How is entropy related to the spontaneity of a reaction?

1 Answer
David Drayer · Truong-Son N.
Dec 23, 2016

Generally a reaction will occur spontaneously if the entropy increases, but the enthalpy is also negative, or small and positive, according to Gibbs free energy equation.

Explanation:

Entropy (#S#) is a measure of the disorder in a system.

In a closed system, entropy always increases over time. In an open system, energy can be added to a system to cause a decrease in entropy, but this is not necessarily a spontaneous reaction.

If entropy (disorder) increases, and the reaction enthalpy is exothermic (#DeltaH < 0#) or weakly endothermic (#DeltaH > 0# & small), the reaction is generally spontaneous.

The Gibbs free energy equation is:

#DeltaG = DeltaH - TDeltaS#

A reaction will be spontaneous if the change in #G#, #DeltaG#, is negative.

For the product of temperature times #DeltaS#, where #DeltaS# is the change in entropy, if the change in entropy is positive (disorder increases), then #TDeltaS#, when subtracted, becomes negative.

Since the cosmos (observable universe) is essentially a closed system, entropy will continue to increase until the universe essentially ceases to exist.

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