19.4 – 19.5 Student Notes

III. Free Energy Concept

A. Free Energy – G –

ΔG is negative à

B. Standard Free Energy Change

1. Equation

ΔG° =

2. When does it occur?

3. Calculating ΔG° from ΔH° and ΔS°

What is the standard free energy change, ΔG°, for the following reaction at 25° C?

N₂ (g) + 3 H₂ (g) à 2 NH₃ (g)

Use values of ΔH°_f and S°, from tables 6.2 and 19.1.

C. Standard Free Energies of Formation

1. Standard Free Energy of Formation - ΔG°_f –

2. ΔG° =

3. Calculating ΔG° from Standard Free Energies of Formation

Calculate ΔG° for the combustion of 1 mol of ethanol, C₂H₅OH, at 25° C.

C₂H₅OH (l) + 3 O₂ (g) à 2 CO₂ (g) + 3 H₂O (g)

Use the standard free energies of formation given in table 19.2.

D. ΔG° as a Criterion for Spontaneity

1. When ΔG° is a large negative number (more negative than about – 10 kJ),

2. When ΔG° is a large positive number (more positive than about 10 kJ),

3. When ΔG° has a small negative or positive value (less than about 10 kJ),

Consider the reaction of nitrogen, N₂, and oxygen, O₂, to form nitric oxide, NO:

N₂ (g) + O₂ (g) à 2 NO (g)

From the standard free energy of formation of NO, what can you say about this reaction?

E. Interpretation of Free Energy

1. Maximum Work

w_max =

Free energy change is the maximum energy available, or FREE, to do useful work

Idealization –

2. Free-Energy Change During Reaction

ΔG° is positive, reaction is non-spontaneous à