Calculating Standard Free Energy Changes

The standard free energy change for a chemical reaction occurring under the standard condition can be calculated in one of three ways.

The first method uses the equation for the standard ΔG of the reaction. Consider the formation of calcium carbonate from calcium oxide and carbon dioxide.

The standard enthalpy change of the reaction can be calculated from the heats of formation of the compounds in the reaction, and the standard entropy change can be calculated from their standard molar entropies.

Using the values from the reference tables, the standard enthalpy change and the standard entropy change for the reaction can be determined.

Substituting these values into the equation for the change in standard free energy, the standard ΔG of the reaction at 298 K equals −130.7 × 10³ J.

In the second method, the change in standard free energy of a reaction is calculated from the difference between the sum of the free energies of formation of the products multiplied by their stoichiometric coefficients and the sum of the free energies of formation of the reactants multiplied by their stoichiometric coefficients.

Take, for example, the reaction between hydrogen and chlorine gases to form tw