**Basic Concept**

**Free Energy**, symbolized by **G**, is a thermodynamic function
related to spontaneity. G is defined by the relationship:

G = H - TS

where H is the enthalpy, T is the temperature in Kelvin, and S is the entropy. Therefore, it is especially useful in dealing with the temperature dependence of spontaneity.

**Standard Free Energy Change (ΔG****º)
**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.

*One way to calculate ΔGº
is to use the equation:*

* Δ**G**º _{
}= *

*Another way is to take
advantages of the fact that free energy is a state function. As calculations in
enthalpy changes (Δ**Hº)
and entropy changes (**Δ**Sº):*

* Δ**G**º _{reaction
}= Σn_{p}*

**Example 1. Calculating ΔG****º
at 25ºC using ****
ΔH****º and **
**ΔS****º
for the following reaction**:

CH_{4}(g) + 2O_{2}(g) = CO_{2}(g) +
2H_{2}O(l)

By the calculations in enthalpy and entropy, the
ΔHº and ΔSº of the above reaction
is -8.91x10^{5} J and -242 J/K, respectively. The ΔGº
can be calculated as:

ΔGº = -8.91x10^{5}
J -(298 K) (-242 J/K) = -8.91x10^{5} J + 0.721x10^{5} J =
-8.19x10^{5} J = -819 kJ

**Example 2. Calculating ΔG****º
using the property of the state function:**

**Substances
ΔG**_{f}º**
(kJ/mol)**

CH_{4}(g)
-51

O_{2}(g)
0

CO_{2}(g)
-394

H_{2}O(l)
-237

The standard free energy change of the reaction can be calculated as:

ΔG_{f}**º**
= [(-394) + (-237) * 2] -
[(-51) + 0 * 2]
= -817 kJ

*For the above example, ** *

1. Select **Free Energy and Chemical Reactions **link either from the front page (Table of Content) or from the **Thermochemistry ** module. The Input and Output screen appears.

2. In the **Input ** area, enter the problem equation; then click **Balance ** button to balance the equation; then enter corresponding Delta G value **(ΔG ****º f ****) **. You may consult the **Selected Thermodynamic Data ** sub-module for reference data.

3. Click **Calculate ** to output the answer with the detailed step-by-step show work on the side as shown below: