Entropy is a state function, so we can calculate values for a process using any path. This allows us to calculate the entropy change of a chemical reaction using standard entropies. Specifically, we sum the entropies of the products and subtract the entropies of the reactions: \[\Delta_{rxn}S^\circ = \sum_{\text{Products}}{v_i S^\circ_i} - \sum_{\text{Reactants}}{v_i S^\circ_i} \nonumber \] Where \(v_i\) is the stoichiometric coefficient. Let's look at the combustion of methane: \[\ce{CH_4} \left( g \right) + 2 \ce{O_2} \left( g \right) \rightarrow 2 \ce{H_2O} + \ce{CO_2}\left(g\right) \nonumber \] The standard entropies are: Molecule Entropy \(\left(\frac{\text{J}}{\text{mol}\cdot\text{K}}\right)\) \(\ce{CH_4}\) \(186.25^1\) \(\ce{O_2}\) \(205.15^1\) \(\ce{H_2O}\) \(188.84^1\) \(\ce{CO_2}\) \(213.79^1\) The entropy for the combustion of methane is: \[\Delta S^\circ = \left[ 2 \left( 188.84 \right) + 1 \left( 213.70 \right) \right] - \left[ 1 \left( 186.25 \right) + 2 \left( 205.15 \right) \right] = -5.17 \: \frac{\text{J}}{\text{mol} \cdot \text{K}} \nonumber \] References Chase, M.W., Jr. , NIST-JANAF Themochemical Tables, Fourth Edition , J. Phys. Chem. Ref. Data, Monograph 9 , 1998, 1-1951.