Free Energy and Chemical Reactions

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) 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 ΔG is to use the equation:


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

ΔGreaction = ΣnpΔG(products) - ΣnrΔG(reactants)

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

CH4(g) + 2O2(g) = CO2(g) + 2H2O(l)             

By the calculations in enthalpy and entropy, the ΔH and ΔS of the above reaction is -8.91x105 J and -242 J/K, respectively. The ΔG can be calculated as:

ΔG = -8.91x105 J -(298 K) (-242 J/K) = -8.91x105 J + 0.721x105 J = -8.19x105 J = -819 kJ

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

Substances                ΔGf (kJ/mol)

CH4(g)                         -51

O2(g)                               0

CO2(g)                       -394

H2O(l)                        -237

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

ΔGf = [(-394) + (-237) * 2] - [(-51)  + 0 * 2] = -817 kJ


User Instructions

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: