Lecture Notes on Gibbs Free Energy

Key Concepts

• Gibbs Free Energy (ΔG): A measure used to predict the spontaneity of a process.
  • ΔG < 0: Spontaneous process.
  • ΔG = 0: Equilibrium state.
  • ΔG > 0: Non-spontaneous process.

Important Formulas

• ΔG = ΔH - TΔS
  • ΔH: Change in enthalpy (kJ/mol).
  • T: Temperature in Kelvin (K).
  • ΔS: Change in entropy (J/mol·K).
  • Convert ΔS from J to kJ by dividing by 1000 when necessary.

Problems and Solutions

Problem 1: Identifying False Statements

• False Statement: "The change in free energy is greater than zero for a spontaneous process."
  • True Statements:
    • ΔG < 0 for spontaneous processes.
    • ΔG = 0 at equilibrium.
    • Maximum work from a spontaneous process equals ΔG.

Problem 2: Calculating ΔG

• Given: ΔH, ΔS, and temperature.
• Convert ΔS from J to kJ by dividing by 1000.
• Use ΔG = ΔH - TΔS to find ΔG.
• Example: ΔG = -109.7 kJ/mol indicates a spontaneous process.

Problem 3: Calculating ΔG for a Reaction

• Use standard enthalpy (ΔH) and entropy (ΔS) values from a table.
• Calculate ΔH and ΔS using formation values.
• Example: ΔG ≈ -190 kJ/mol after calculations.

Problem 4: Using Formation Values

• Direct use of standard ΔG values of formation to calculate ΔG for reactions.

Problem 5: Adjusting Equations

• Adjust reaction equations to find ΔG for new set of reactions.

Problem 6: Reaction Direction

• Negative ΔG: Reaction shifts right, ΔG increases toward equilibrium.

Problem 7: Estimating Boiling Points

• Use ΔG equation with ΔH and ΔS to find boiling point.

Problem 8: Identifying False Reaction Statement

• False Statement: "For a spontaneous process, K < 1."
  • True: K > 1 for spontaneous reactions.

Problem 9: Calculating ΔG from Kp

• Use: ΔG = -RT ln(Kp)
• Convert ΔG to appropriate units.

Problem 10: Calculating K from ΔG

• Use: K = e^(-ΔG/RT)
• Convert ΔG to joules when using R in joules.

Problem 11: Temperature Effect on Reactions

• Spontaneity depends on ΔH and ΔS:
  • Negative ΔH and ΔS: Spontaneous at low temperatures.
  • Equilibrium at 0°C: ΔG = 0 at equilibrium, process spontaneous below freezing point.

Additional Insights

• Importance of consistent units when using thermodynamic equations.
• Strategies for solving thermochemical equations, focusing on proper conversions and understanding of the relationships between ΔG, K, and spontaneity.