Chem 2 - Std Free Energy of Formation VII

1. Std Free Energy of
Formation and G (Pt 7)
By Shawn P. Shields, Ph.D.
This work is licensed by Shawn P. Shields-Maxwell under a Creative Commons
Attribution-NonCommercial-ShareAlike 4.0 International License.

2. Gibbs Free Energy (G)
The Gibbs Free Energy change for
a given process is
G = H  T S (constant T and P)
The “free energy” available for work
and the extent of reaction are
described by G.

3. Standard Free Energy of Formation (Gf)
The standard Gibbs free energy of
formation (∆𝐺𝑓
°
) is defined as the free
energy change for the formation of one
mole of a given compound from its
constituent elements in their standard
states.
Pressure (1 atm) or Molarity (1 M)
(usually tabulated at 25C)

4. Gf for Elements in the Std State
Recall: Standard enthalpy of formation
(Hf) for a pure element in its standard
state is zero.
The standard free energy of formation
(Gf) for pure elements in the
standard state is also zero.
Gf = 0 for elements in the std state.

5. Examples of Standard Free Energies of
Formation (∆𝐺𝑓
°
) in a Table
An example is given below.
Here is the column that provides ∆𝐺𝑓
°
values (in kJ/mol).

6. Calculating ∆G0
Using Gf for
We can calculate ∆G0
from the tabulated
standard free energy of formation (Gf).
For the reaction
aA + bB  cC + dD
∆𝐆 𝟎
= 𝐜 ∆𝑮 𝒇
𝟎
𝐂 + 𝐝 ∆𝑮 𝒇
𝟎
𝐃 − 𝐚 ∆𝑮 𝒇
𝟎
𝐀 + 𝐛 ∆𝑮 𝒇
𝟎
𝐁

7. Calculating ∆G0
Using Standard
Molar Entropies (S)
Another way to write the equation:
For the reaction aA + bB  cC + dD
∆𝐆 𝟎
= 𝐜 ∆𝑮 𝒇
𝟎
𝐂 + 𝐝 ∆𝑮 𝒇
𝟎
𝐃 − 𝐚 ∆𝑮 𝒇
𝟎
𝐀 + 𝐛 ∆𝑮 𝒇
𝟎
𝐁
∆𝐆 𝟎
= 𝐧 𝐩 ∆𝑮 𝒇
𝟎
𝐩𝐫𝐨𝐝𝐮𝐜𝐭𝐬 − 𝐧 𝐫 ∆𝑮 𝒇
𝟎
𝐫𝐞𝐚𝐜𝐭𝐚𝐧𝐭𝐬

8. Standard Free Energy Change (G)
The standard Gibbs free energy (∆𝐺°
)
can also be calculated using tabulated
standard enthalpies of formation (∆𝐻𝑓
°
) and
standard molar entropy (S) values
∆G° = ∆H° − T∆S°
Calculate H and S using tabulated data,
temperature T in K, and plug into the
equation above.

9. Next up…
Free Energy G and the
Equilibrium Constant K (Pt 8)