Standard entropy

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The standard entropy (symbol S 0 ) of a chemical substance is the entropy of this substance under standard conditions . If the value of the standard entropy relates to the amount of substance of one mole , one also speaks of the molar standard entropy (symbol S 0 m ).

Unit of measurement

The unit of measurement for standard molar entropy is J / ( K · mol ).


As a measure of the disorder in a system , one can imagine that this disorder increases with increasing temperature - such as water molecules , which in ice crystal initially in a certain order are available, solve by heating in the liquid water from this association until they move around almost completely independently of each other after boiling .

A formulation of the third law of thermodynamics says: For every pure substance that crystallizes in an ideal order , the entropy at absolute zero is the same. These substances are given zero entropy at T = 0 .

Therefore, standard molar entropies can be given in absolute values.

However, depending on the system under consideration, other zero points are chosen for the standard entropy. For electrochemical measurements in aqueous solutions containing ions , e.g. B. one assigns the proton the standard entropy zero.

See also