Gibbs Free Energy

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# Gibbs Free Energy - PowerPoint PPT Presentation

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

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### Gibbs Free Energy

Third Law of Thermodynamics

Standard Molar Entropy (S°)

Gibbs Free Energy Equation

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.

Standard Entropies
• Molar entropy values of substances in their standard states.

p. 801

• Standard entropies tend to increase with increasing molar mass.
Standard Entropies

Larger and more complex molecules have greater entropies.

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.

Sample Exercise 19.5 p. 802
• Synthesis of Ammonia Example
• USE TABLE of Standard Molar Entropy
Chemical Reactions
• Spontaneity of reactions involves the investigation of two thermodynamic concepts:

#1. EnthalpyDH

#2. Entropy DS

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

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

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.