Illustrated Glossary of Organic Chemistry

ΔS: The difference in entropy between two systems.

CH4 + 2 O2

CO2 + 2 H2O
By definition, ΔS = S(products) - S(reactants). So for the combustion of methane, ΔS = [S(CO2) + 2 S(H2O)] - [S(CH4) - 2 S(O2)].