Chemical Equilibrium Lecture Notes

Introduction to Chemical Reactions and Equilibrium

• Chemical Reactions: Typically move spontaneously in the forward direction from reactants to products.
• Molecular Collisions: Reactions require successful reactant collisions; reverse reactions occur if product collisions are thermodynamically favorable.
• Reaction Rates: Reflect forward and reverse reaction frequencies.

Equilibrium

• Definition: Equilibrium is achieved when the rates of the forward and reverse reactions are equal.
• Dynamic Condition: Even at equilibrium, atoms and molecules are constantly moving and colliding.

Understanding Equilibrium through Physical Systems

• Example: Methanol in a closed container at constant temperature.
  • Process: Methanol molecules vaporize and condense continuously.
  • Equilibrium State: Vaporization rate equals condensation rate; the number of liquid and gas phase molecules remains constant.

Characteristics of Equilibrium in Chemical Reactions

• Graphical Representation:
  • Concentration changes of reactants and products are graphed over time.
  • At equilibrium, concentrations of reactants and products become constant.
• Product vs. Reactant Favored Reactions:
  • Product Favored: More products than reactants.
  • Reactant Favored: More reactants than products.

Equilibrium Expression and Constant (K)

• Equilibrium Expression: Derived from a balanced chemical equation.
  • Example Equation: (2N + Cl_2 \rightarrow 2NOCl)
  • Expression: (K = \frac{[NO]^2}{[N]^2[Cl_2]})
  • Concentrations: Expressed in molarity (mol/dm³).

Rules for Equilibrium Expressions

• Pure Solids and Liquids: Omitted from expressions as they have fixed concentrations.
• Concentration Units: Only aqueous and gas states included.
• Value of K:
  • Greater than 1: Product concentration > Reactant concentration.
  • Less than 1: Reactant concentration > Product concentration.
  • Equal to 1: Reactant and product concentrations are approximately equal.

Manipulating the Equilibrium Constant (K)

• Temperature Dependence: K is constant only at a specific temperature.
• Reversing Reactions: Flips the expression and takes the reciprocal of K.
  • Example: (K_{forward} = 2.40 \times 10^{33}); (K_{reverse} = 1/K_{forward})
• Altering Coefficients: May require squaring K or multiplying K values when combining reactions.

Summary

• Dynamic Equilibrium: Forward and reverse reaction rates are equal.
• Graphical Representation: Concentrations of reactants/products remain constant over time at equilibrium.
• Equilibrium Constant (K): Predicts concentrations at equilibrium and informs about reaction extent.
• Manipulating K: Adjusts for different reactions and temperature changes.

Real-Life Connection

• Equilibrium in chemistry mirrors balance in life, adjusting to stress and change.