Chemical Equilibrium Lecture Notes

Introduction to Chemical Equilibrium

Equilibrium Definition: A system (often a reaction) can be in one of two states: changing or not changing.
- Equilibrium: When a system is not changing; macroscopic properties like color, pressure, and concentration don't change.
- Kinetics Relation: Equilibrium is achieved when the rate of the forward reaction equals the rate of the reverse reaction.

Dynamic Equilibrium: Even though reactions continue to occur, the concentrations of reactants and products remain constant because the forward and reverse reaction rates are equal.
Reversible Reaction Notation: Represented by a double arrow (↔) indicating the reaction can proceed in both directions.

Experiment 1: Starting with 1 atm NO2 at 298K
- Initial: 1 atm NO2, 0 atm N2O4
- As reaction proceeds, NO2 pressure decreases, N2O4 pressure increases until they reach equilibrium (pressures constant).
Experiment 2: Starting with 0.5 atm N2O4 at 298K
- Initial: 0.5 atm N2O4, 0 atm NO2
- Reaction proceeds to form NO2 until equilibrium is reached, where pressures stabilize.

Definition: Concisely describes the equilibrium state of a reaction.
- Expression: For a reaction A + B ↔ C + D, KEQ = ([C]^c [D]^d) / ([A]^a [B]^b)
- Concentration and Pressure: Concentrations in molarity or pressures can be used interchangeably using the ideal gas law.

Combustion Reaction: C3H8 + 5O2 ↔ 3CO2 + 4H2O
- KEQ Expression: ([CO2]^3 [H2O]^4) / ([C3H8] [O2]^5)
Reaction Favorability:
- Large KEQ (e.g., 1 x 10^19): More products than reactants; reaction favors products.
- Small KEQ (e.g., 4.1 x 10^-31): More reactants than products; reaction favors reactants.
Interpretation:
- KEQ > 1: Favors products.
- KEQ < 1: Favors reactants.
- KEQ = 1: Neither products nor reactants are favored; balanced.

Understanding equilibrium concepts and learning to write equilibrium constant expressions are crucial.
Practice with different reactions and equilibrium constant expressions is important for mastery.