Gibbs Free Energy

1. Gibbs Free Energy Third Law of Thermodynamics Standard Molar Entropy (S°) Gibbs Free Energy Equation

2. Third Law of Thermodynamics The entropy of a pure crystalline substance at absolute zero is S =0 at 0 K. P. 799 All our entropy values are relative to this “standard” value.

3. Standard Entropies • Molar entropy values of substances in their standard states. p. 801 • Standard entropies tend to increase with increasing molar mass.

4. Standard Entropies Larger and more complex molecules have greater entropies.

5. Entropy Changes Entropy changes (S)for a reaction can be estimated in a manner analogous to that by which H is estimated: S = nS(products) — mS(reactants) where n and m are the coefficients in the balanced chemical equation.

6. Sample Exercise 19.5 p. 802 • Synthesis of Ammonia Example • USE TABLE of Standard Molar Entropy

7. Chemical Reactions • Spontaneity of reactions involves the investigation of two thermodynamic concepts: #1. EnthalpyDH #2. Entropy DS

8. Josiah Gibbs • Predicted whether a reaction would be spontaneous by examining DH and DS, at constant P and T GIBBS FREE ENERGY, focuses on system only, ignores surroundings

9. Gibbs Free Energy

10. Gibbs Free Energy • If DG is negative, the forward reaction is spontaneous. • If DG is 0, the system is at equilibrium. • If G is positive, the reaction is spontaneous in the reverse direction. P. 804 and summary sheet

11. Sample Exercise 19.6 p. 805

12. Graphical Interpretation Watch This!

13. DG = SnDG (products)  SmG (reactants) f f Standard Free Energy Changes Analogous to standard enthalpies of formation are standard free energies of formation, G. f where n and m are the stoichiometric coefficients.