Unit 9.3: Gibbs Free Energy and Thermodynamic Favorability Notes

Understanding Gibbs Free Energy

• Gibbs Free Energy (G): A thermodynamic potential that indicates the favorability of a process.
• ΔG: Change in Gibbs Free Energy for a reaction.
  • If ΔG < 0: The process is thermodynamically favored (will occur spontaneously).
  • If ΔG > 0: The process is not thermodynamically favored (requires intervention).

Key Concepts

• Enthalpy (ΔH): The change in heat energy of a system during a process.
  • Key points:
  • If ΔH < 0: Exothermic reaction (heat is released), likely thermodynamically favored.
  • If ΔH > 0: Endothermic reaction (heat is absorbed), likely not favored.
• Entropy (ΔS): A measure of disorder or chaos in a system.
  • Key points:
  • If ΔS > 0: Process is becoming more disordered, likely thermodynamically favored.
  • If ΔS < 0: Process is becoming more ordered, likely not favored.

Equations for Gibbs Free Energy

1. Standard Gibbs Free Energy Equation: $ext{ΔG} = ext{ΔG}<em>{ ext{products}} - ext{ΔG}</em>{ ext{reactants}}$
   • Use values of ΔG of formation found in reference materials.
2. Temperature-dependent Gibbs Free Energy Equation: $ext{ΔG} = ext{ΔH} - T imes ext{ΔS}$
   • Useful when ΔH, ΔS, and temperature (T) are known.

Favorability Predictions

• Combination Predictions: Understand how ΔH and ΔS interact:
  1. ΔH positive, ΔS negative: Never favored (endothermic + more ordered).
  2. ΔH negative, ΔS positive: Always favored (exothermic + more disordered).
  3. ΔH positive, ΔS positive: Favored at high temperatures.
  4. ΔH negative, ΔS negative: Favored at low temperatures.
• Equilibrium: If ΔG = 0, the system is at equilibrium, and the reaction may occur at a phase change (e.g., melting or boiling).

Example Scenarios

• Example Question: Given a reaction, identify if it is thermodynamically favored based on ΔG and ΔS values.
• Analyzing the Reaction:

1. Given ΔH: Determine if it is exothermic or endothermic.
2. Determine ΔS: Analyze the states of matter to see if there's an increase or decrease in disorder.
3. Choosing the Correct Option: Based on the analysis, conclude if the reaction is thermodynamically favorable.

Practical Applications

• Calculations for Melting Points:
  • For phase changes, set ΔG = 0 to calculate melting points and predict reactions under various temperatures.
• Hess's Law for ΔG:
  • Similar to ΔH, can apply Hess's Law. Combine elementary steps to determine the overall ΔG for a reaction.
  • A negative ΔG from the overall reaction indicates the reaction is thermodynamically favored, even though the speed of the reaction may vary.

Conclusion

• Gibbs Free Energy is crucial to predict whether a reaction is spontaneous or requires assistance. Remember that thermodynamic favorability does not equate to reaction speed, which is a separate concept.