Overview
This lecture explains the real-world applicability of Gibbs Free Energy (ΔG), focusing on how to calculate and interpret ΔG under non-standard conditions, using evaporation and a chemical reaction example.
Applicability of ΔG
- Standard ΔG calculations assume standard conditions (25°C, 1 atm, 1 M concentrations).
- Real-life scenarios often differ from standard state conditions, affecting the spontaneity of processes.
- Example: Calculated ΔG for water evaporation at standard conditions is +8.59 kJ/mol (non-spontaneous), but water does evaporate in reality due to lower than 1 atm partial pressure.
Calculating ΔG at Non-Standard State Conditions
- Use the equation: ΔG = ΔG° + RT ln Q, where Q is the reaction quotient.
- R (ideal gas constant) = 8.314 J/mol·K (or 8.314 × 10⁻³ kJ/mol·K for consistency with ΔG in kJ).
- Q represents the ratio of products to reactants, using concentrations or partial pressures (not at equilibrium).
- For partial pressure calculations, standard state means 1 atm for gases.
Example—NO Reaction Calculation
- Given: ΔG° = –71.2 kJ, T = 298 K, Q calculated using provided partial pressures.
- Q = (P(NO₂))² / [(P(NO))² × P(O₂)].
- Plug values into the ΔG equation to find ΔG under non-standard conditions.
- Result: ΔG = –50.7 kJ, less negative than standard, meaning less spontaneous.
Interpreting Spontaneity and Work
- Negative ΔG indicates a spontaneous reaction.
- The more negative ΔG is, the more spontaneous the process, and the more work can be extracted.
- A less negative ΔG under non-standard conditions means the system can do less work.
Key Terms & Definitions
- ΔG (Gibbs Free Energy Change) — energy available to do work at constant temperature and pressure.
- Standard State Conditions — 25°C, 1 atm pressure, 1 M concentration.
- Reaction Quotient (Q) — ratio of product/reactant concentrations or partial pressures, not at equilibrium.
- ΔG° (Standard Gibbs Free Energy Change) — ΔG calculated at standard state conditions.
- R (Ideal Gas Constant) — 8.314 J/mol·K or 0.008314 kJ/mol·K.
Action Items / Next Steps
- Practice calculating ΔG under non-standard conditions using the ΔG = ΔG° + RT ln Q equation.
- Review definitions and units for ΔG, R, Q, and standard state conditions.
- Prepare for problems involving real-world conditions, not just standard state scenarios.