Gibbs Free Energy

Third Law of ThermodynamicsStandard Molar Entropy (S°)Gibbs Free Energy Equation

Third Law of ThermodynamicsThe 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 = nS(products) — mS(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. Enthalpy H

#2. Entropy S

Josiah Gibbs• Predicted whether a reaction would be spontaneous by

examining H and S, at constant P and T

GIBBS FREE ENERGY, focuseson system only, ignores surroundings

Gibbs Free Energy

Gibbs Free Energy

1. If G is negative, the forward reaction is spontaneous.

2. If G is 0, the system is at equilibrium.

3. If G is positive, the reaction is spontaneous in the reverse direction.

P. 804 and summary sheet

Sample Exercise 19.6 p. 805

Graphical Interpretation

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Standard Free Energy Changes

Analogous to standard enthalpies of formation are standard free energies of formation, G.

f

G = nG (products) mG (reactants)f f

where n and m are the stoichiometric coefficients.