Study Notes on Thermodynamics and Thermodynamic Favorability

Applications of Thermodynamics

9.1 Introduction to Entropy

  • Entropy Definition: The measure of disorder or randomness in a system. Higher entropy indicates a greater degree of disorder.
  • Entropy increases are favored in nature and are often associated with spontaneous processes.
  • Changes in entropy can be impacted by various factors, such as temperature and state changes.

Examples of Entropy Increase:

  • Spilling a glass of milk (increased disorder).
  • Melting and vaporization of ice (solid to liquid to gas).
  • Mixing of different substances.

Entropy Changes During Phase Transitions:

  • 3 Solid (low disorder) → Liquid → Gas (high disorder).

9.2 Absolute Entropy and Entropy Change

  • Absolute Entropy: The entropy of a substance at absolute zero (0 K) is defined as zero. As the temperature increases, the absolute entropy also increases.
  • Entropy Change (ΔS): Changes in entropy between reactants and products can determine the spontaneity of a reaction.
  • Processes that increase the number of gas molecules or dissolve ionic compounds typically increase entropy:
    • Melting, vaporization, and addition of heat.

Laws of Thermodynamics

  1. First Law: Energy cannot be created or destroyed, only transformed (The Law of Conservation of Energy).
  2. Second Law: The entropy of the universe is always increasing; processes occur spontaneously in the direction of increasing entropy.
  3. Third Law: The entropy of a perfect crystal approaches zero as the temperature approaches absolute zero.

9.3 Gibbs Free Energy and Thermodynamic Favorability

  • Gibbs Free Energy (G): A thermodynamic potential that indicates whether a process is spontaneous.
  • Formula: G = H - TS, where H = enthalpy, T = temperature, and S = entropy.
  • If G < 0, the process is thermodynamically favored.
  • Example of calculating change: Gf⁰ = ∑nGf⁰ (products) - ∑nGf⁰ (reactants).

9.4 Thermodynamic and Kinetic Control

  • Kinetic Control: Refers to processes that are influenced by the rates of reaction rather than the energy or favorability of the products.
  • Thermodynamic Control: Processes that proceed to form the more stable product (lower G).

9.5 Free Energy and Equilibrium

  • At equilibrium, the system’s Gibbs free energy is at a minimum, and no net changes occur in concentrations.
  • The relationship between Gibbs free energy and equilibrium constant (K) is given by: G = -RT ln K.

9.6 Free Energy of Dissolution

  • Dissolution of ionic compounds often results in spontaneous reactions if ΔG < 0. This indicates solubilization.
  • Compounds like NaCl are generally soluble in water, while others might be too stable to dissolve.

9.7 Galvanic and Electrolytic Cells

  • Galvanic Cell: Converts chemical energy to electrical energy spontaneously (ΔG < 0).
  • Electrolytic Cell: Uses electrical energy to drive a non-spontaneous chemical reaction (ΔG > 0).
  • Cathode and Anode: In galvanic cells:
    • Anode: oxidation (loss of electrons), negative terminal.
    • Cathode: reduction (gain of electrons), positive terminal.

Cell Potential and Free Energy

  • Standard Reduction Potentials indicate the tendency of a species to be reduced. The more positive the E°, the more favorable the reduction.
  • The relationship between Gibbs free energy and cell potential is critical for predicting the spontaneous direction of reactions in electrochemical cells.

9.10 Cell Potential Under Nonstandard Conditions

  • Cell potential changes with concentration, pressure, and temperature. Use the Nernst equation to calculate cell potentials under nonstandard conditions:
    • Ecell = E°cell - (RT/nF) * ln(Q).

9.11 Electrolysis and Faraday’s Law

  • Faraday's laws relate the amount of substance transformed in electrochemical reactions to the current and time:
    • I = q/t, where q is the charge.
  • The charge can be converted using Faraday’s constant (96485 C/mol e-).

Practice Problems and Applications

  • Various practice problems involving calculation of ΔG°, ΔS°, ΔH°, and predicting thermodynamic favorability are included in the notes.
  • Examples:
    • Calculate entropy differences for specific reactions.
    • Predict signs of ΔG based on reaction composition.
    • Apply Faraday's Law to real-world electrolysis scenarios.