In limestone calcination, a decomposition process, the chemical reaction is CaCO 3 → CaO + CO 2 (g). The standard Gibbs free energy of reaction is approximated as ΔG° r ≈ 177,100 − 158 T (J/mol). The standard free energy of reaction is 0 in this case when the temperature, T, is equal to 1121 K, or 848 °C. Oxidation. In some cases, calcination of a metal results in oxidation of the metal.