The determination of entropy change during a chemical reaction relies on the principle that entropy is a state function. This allows for the calculation of reaction entropy by considering the difference between the total standard molar entropies of the products and the total standard molar entropies of the reactants. Standard molar entropy values, typically denoted as S, are readily available in thermodynamic tables for various substances under standard conditions (298 K and 1 atm pressure). The calculation involves summing the standard molar entropies of all products, each multiplied by its stoichiometric coefficient in the balanced chemical equation, and then subtracting from this sum the analogous sum for the reactants. For example, in a reaction aA + bB cC + dD, the change in entropy (S) is calculated as [c S(C) + d S(D)] – [a S(A) + b S(B)].
Accurate knowledge of entropy changes is crucial for predicting the spontaneity of chemical reactions. The Gibbs free energy equation (G = H – TS) highlights the relationship between enthalpy change (H), entropy change (S), and temperature (T) in determining whether a reaction will proceed spontaneously at a given temperature. Reactions with a positive entropy change are favored, as this contributes to a more negative Gibbs free energy, indicating a greater tendency for the reaction to occur spontaneously. Furthermore, this type of calculation provides insight into the degree of disorder or randomness introduced or reduced as a result of a chemical transformation. Historically, the development of these calculations has been instrumental in establishing fundamental principles of thermodynamics and chemical kinetics.
