23.3.3—The entropy change for a reaction, ΔS

For a reaction, ΔS° = ΣS°(products) − ΣS°(reactants). Multiply each standard molar entropy by its stoichiometric coefficient before summing.

Keep units consistent, usually J K⁻¹ mol⁻¹, and interpret the sign after the subtraction. A positive result means the system has more accessible arrangements overall.

For A + 2B → C, ΔS° = S°(C) − [S°(A)+2S°(B)]. The coefficients belong inside the brackets, not only in the balanced equation.

Do not reverse the subtraction or use ΔS° = ΔSsurr + ΔSsys when the syllabus only asks for the tabulated-state calculation.