperature, and pressure.

  • Common disturbances:

    1. Adding reactants/products

    2. Removing reactants/products

    3. Changes in volume/pressure

    4. Temperature changes

Responses to Stresses

  • System shifts to counteract the imposed change and establish a new equilibrium.

  • Example scenarios illustrating predictive responses of equilibrium.

Entropy Change vs. Enthalpy Change

Key Terms

  • Enthalpy: Heat of a substance, favoring lower energy states.

  • Entropy (S): Degree of disorder; affects the direction of chemical processes.

Factors Affecting Entropy

  • Substance states (solid, liquid, gas).

  • Temperature increases lead to higher entropy due to increased molecular motion.

Thermodynamic Drives

  1. Decrease in Enthalpy: Systems move toward lower energy states.

  2. Increase in Entropy: Systems move toward greater disorder.

Equilibrium Constant (Keq)

Definition

  • Ratio of concentrations of products to reactants at equilibrium, constant at a given temperature.


  • Formula: For general reaction aA + bB ⇌ cC + dD,Keq = [C]^c [D]^d / [A]^a [B]^b

Importance of Keq

  • Indicates whether reactants or products are favored at equilibrium.

  • Only gases and aqueous solutions included in Keq calculations.

Calculating Keq

  • Methods to calculate based on equilibrium concentrations using the formula for Keq.

Practice Scenarios

  • Example reactions to determine concentration-based Keq values at equilibrium using varying conditions (initial concentrations and amounts of reactants/products).

Special Cases

  • Reverse reactions yield Keq as a reciprocal of the forward reaction.

Conclusion and Practice Problems

  • Review problems that apply concepts of Le Chatelier's principle, equilibrium conditions, and calculating Keq to solidify understanding.