What is Standard Entropy? - QS Study
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The absolute entropy of a pure substance at 25°C (298 K) and 1 atm pressure is called the standard entropy, S°. The standard entropies of all substances either elements or compounds at any temperature above 0°K always have positive values.

When the standard entropies, S° of various substances are known, the standard entropy change of a chemical process or reaction is written as

ΔS° = Σ S°products – Σ S°reactants

This ΔS° is the standard entropy change of the reaction.

Standard entropy change of formation, ΔS°f is defined as the entropy of formation of 1 mole of compound from the elements present in the standard conditions.ΔS°fcan be calculated for chemical compounds using the S° values of elements from which the compound is formed.

ΔS°f compound = ΣS°compound  – Σ S°elements