1. Free Energy
Also called Gibbs Free Energy
Thermodynamic function also related to spontaneity and is useful dealing with the temperature dependence of spontaneity.
Symbolized by “G” and for a process that occurs at constant temperature, the change in free energy is given by:
ΔG = ΔH - TΔS

2. Processes carried out at constant temperature and pressure are spontaneous only if ΔG is negative.
That is, a process (at constant T and P) is spontaneous in the direction in which the free energy decreases (-ΔG means +ΔSuniv).

3. H, S, G and Spontaneity
G = H - TS H is enthalpy, T is Kelvin temperature
Value of H
Value of TS
Value of G
Spontaneity
Negative
Positive
Spontaneous
Nonspontaneous
???
Spontaneous if the absolute value of H is greater than the absolute value of TS (low temperature)
Spontaneous if the absolute value of TS is greater than the absolute value of H (high temperature)

4. Free Energy and Chemical Reactions
Standard Free Energy Change (ΔGo)
Go is the change in free energy that will occur if the reactants in their standard states are converted to the products in their standard states.
Go cannot be measured directly.
Knowing ΔGo values for reactions can help predict the relative tendency of reactions to occur
The more negative the value for Go, the farther to the right the reaction will proceed in order to achieve equilibrium.
Equilibrium is the lowest possible free energy position for a reaction.

5. Calculating Free Energy Change For reactions at constant temperature:
There are several ways to calculate ΔGo.
Method #1:
For reactions at constant temperature:
G0 = H0 - TS0

6. Calculating Free Energy: Method #2
An adaptation of Hess's Law:
Cdiamond(s) + O2(g)  CO2(g) G0 = -397 kJ
Cgraphite(s) + O2(g)  CO2(g) G0 = -394 kJ
Cdiamond(s) + O2(g)  CO2(g) G0 = -397 kJ
CO2(g)  Cgraphite(s) + O2(g) G0 = +394 kJ
Cdiamond(s)  Cgraphite(s)
G0 =
-3 kJ

7. Calculating Free Energy Method #3
Using standard free energy of formation (Gf0):
Gf0 of an element in its standard state is zero