How is the standard reaction entropy calculated?

The standard reaction entropy, denoted as Δ_rS®, is calculated using the formula that involves the standard molar entropies of both the products and reactants. The correct approach is to sum the standard molar entropies of the products, each weighted by their stoichiometric coefficients in the balanced chemical equation, and then subtract the sum of the standard molar entropies of the reactants, similarly weighted. This is represented as:

Δ_rS® = Σ(Products) νS®_m - Σ(Reactants) νS®_m

This calculation reflects the change in entropy as the system transitions from reactants to products under standard conditions. The reason this formulation is used is that entropy is a measure of disorder, and this approach quantifies how the overall disorder changes during a reaction. If the products are more disordered than the reactants, the reaction entropy will be positive, indicating an increase in disorder, and vice versa.

The other proposed formulas do not correctly reflect how standard reaction entropy is computed in the context of thermodynamics and do not involve the proper sums of entropies of products and reactants, making them unsuitable for calculating Δ_rS®.